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Terahertz Spectroscopy Jason B. Baxter* and Glenn W. Guglietta Department of Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania, 19104, United States
’ CONTENTS Introduction Overview and Scope Introduction to THz Spectroscopy Resources for THz Information Advances in Analytical Capabilities THz Generation and Detection Photoconductive Antennas Electro-optic Effects Pulse Front Tilting in LiNbO3 Air and Gas Plasma Photonics THz Generation and Detection with Fiber Lasers Other Electronic Detection Schemes Potential Emitters and Detectors Based on Nanomaterials Improving Sensitivity and Scanning Rates in Coherent Detection Sample Detection Geometries: Transmission, Reflection, ATR General Considerations for THz Optics Transmission Reflection Attenuated Total Reflection (ATR) Parallel Plate Waveguides Imaging Far-Field Imaging Near-Field Imaging Applications Chemical Analysis and Molecular Spectroscopy Gases Liquids Solids Solid State Physics Semiconductors Nanomaterials Correlated Electron Materials Biology, Medicine, and Pharmaceuticals Biomolecules Biomedicine Pharmaceuticals Terahertz Imaging r 2011 American Chemical Society
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Conclusions and Future Outlook Author Information Biographies Acknowledgment References
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’ INTRODUCTION Overview and Scope. THz radiation, with frequencies in the range of 0.1 20 THz, can probe physical phenomena such as low-energy excitations and carrier dynamics in electronic materials, collective vibrational or torsional modes in condensed-phase media, and rotational and vibrational transitions in molecules. In other units, 1 THz is equivalent to 33.3 cm 1 (wavenumbers), or 0.004 eV photon energy, or 300 μm wavelength. The variety of phenomena that can be investigated using terahertz radiation, shown schematically in Figure 1, is of great importance to scientists and engineers. However, until recently, it has been very challenging to access this region of the electromagnetic spectrum, leading to what has often been called the “terahertz gap.” The low frequency end is still beyond what can be generated using electronic circuitry while the high frequency end is below what can be easily accessed optically. Fortunately, developments in THz sources and detectors over the last two decades have provided unprecedented means to probe this spectral region. Research in terahertz science and technology has exploded during this time as new spectroscopy and imaging applications have emerged in areas including solid state physics, biology, pharmaceuticals, and security screening. A Web of Science search using “terahertz or THz” returns less than 20 papers published in 1990, over 350 in 2000, and nearly 1400 in 2010. This critical Review focuses on developments in THz spectroscopy over the years 2008 2011. THz spectroscopy is an important tool for analytical chemistry. However, a Web of Science search in January 2011 showed only three articles containing the keywords “THz or terahertz” in Analytical Chemistry. This article is the first Analytical Chemistry Review of terahertz spectroscopy, and the technique may not be well-known to much of the readership. Therefore, we will devote some space to introductory material and seminal work published before 2008 in addition to more recent developments. We focus specifically Special Issue: Fundamental and Applied Reviews in Analytical Chemistry Published: May 02, 2011 4342
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Figure 1. Electromagnetic spectrum on the THz scale, with images highlighting the wide variety of molecules, materials, and phenomena that can be explored using THz spectroscopy. A few examples include charge carrier dynamics in semiconductors, nanomaterials, and correlated electron materials; hydration of solutes and librational modes in water and other liquids; and collective modes in proteins and amino acid crystals.
on THz generation and detection techniques, advances in spectrometer design, and applications of THz spectroscopy, with additional coverage of THz imaging. Terahertz radiation can be generated and detected using many different methods. Terahertz sources include free electron lasers, synchrotrons, narrowband quantum cascade lasers, and broadband generation from ultrafast pulsed lasers. This Review will primarily focus on time-domain spectroscopy performed using benchtop pulsed Ti:Sapphire lasers, which are the most common broadband terahertz sources. The Introduction will briefly review conventional approaches to terahertz spectroscopy in order to put recent advances into context. We then provide a compact listing of additional THz resources by highlighting books, journal review articles, and conferences devoted to terahertz science. Advances in Analytical Capabilities describes recent advances in analytical capabilities including terahertz generation and detection and spectrometer design with improved intensity, resolution, bandwidth, dynamic range, and speed. New methods of data analysis and interpretation are discussed. Applications highlights applications of THz spectroscopy in areas including molecular spectroscopy, semiconductors and other condensed matter physics, biology, pharmaceuticals, standoff detection and security, and art conservation. THz radiation can be transmitted reasonable distances through air, many plastics, cardboard, paper, clothing, and many other materials, with the notable exceptions of metals and water. THz radiation is therefore useful for imaging applications, which also will be briefly discussed. Conclusions and Future Outlook succinctly summarizes recent advances and applications and provides a future prospective. Introduction to THz Spectroscopy. This Review centers on THz spectroscopy based on generating and detecting subpicosecond far-IR pulses with an ultrafast laser. In conventional benchtop experiments, THz pulses are created and detected using short-pulsed Ti:Sapphire lasers with pulsewidths ranging from ∼10 to 100 fs and center wavelengths around 800 nm. THz pulses have a duration of ∼1 ps, and coherent detection in the
time domain allows measurement of the transient electric field, not only its intensity. Fourier transform of the transient electric field enables direct determination of both the amplitude and the phase of each of the spectral components that make up the pulse, in contrast to incoherent sources of far-IR radiation such as arc lamps or globars. The amplitude and phase are used to compute the absorption coefficient and refractive index of the sample. Thus, the complex-valued permittivity of the sample is directly obtained without requiring a Kramers Kronig analysis. In addition to the advantages of coherent detection, THz spectrometers based on pulsed Ti:Sapphire lasers enable timeresolved THz spectroscopy with subpicosecond time resolution. Hence, dynamics induced by near-instantaneous photoexcitation or heating with a laser pump pulse can be investigated. Pulsed sources such as free electron lasers or synchrotrons have also recently achieved subpicosecond pulse durations with very high intensities, but these are large, expensive user facilities and are less accessible than a Ti:Sapphire laser in an individual laboratory. We will focus on three forms of terahertz spectroscopy: THz time-domain spectroscopy (THz-TDS), time-resolved THz spectroscopy (TRTS), and THz emission spectroscopy (TES); we will also briefly cover THz imaging. THz-TDS is the most commonly used technique. In a typical configuration, THz-TDS enables determination of the complex permittivity of a sample over a frequency range of 0.2 3 THz. In THz Generation and Detection, we will describe how this range can be extended beyond 30 THz and even to 100 THz. THz-TDS provides information about the static properties of the sample, which is sufficient for many applications including molecular spectroscopy, biology, and quality control. In contrast, TRTS probes the dynamic, evolving properties of the material. In TRTS, the sample is typically photoexcited with an ultrafast laser pump pulse (with energy ranging from UV to mid-IR and even THz) and the time-dependent THz spectrum is obtained with the THz probe pulse after delay times ranging from less than 100 fs to several nanoseconds. One important application of TRTS is in semiconductor photophysics. Photoexcitation above the bandgap increases 4343
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Figure 3. (a) THz pulses in the time domain for an air reference and a sample, a ZnO wafer in this case. The pulse traveling through the sample is attenuated, delayed, and dispersed according to the sample permittivity. THz radiation was generated and detected using 1 mm thick ZnTe crystals and ∼100 fs pulse duration. (b) Power spectrum in the frequency domain, calculated by Fourier transform of the time domain pulse. Broadband absorption by mobile electrons in the sample as well as sharper absorption features at 1.1 and 1.6 THz from water vapor in the atmosphere are evident.
Figure 2. Schematics of typical transmission spectrometer configurations for (a) THz-TDS with photoconductive antennas and (b) TRTS with electro-optic generation and detection.
the conductivity of the semiconductor, which is directly related to its permittivity. Therefore, TRTS can serve as a noncontact electrical probe with subpicosecond temporal resolution. A few vendors offer either one-box THz spectrometers with included pulsed fiber laser or a spectrometer that is optically coupled to a Ti:Sapphire laser by the user. However, the vast majority of systems are home-built. A typical THz-TDS spectrometer is shown schematically in Figure 2a. This spectrometer design was first reported in 1989 by Grischkowsky and coworkers for measuring water vapor and in 1990 for semiconductors and dielectrics.1,2 The 800 nm, 80 MHz beam from a Ti: Sapphire oscillator is split into two parts, the THz generation arm and the THz detection arm. Generation occurs by focusing the 800 nm pulse onto a GaAs photoconductive antenna. THz radiation is collimated and then focused onto the sample and then recollimated and focused onto the detector using a set of four off-axis parabolic mirrors. If the sample is uniform over a large enough area, focusing at the sample is not required and two parabolic mirrors are sufficient. The THz pulse is detected by a second GaAs photoconductive antenna. The physics of THz generation and detection are described in THz Generation and Detection. The THz pulse is single cycle oscillation of the electric
field with duration of ∼1 ps. It is coherently measured in the time domain using a gated detection scheme, whereby an optical delay line adjusts the time delay between the THz generation and detection arms to reconstruct the THz waveform in the time domain. A computer controls the delay lines and records data from the lock-in amplifier, which is coupled to an optical chopper for improved signal-to-noise. THz radiation passing through a sample is attenuated and delayed, as in Figure 3, according to the sample permittivity. Fourier transform of the time-domain reference and sample waveforms enables determination of the sample permittivity, which is the basis of THz-TDS. Careful modeling of the transmission and reflection of the THz radiation through the dielectric stack is essential for correct transformation of the time-domain waveform into frequency-dependent material properties. Several methods for modeling THz propagation have previously been published.3,4 TRTS is very similar in concept to THz-TDS but requires the pulsed laser beam to be split into three parts, with the third part being the pump arm, as shown in Figure 2b. An amplified Ti: Sapphire laser operating at 1 kHz is typically used for pump probe studies because of the higher pulse energies and lower repetition rates compared to a Ti:Sapphire oscillator. Lower repetition rates are important so that the sample has time to relax back to its ground state (1 ms for 1 kHz vs 12 ns for 80 MHz) before the next pump pulse arrives. Larger pulse energies (by ∼105) in amplified systems are useful for achieving large pump fluence for photoconductivity measurements. Pump wavelength 4344
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Analytical Chemistry can be tuned from UV to mid-IR using nonlinear optical crystals and optical parametric amplifiers (OPAs). The higher intensity pulses would damage the photoconductive antennas, so optical rectification in a ZnTe nonlinear crystal is used to generate THz radiation which is then coherently detected by free space electrooptic sampling in a second ZnTe crystal. The THz radiation induces birefringence in the detector crystal, which rotates the polarization of the optical detection beam. The optical beam passes through a polarizing beam splitter and is detected using balanced photodiodes. Expected bandwidth is similar to photoconductive antennas, ∼0.2 3.0 THz. A second delay line adjusts the time delay between the THz generation pulse and the pump pulse for pump probe studies. TRTS measures dynamical properties by introducing an optical pump beam with variable time delay between the optical pump and the THz probe. In this way, the THz frequencydependent, complex-valued conductivity can be determined as a function of pump probe delay time. Two types of experiments are possible. “Pump” scans give the average change in THz absorption as a function of delay time. “Probe” scans give the frequency-dependent permittivity at a given pump probe delay time. The most complete information is generated by performing a 2D scan of both the pump and probe. “Difference scans” measure the change in THz transmission between photoexcited and nonphotoexcited states by chopping the pump beam and thus provide the most accurate data collection. TES shares many of the essential features of THz-TDS, wherein pulsed light irradiates a sample which generates THz radiation. However, THz-TDS simply uses radiation from a wellcharacterized source to probe a separate sample, while TES centers on analysis of the amplitude and shape of the transient electric field emitted from the sample under study. TES is often performed with an amplified laser and the detector in the nearfield of the emitter, and it should always be done without focusing optics to avoid artifacts.3 THz imaging makes use of the concepts of THz-TDS with the additional feature of spatial mapping. In the simplest case, the sample is rastered across the focus of the THz beam and amplitude and phase of either the broadband pulse or individual frequency components can be mapped. However, this procedure is slow and often suffers from poor spatial resolution due to the long wavelength of the THz radiation. Recent advances to be discussed in Imaging include real-time 2D imaging as well as reflection tomography to achieve 3D mapping. The use of continuous wave (cw) THz sources will also be described. THz imaging has potential applications including security screening, biomedical testing, pharmaceuticals, and art conservation. Resources for THz Information. Not surprisingly, the emergence of THz spectroscopy and its many potential applications has led to recent publication of many books and review articles. A number of specialized conferences fully or partially devoted to THz spectroscopy have also arisen. These will be briefly summarized here to provide direction toward resources that go beyond the scope of this Review article. At least three books devoted to THz spectroscopy have been published since 2008. Terahertz Spectroscopy: Principles and Applications devotes sections to instrumentation and methods including photoconductive antennas, optical rectification, electro-optic sampling, TRTS, and TES; as well as applications in physics, materials science, chemistry, biomedicine, and security.3 Edited by Susan Dexheimer, the book focuses primarily on time-
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domain spectroscopy methods using femtosecond laser sources, with each chapter written by experts in the field. Principles of Terahertz Science and Technology, by Yun-Shik Lee, describes generation and detection of both broadband THz using pulsed lasers and continuous wave THz radiation.5 It also covers THz optics and interaction of THz radiation with molecules and condensed matter, with an additional chapter on imaging. Introduction to THz Wave Photonics, by X.-C. Zhang and Jingzhou Xu, also covers THz generation and detection and interaction with matter.6 However, it also includes a much more extensive description of THz imaging and stand-off detection than the other books. A number of other THz books were published earlier than 2008. Sensing with Terahertz Radiation, edited by Daniel Mittleman, covers pulsed and continuous THz generation and detection, spectroscopy, and imaging. It also provides a brief history of the developments over the last four decades that led to the emergence of THz spectroscopy as we knew it in 2002.7 Terahertz Optoelectronics, edited by Kiyomi Sakai, focuses on work from Japan on THz generation by various means, with additional chapters on spectroscopy and imaging.8 Other edited books published since 2000 also include chapters detailing many different aspects of THz generation, detection, imaging, and spectroscopy, although they do not necessarily focus on the pulsed broadband THz sources reviewed here.9,10 A large number of journal review articles have also appeared since 2008. Jepsen et al. provided a very thorough and broad review of modern techniques in terahertz spectroscopy and imaging.4 Other reviews focus on different aspects including THz generation, detection, spectroscopy, imaging, and specific applications. Kitaeva reviewed THz generation by means of optical lasers, including both resonant and nonresonant excitations such as photoconductive antennas and optical rectification, respectively.11 Blanchard, Hoffmann, and Hebling and their respective coauthors focus more specifically on generation of THz pulses with high intensity and high field amplitudes that are useful for nonlinear THz spectroscopy.12 14 Krotkus focused on semiconductors for generation and detection of THz radiation in combination with ultrafast laser pulses, including an in-depth look at conventional low-temperature-grown GaAs as well as other novel materials.15 Karpowicz et al. reviewed generation and detection of THz radiation in gases via focusing of an ultrafast laser pulse, including both a discussion of the relevant physics and suggestions for implementation in spectrometers.16 Sizov and Rogalski provide a detailed review of THz detectors, including many different schemes and materials for both broadband and narrowband radiation.17 Walther et al. reviewed chemical sensing and imaging by THz-TDS.18 Capabilities and limitations of spectral and spatial resolution are described in the context of analytical and bioanalytical chemistry. Ueno reviewed essential concepts in THz-TDS and imaging with specific focus on applications in detecting hydrogen bonding, especially in amino acids and ice.19 Pickwell-MacPherson reviewed the potential of pulsed THz imaging for medical applications, and Zeitler reviewed tomography and imaging of pharmaceuticals.20,21 THz spectroscopy for explosives detection was reviewed by LeahyHoppa.22 THz is particularly useful in explosives detection because many common explosives have unique THz fingerprints, and THz can penetrate most dielectrics, clothing, plastics, and cardboard that might be used to conceal the explosives. A review by Davies described the use of THz spectroscopy for explosives and also for drugs of abuse.23 Markelz reviewed the use of terahertz spectroscopy to understand structure and function of 4345
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Analytical Chemistry biomolecules.24 The dependence of the dielectric response on collective vibrational modes and relaxational contributions was discussed, with particular attention on the effects of temperature, hydration, binding, and conformation. Son reviewed the application of THz spectroscopy and imaging for various biological molecules including DNA, proteins, peptides, and biological liquids.25 McGoverin and Wagh separately reviewed the use of THz spectroscopy in pharmaceuticals.26,27 Nemec reviewed TRTS of nanostructured materials for solar energy applications.28 We would also like to point out a number of particularly important reviews published before 2008. Schmuttenmaer described the essential features and implementation of THz spectroscopy and its application in semiconductor carrier dynamics, collective solvent modes in liquids, and intramolecular charge transfer.29,30 Ferguson reviewed materials for terahertz science and technology including broadband and narrowband THz sources and spectroscopy as well as imaging.31 Woolard critically evaluated the progress and promise of THz spectroscopy and imaging, as well as potential difficulties that must be overcome to advance the technology.32 Chan comprehensively reviewed techniques for THz imaging and presents a number of examples in emerging technologies.33 Zeitler reviewed the use of both pulsed THz imaging and spectroscopy for pharmaceutical applications such as identification of polymorphs and measurement of coating thickness and uniformity.34 Plusquellic reviewed application of THz spectroscopy to condensed-phased biological systems ranging from crystalline amino acids and polypeptides to aqueous proteins and DNA.35 Averitt reviewed THz spectroscopy of correlated electron materials including colossal-magnetoresistance manganites and high-temperature superconductors.36 Several specialized conferences and workshops are now completely or partially devoted to terahertz spectroscopy. The International Workshop on Optical Terahertz Science and Technology was recently held in March 2011 in Santa Barbara, CA. The 4-day workshop was exclusively focused on development of terahertz instrumentation based on optical sources and on applications of THz radiation in spectroscopy and imaging. The meeting is held biennially, with the 2013 workshop slated for Japan. The annual International Conference on Infrared, Millimeter, and Terahertz Waves will be held in Houston in October 2011. SPIE features numerous terahertz-related conferences, some portions of which are devoted to spectroscopy. These include “Terahertz Physics, Devices, and Systems,” “Terahertz Emitters, Receivers, and Applications,” and “Terahertz Wave Technologies and Applications”. Terahertz spectroscopy has recently also appeared prominently in the Optical Society of America’s “Ultrafast Phenomena” conference and the Gordon Research Conferences on “Ultrafast Phenomena in Cooperative Systems” and “Vibrational Spectroscopy”, all of which occur biennially. THz-related talks also now frequently can be found at OSA, APS, MRS, CLEO, and ACS meetings and at many other conferences as well. A number of THz organizations and Web sites exist, including the Terahertz Science and Technology Network (http://thznetwork.net). The THz Network’s goal is to lower barriers to entry into THz science and to foster collaboration and advancement of the technology. The thznetwork.net site includes listings of THz news, conferences, career center, buyer’s guide for THz components and spectrometers, and a link to the Virtual Journal of Terahertz Science and Technology. The virtual journal compiles THz-related articles published in other journals for ease of use.
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Multiple online databases containing THz spectra of a variety of materials now exist as well. These include the “Terahertz Database: http://thzdb.org,” operated by RIKEN and NICT in Japan, and the “THz Spectral Database” found in the NIST Chemistry Webbook.
’ ADVANCES IN ANALYTICAL CAPABILITIES In over 20 years since the first demonstration using THz radiation to investigate materials, many advances in THz generation, detection, spectroscopy, and sample preparation techniques have been made. These advances have led to improved THz bandwidth, power, field intensity, and signal-to-noise ratio and have enabled a wide variety of applications. This section will describe methods to generate and detect broadband THz radiation using femtosecond optical pulses, different spectrometer configurations, and techniques for imaging. We will provide an overview of these areas with particular focus on recent advances. Further information on tunable and continuous wave generation and detection, as opposed to broadband and pulsed THz, can be found elsewhere.4,37 The most common form of THz spectroscopy is THz-TDS. The measurement is made in the time domain but is not time-resolved; it provides information on static material properties. Alternatively, TRTS enables time-resolved measurement of transient permittivity, typically after photoexcitation with an optical pulse. The majority of the cost in a THz spectroscopy system comes from the ultrafast laser. TRTS measurements require an amplified Ti:Sapphire laser to provide sufficient power for optical excitation of the sample. Amplified lasers provide ∼1 mJ/pulse while oscillators provide ∼10 nJ/pulse. TRTS is therefore a more expensive undertaking than THz-TDS or THz imaging, which can be done with just a Ti:Sapphire oscillator or even a fiber laser. An amplified system can, of course, also be used for THz-TDS; however, an oscillator-only system usually will not work well for optical-pump TRTS. THz Generation and Detection. Oscillator-based spectrometers and imaging systems typically use photoconductive antennas for generation and detection, while amplifier-based spectrometers generally use electro-optic materials like ZnTe, DAST, LiNbO3, or even air. Photoconductive antennas, usually GaAs based, work well for oscillators but would be damaged by the high pulse energies provided by amplifiers. Nonlinear electrooptic effects are generally too weak to be useful for THz generation by Ti:Sapphire oscillators. With amplified systems, the most common electro-optic material has been ZnTe because it has relatively low group velocity mismatch between the copropagating optical and THz pulses. Group velocity mismatch can limit the optical-to-THz conversion efficiency and THz bandwidth. Thinner crystals and different materials have been used to increase bandwidth but at the expense of THz pulse energy. A relatively new development to overcome the group velocity mismatch is to use tilted-front pulses in LiNbO3, where the group velocities are matched by tilting the wavefront relative to the direction of propagation. The tilted pulse front method increases both bandwidth and field intensity of the THz pulse. More recently, high intensity THz pulses with very large bandwidth can be generated and detected using air or gas plasma-based techniques. Comparisons of some optical methods for high-field generation techniques have recently been reported by Blanchard et al.12 and Hoffmann and F€ul€op.13 Increasing the field intensity and bandwidth of THz pulses generated using Ti: Sapphire benchtop lasers is a major direction of current research. 4346
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Analytical Chemistry However, a significant amount of work is also being done to increase the portability and reduce the cost of terahertz systems. One focus has been on the development of sources and detectors that are compatible with telecom wavelength (1.5 μm) femtosecond fiber lasers. The development of electronic devices for THz generation and detection has also seen significant progress, primarily for communications. Additionally, various novel methods have recently been demonstrated with potential for future THz applications. In this section, we will discuss the basic physics of each of these generation and detection schemes, their respective advantages and disadvantages, and significant recent advances. Further details on the physics can be found in several references.3,5,6 Photoconductive Antennas. Photoconductive antennas were first reported by Auston et al. in 1984 for generation of THz radiation from ultrafast optical pulses.38 Today, photoconductive antennas remain the most widely used source and detector for THz-TDS due to their good performance, low cost, and relative simplicity. A photoconductive antenna typically consists of a pair of parallel electrodes with a small gap, as a dipole or coplanar stripline antenna, on a semi-insulating (but photoconductive) substrate with a silicon lens bonded to the opposite side. In the case of THz generation, an impinging ultrafast, above-bandgap, optical pulse creates photoexcited carriers that are accelerated by the dc-bias and then recombine. Applied dc-bias and electrode gap depend on material; low temperature grown GaAs (LTGaAs) antennas typically have a 5 10 μm gap with a bias of about 30 40 V. The short duration of the excitation and the short lifetime of the photoexcited carriers generate a current transient, emitting a THz frequency pulse. The THz pulse, about a picosecond in duration, is collected and directed by the attached lens. Another photoconductive antenna can be used for detection of the freely propagating THz signal. In the detector, the optical and THz pulses are both focused into the gap between electrodes and no dc bias is applied. Instead, the optical pulse creates photoexcited carriers, and the measured current depends upon the instantaneous electric field provided by the THz pulse. The optical pulse is much shorter than the THz pulse and coherently gates the detection by photoexcitation. The THz pulse is reconstructed in the time domain by measuring the detector current while varying the time delay between the optical and THz pulses. In general, shorter optical pulses and shorter carrier lifetimes are advantageous for higher bandwidth. Photoconductive switches used with oscillators generate a THz pulse with a field amplitude of around tens of V/cm, while large area switches used with amplified systems can approach tens of kV/cm.13 Further detail can be found in the respective sections of Jepsen et al.,4 Kitaeva,11 or Reimann.39 Photoconductive antenna materials continue to be investigated for improving detector and emitter performance. Conventional materials with high carrier mobility and low lifetimes include LT-GaAs on GaAs and radiation-damaged silicon-onsapphire. Kasai et al. have demonstrated a photoconductive antenna of LT-GaAs on high resistivity Si substrate that nearly eliminates the resonant phonon absorption that gives a gap in the spectrum from 7 to 10 THz with GaAs substrates.40 The contact metallization also plays a role in emitter performance. Two of the most common materials are low resistance AuGe alloys and Ti/ Al stacked layers. Recently, a systematic comparison of coplanar stripline emitters found that AuGe contacts provided twice the power of Ti/Au, although thermal stability was reduced.41
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Potential improvement of oxidative stability for GaAs switches has been reported for surface passivation followed by silicon nitride encapsulation.42 Recent progress has also been made to facilitate system alignment and to increase power with arrays of photoconductive antennas. One of the difficulties in setting up an antenna system is the optical alignment of the generation and detection pulses, which have to be focused onto the photoconductive gap that is only ∼10 μm wide. One solution is to fabricate an array of interdigitated antennas that could utilize a spot size of a few hundred micrometers.43 Another array emitter geometry uses the lateral photo-Dember effect by employing wedged metal stripes to create large unidirectional carrier gradients.44 This design produces higher bandwidth than the tested photoconductive antennas, but further improvement in power is needed. A microlens array to split and focus the optical pulse into alternating gaps in an antenna array uses a large portion of the generation pulse for high efficiency.45 Increased THz field strength from a femtosecond oscillator was also achieved using an RF bias across photoconductive electrodes that are insulated from the substrate.46 A similar method at lower bias frequencies for an amplified system has also been reported.47 Electro-optic Effects. At the same time as the development of photoconductive antennas, THz radiation was also generated using femtosecond laser pulses incident on the electro-optic (EO) crystal LiNbO3.48 Collinear THz generation by optical rectification requires only that femtosecond optical pulses are incident on an appropriate EO crystal. Optical rectification can be considered as difference frequency mixing of all of the spectral components within the optical pulse. In principle, a pulse with 100 fs duration has the spectral width to generate bandwidth of nearly 10 THz, although phonon absorption and velocity mismatch can reduce this significantly. THz detection by free space electro-optic sampling was reported in 1995 1996 after independent efforts of Nahata et al.,49 Jepsen et al.,50 and Wu and Zhang.51 Detection by free-space electro-optic sampling requires that the THz field is focused on an EO detection crystal. As the optical detection pulse passes through the crystal, its polarization is rotated according to the birefringence induced by the electric field of the THz pulse at that time. The relatively long THz pulse (∼1 ps) is coherently reconstructed by sweeping the relative time delay of the shorter optical pulse (∼100 fs) through the duration of the THz waveform. Considerations for selection of an EO crystal include nonlinearity, absorption of optical and THz radiation, and coherence length. In general, thicker EO crystals generate stronger THz pulses because of longer interaction lengths. However, thinner crystals can produce broader bandwidth because the optical and THz pulses accumulate less phase mismatch. ZnTe is currently the most commonly used EO material for both generation and detection because it has low group velocity mismatch between the 800 nm Ti:Sapphire pulse and THz frequencies. A typical 300 μJ, 100 fs Ti:Sapphire pulse incident on a 1 mm thick ZnTe crystal will produce a THz pulse with ∼1 kV/cm field intensity and bandwidth of ∼0.2 3 THz.3 Bandwidth is limited by group velocity mismatch and a ZnTe phonon mode at 5.3 THz. The bandwidth attainable using ZnTe has continued to increase by thinning the crystal for better phase matching and reduced phonon absorption. Moving beyond ∼10 THz also requires shorter laser pulses so that a broader spectrum of visible light can be mixed. Bandwidth over 30 THz was reported with 4347
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Analytical Chemistry 30 μm thick ZnTe crystals and pulse duration of 12 fs.52 In 2005, ultrabroad bandwidth exceeding 100 THz was achieved using tilted 90 μm thickness GaSe crystals and sub-10 fs pulses.53 GaP is also widely used, despite the lower sensitivity, because the phonon mode is not reached until around 7 THz. Further details regarding mechanisms involved in EO THz generation and theoretical limits of bandwidth using semiconductor materials are reported by Mu et al.54 The very thin crystals required to minimize phonon absorption are fragile and expensive. Recent work focuses on organic EO materials. These materials are either organic crystals that have a phonon resonance outside the generated THz frequencies or amorphous polymers that do not have a phonon resonance at all. The organic ionic salt, 4-dimethylamino-N-methylstilbazolium tosylate (DAST), the first and still most widely used organic material, generates bandwidth approaching 20 THz. DAST and other organic EO material performance were reviewed by the Hayden group in 2007.55 Rapid progress in nonlinear optical generation was made up to the ultrabroadband work in 2005. Since that time, a few techniques that had been previously reported but not widely pursued were rediscovered and developed into robust new directions for benchtop THz generation and detection. Recent emphasis on THz generation and detection has been on the use of tilted pulse fronts in LiNbO3 to achieve orders of magnitude increases in field strength, air plasma photonics with ultrabroadband and gap-free bandwidth, and the use of fiber-based femtosecond lasers for low-cost and compact spectrometers. We next discuss each of these three directions in turn. Pulse Front Tilting in LiNbO3. The ability to generate very high field intensities without large user facilities was significantly improved by the use of tilted-front optical pulses with a 63° 65° cut LiNbO3 crystal.56 While ZnTe exhibits good collinear THz phase matching with 800 nm light, phase matching with LiNbO3 requires the optical and THz beams to propagate in different directions. Phase matching can be achieved in the direction of THz propagation if the intensity front of the optical pulse is tilted using a grating. The optical pulse front is tilted such that the THz pulse is generated at an angle relative to the optical pulse, enabling group velocity matching and forming a continuous THz wavefront. Tilted pulse fronts enable the use of materials with high EO coefficients that would otherwise be unsuitable for optical rectification due to the large mismatch between the optical and THz group velocities. By 2007, tilted pulse front methods were able to generate intense THz pulses with energy up to 10 μJ.57 This THz intensity regime enables benchtop nonlinear THz spectroscopy that was previously limited to large-scale user facilities. Further details and progress up to 2008 were reviewed by Hebling et al.14 Much recent effort has been devoted to increasing THz intensity from LiNbO3 by manipulating the optical configuration, particularly in distributing larger pump energies over larger spot sizes. In the typical configuration, the optical pulse is tilted with a grating and then focused onto the LiNbO3. The use of imaging optics causes the pulse duration to increase dramatically with spatial chirp, reducing efficiency. A promising new method mounts the grating directly to the LiNbO3 crystal, which can be used to greatly increase generated THz power, potentially up to the millijoule level.58 The potential for increased intensity through the use of larger spot sizes was demonstrated by the use of a 6 mm optical spot to generate THz pulses with 30 μJ energy at 100 Hz.59
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The ability to shape terahertz pulses has value for a wide variety of spectroscopic and imaging applications. Proof-ofprinciple generation of tunable shaped pulses, both single cycle and multiple cycle, with multiple tilted-front optical pulses has been demonstrated by Yeh et al.60 They generated 3 μJ of THz energy at 1 kHz for a single cycle pulse with a focused intensity of 200 MW/cm2. Multiple optical pulses can enable cascaded optical rectification processes that increase bandwidth. This effect has been reported in conjunction with phase modulation of the copropagating optical pulse by the generated THz pulse to effectively result in pulse narrowing and increased THz emission at higher frequencies.61 Pulsed THz systems can be characterized in a variety of ways. The total pulse energy can be calculated by dividing the average power by the repetition rate. The pulse energy alone is not sufficient to determine the applied field strength; the focused THz spot size is also very important. The optical design and THz beam quality can be optimized to get a spatially uniform pulse. A uniform pulse can be focused to the smallest spot and therefore highest field strength. Careful implementation of a well designed optical system has been used very recently to generate single cycle pulses with field strengths over 1 MV/cm.62,63 Further details regarding optimization of the optical design as well as consideration and comparison of semiconductor materials can be found in F€ul€op et al.64 Detection of these high intensity fields is generally straightforward because the strong field is easily detected by EO sampling or a photoconductive antenna. THz absorption and velocity mismatch are the primary considerations for broad bandwidth detectors. Typically, ZnTe or GaP EO crystals with a thickness of a few hundred micrometers are used. These thinner crystals lead to increased bandwidth but lower sensitivity. Methods to retrieve the original waveform after distortion by propagation through a thicker, more sensitive, detection crystal have been reported.65 Air and Gas Plasma Photonics. The use of THz gas photonics offers many unique possibilities. The bandwidth is limited only by optical pulse duration and the field strengths are 100 those from ZnTe. Remote generation and detection are also possible. Second order optical rectification does not occur in a centrosymmetric medium such as a gas, but third order effects can be significant. Third order effects can generate THz radiation from three photons if one photon has approximately twice the energy of the other two. This process was initially considered under the framework of four-wave mixing, although this cannot explain all of the observed phenomena. Third order nonlinear coefficients are very small for normal air, but THz generation efficiency increases significantly if the air is ionized. THz generation in gas was first reported by Hamster.66 A more efficient method of THz generation is to mix the fundamental and second harmonic wavelengths in air plasma, as was demonstrated in 2000.67 The simplest way to mix the fundamental and second harmonic is to focus the optical beam to a point just after a thin beta barium borate crystal. Accurate phase control improves the efficiency of THz generation, particularly for remote generation,68 although two different reasons were suggested.67,69 Recently, a understanding of the generation process has been described with a full quantum-mechanical model.70 The group of X.-C. Zhang has driven much of the recent development of air photonics for THz generation and detection. The primary advantage of gas-photonics over EO techniques is gap-free bandwidth out to 20 THz with an 80 fs pulse.71 4348
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Figure 4. Broadband THz generation using air plasma generation and ABCD detection. (a) Time domain waveform and (b) its Fourier transform with an 85 fs amplified laser. (c) Time domain waveform and (d) its Fourier transform spectrum with a 32 fs amplified laser. Reprinted with permission from ref 75. Copyright 2010 Optical Society of America.
EO crystals suffer from phonon absorption, phase-matching limitations, and thermal degradation, but air does not. Figure 4 shows the air-plasma-generated THz pulse in the time domain and its spectral amplitude for two different pulse durations. Pulses with sub-20 fs duration can be used to generate ultrabroadband THz, exceeding 100 THz in ambient air.72 Field strengths can also be much larger with air plasma generation than with typical EO crystals, exceeding a few hundred kV/cm. Remote generation of THz radiation is possible by frequencydoubling locally and propagating the two optical beams in free space where they focus and mix at a distance to generate THz radiation. This strategy relieves the concern that broadband remote THz sensing would be limited by THz absorption by water vapor in the atmosphere. Finally, data analysis with air photonics is simpler than with EO because no generation or detection surfaces exist to create reflection echoes in the time domain waveform. Gas-phase detection of terahertz radiation was reported in 2006, when THz field-induced second harmonic (TFISH) generation was used for coherent detection.73 A detection technique similar to TFISH used an ac biased electrode pair to modulate the ionization of the air at the THz focus for coherent detection and enabled recycling the optical generation pulse for detection.71 Lu et al. further optimized the detection by investigating the role of laser intensity, bias field strength for ionization, gas species, pressure, and focusing.74 Later, the technique was renamed by the same group as THz-air-biased-coherentdetection (ABCD) and did not include the recycle.75 In that paper, Ho et al. also compare air plasma generation and ABCD detection with traditional THz-TDS techniques. With an 85 fs optical pulse, air photonics methods generated 10 THz of bandwidth compared to 3 THz for EO, as well as an order of magnitude increase in peak power. The only significant deficit compared to EO is losing 2 orders of magnitude of dynamic range in the overlapping bandwidth below 3 THz. Remote detection of THz pulses can be accomplished through THz-radiation-enhanced emission of fluorescence (THz-REEF).76 In this method, a femtosecond optical pulse is focused at the desired measurement location to generate an air plasma. THz radiation
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passes through the plasma and enhances the optical fluorescence. The THz waveform in the time domain can then be reconstructed from the transient fluorescence, enabling coherent and omnidirectional THz-TDS. A similar technique was demonstrated that could coherently detect the remote terahertz pulse at 10 m by changing the relative phase of the two colors that generate the plasma to delay the net electron drift through the terahertz waveform. Changing the relative phase modulates the time-dependent fluorescence intensity change. Hence, it is possible to reconstruct the THz waveform by “looking” at the plasma.77 Alternatively, recording the acoustic pressure change from the passage of the THz pulse through phase-controlled two color plasma enables reconstruction of the THz waveform by “listening” to the plasma.78 THz Generation and Detection with Fiber Lasers. Fiber lasers are stable, robust, and compact. They are also typically less expensive than comparable Ti:Sapphire technology. Emitters and detectors compatible with fiber lasers will help expand the application of THz spectroscopy. Ytterbium and erbium doped fiber lasers usually operate around telecom wavelengths of 1 to 1.55 μm. However, most contemporary THz emitters and detectors have been optimized to operate near the 800 nm wavelength generated by Ti:Sapphire oscillators and amplifiers. The telecom wavelengths can be frequency doubled to work with current materials, but power losses are high. Different emitter and detector materials are required to take advantage of the fundamental wavelengths of fiber-based systems. Much of the recent work has focused on development of photoconductive antennas and EO materials to operate at these wavelengths. Telecom wavelength devices require photoconductive antennas with small bandgap, high resistivity, and short carrier lifetime. One of the primary candidate materials is In1-xGaxAs, although the dark resistivity needs to be increased and carrier lifetime needs to be decreased. Early work utilized doping79,80 or lowtemperature growth81 to improve performance. The potential of InN as a THz emitter was also studied as a function of the growth conditions.82 Progress has been made, but improvement is still needed to match the performance of LT-GaAs with 800 nm pulses. One approach is to use composites. ErAs nanoparticles in In.53Ga.47As were investigated and measured to have a higher average power at cryogenic temperature than structures without nanoparticles at room temperature.83 These materials were later used in a superlattice configuration with substantially improved power and bandwidth to 3 THz.84 Telecom wavelength emitters can also be improved by etching the photoconductive material between the electrodes and leaving a mesa-like conductive region in the middle.85 This method increased the THz output power by about 5 and sensitivity as a receiver by 11, compared to their measured planar geometries. Bandwidth was ∼4 THz. An alternative method that circumvents the low dark resistivity of these materials is to use the lateral photo-Dember effect86 as mentioned in Photoconductive Antennas, with the additional potential of use with even smaller band gap materials. The potential advantages of fiber-based laser systems have also stimulated work to develop organic EO materials that work well at telecom wavelengths. The commonly used DAST was photoexcited with an ultrashort 17 fs pulse Er-doped fiber laser to generate and detect up to 25 THz.87 Alternatively, a hydrogen bonded, configurationally locked polyene crystal was shown to be comparable to DAST but without the gap near 1.1 THz.88 The recent development of DASC (4-dimethylamino-N-methyl-4stilbazolium p-chlorobenzenesulfonate) may offer higher power 4349
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Analytical Chemistry than DAST.89 EO polymers have also been under development. Poled polymer films excited at telecom wavelengths have shown bandwidth to 15 THz.90 Other Electronic Detection Schemes. A wide variety of electronic devices are capable of detecting THz frequency radiation. They are not widely used for THz spectroscopy due to generally poor performance approaching 1 THz but are becoming more prevalent for incoherent imaging. The extremely diverse nature of these devices prohibits more than a very limited review to provide a reference for further investigation. Electronic devices for THz generation rapidly lose power beyond 1 THz, with significant losses after 0.1 THz. Diodes are widely used. Common types are Gunn, impact-avalanche and transit-time, and resonant tunneling diodes. Frequency multipliers are also common. Further detail on electronic generation at THz frequencies can be found in Eisele.91 Electronic detectors have a larger role in characterization of THz sources and in imaging. Detectors can be split generally into incoherent (amplitude only) and coherent (amplitude and phase) configurations. Incoherent measurements are typically accomplished with sensors that directly detect the THz radiation. Direct detectors are used for THz power measurements, operate at or near room temperature, and have millisecond response time. Common detector types are Golay cells, pyroelectrics, or microantenna coupled bolometers. Direct detectors with faster response times and greater sensitivity typically require cooling below 4 K. Alternatively, coherent detectors use an electronically accessible modulation frequency of the signal to carry the complex-valued measurement (heterodyne detection). Coherent detection requires a nonlinear response to the THz electric field to mix, or down-convert, the measurement. Low noise is desired for efficient conversion. The most common devices are Schottky barrier diodes, superconducting tunnel junctions, and hot electron bolometers. Further detail regarding electronic THz detection and progress through 2009 is reviewed by Sizov and Rogalski.17 Potential Emitters and Detectors Based on Nanomaterials. Considering the large amount of work on nanostructured materials, it is not surprising that some potential applications in the terahertz region have been found. One novel method of generating THz frequency radiation uses self-assembled ZnO nanoparticles that are decorated with nanocantilevers. Photoexcitation with a mechanically modulated green cw laser causes the cantilevers to resonate, emitting radiation at about 0.36 THz.92 Carbon nanotubes have been proposed as both generators and detectors of THz radiation. High power densities are suggested to be possible from arrays of single wall carbon nanotubes.93 Carbon nanotubes can be used for detection of THz radiation, as discussed by Portnoi.94 Metallic single wall carbon nanotubes have also been reported as a feasible hot electron bolometer for heterodyne detection of THz radiation.95 This detector offers the potential to increase the effective bandwidth of typical bolometer detection schemes with very low power requirements at room temperature, although improvements are needed before it can compete with other detection methods. In general, it appears that antenna structures are needed to couple the THz radiation to the carbon nanotubes. With the rapidly expanding amount of research on graphene, it is no surprise to find that it has also been proposed as a THz detector.96 Improving Sensitivity and Scanning Rates in Coherent Detection. Coherent detection of the THz pulse is critical for all
spectroscopic detection schemes as well as broadband imaging.
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Increasing the measurement sensitivity and speed will improve signal-to-noise ratio and reduce measurement time. Most benchtop THz systems use gated detection with a mechanical delay stage to obtain the THz waveform. Mechanical delay lines can limit the data collection speed, introduce noise, and are not very compact or portable. Recent work has reported methods to overcome the various contributors to noise for increased measurement speed and sensitivity. When very small signals are measured, the technique of double-modulated differential detection can greatly increase sensitivity. Double modulation reduces noise by increasing the effective modulation frequency.97 More recently for time-resolved experiments, separately chopping the pump and THz generation at nonharmonic frequencies allowed simultaneous detection of the reference and signal scans.98 This technique was effective for minimizing noise from laser power output and pulse detection timing errors. Measurement stability can be increased by incorporating a control loop into the system. In most EO detection schemes, a pair of photodiodes are used to measure the polarization change of the probe pulse caused by the coincident THz electric field. The detectors are balanced with a quarter-wave plate such that the difference signal is zero when no THz field is present. The optical alignment and polarization quality are important, and small changes in the environment can reduce the signal-to-noise ratio of this detection scheme. To compensate for environmental changes, Schulkin and Zhang demonstrated a control mechanism for the quarter-wave plate to actively maintain photodiode balance.99 Detection of small signals usually requires extensive averaging to achieve good signal-to-noise levels. The upper limit on averaging time depends on the rate of drift due to environmental changes. For a given detection time, the signal-to-noise ratio can be improved by a factor of 21/2 when doubling the detection speed. The increased signal-to-noise ratios that can be achieved with high scan rates have been the subject of several recent efforts. Linear delay lines typically require tens of seconds to scan over an optical delay time of tens of picoseconds. In contrast, a rotary delay line with six curved mirrors with rates up to 400 Hz for a 70 ps time window was reported by Kim et al.100 A later design with a screw auger delay line could be more compact but was not balanced for high-speed operation.101 Most recently, a monolithic rotary optical delay line with opposing helicoid mirror surfaces achieved high speed scanning with a spectral capture rate of 250 Hz for a 140 ps scan.102 This design has the advantages of the use of a pair of mirrors to achieve a stable center ray and symmetric design for low-noise, high-speed operation. Additionally, the design can be scaled to alter the scan length and the measurement speed by changing the number and/or angle of mirror pairs and the diameter of the cylinder. The use of high-speed electronics in conjunction with two lasers, as with asynchronous optical sampling (ASOPS) and electronically controlled optical sampling (ECOPS), may offer the fastest collection times. These techniques do not use mechanical delay lines or lock-in detection, but they require a second pulsed laser. ASOPS uses two lasers that are synchronized with slightly different repetition rates. This offset results in a varying time delay between the pulses. The difference in repetition rate determines the rate at which the faster laser samples the slower. The scan rate is on the order of a few kHz, much faster than a mechanical delay. This technique can provide sufficient spectral 4350
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Analytical Chemistry resolution to capture the 100 spectral lines of water vapor between 0.2 and 6.5 THz with a 60 s scan. A 0.6 s scan still has bandwidth out to ∼3.5 THz.103 Further system improvements have been reported such as reducing the timing jitter between the synchronized lasers so that the temporal resolution is limited only by the optical pulse duration.104 This system was also recently used to extract the spectra of D2O with 1 GHz resolution from the H2O and HDO components.105 In ECOPS, the two lasers are synchronized with piezo-controlled cavity mirrors. The sweep is then set by application of an offset voltage to the locking electronics of one laser.106 The advantage over ASOPS is that the scan rate and length can be adjusted electronically. The temporal scan length can then be chosen to achieve precisely the desired resolution, whereas ASOPS has a fixed scan length. The ECOPS system is reported to have an improved signal-to-noise ratio compared to the ASOPS for a given measurement time. Despite the exciting potential offered by ASOPS and ECOPS, the requirement of two lasers may limit their use. Sample Detection Geometries: Transmission, Reflection, ATR. THz spectrometers can be configured in various ways
depending upon the sample properties and geometrical constraints. Transmission THz-TDS is both the most straightforward and the most common. Transmission measurements work well on samples that are moderately absorbing and of low dispersion. Samples that are highly absorbing, reflective, or dispersive may instead be best measured in a reflection or attenuated total reflection (ATR) geometry. The parallel plate waveguide geometry can provide high spectral resolution and is useful for low-volume, highly scattering, or weakly absorbing samples. Each of these geometries will be discussed in this section. Details on the mathematics behind parameter extraction in each geometry can be found in the review by Jepsen.4 General Considerations for THz Optics. Transmissive optics like polyethylene lenses make the spectrometer more compact, with some absorption losses at higher frequencies. High-resistivity silicon has very low absorption over most of the THz range and is commonly used for beam splitters and filters. Reflective optics, typically metal-coated off-axis parabolic mirrors, offer very low losses but require more table space and can be more difficult to align than transmissive optics. THz-TDS of gases or solids that are uniform over a large area allows for a two-lens configuration. Small samples and TRTS require a four-lens system with the sample placed at the THz focus. The very wide frequency range within a single THz pulse leads to a frequency-dependent beam waist. To avoid frequency-dependent artifacts, the sample should be uniform across the entire beam waist. Additionally for TRTS, the spot size of the optical pump should be significantly larger than the THz probe. Strong THz absorption by water vapor typically requires the THz beam path to be purged with dry air or nitrogen or even evacuated. An alternative configuration placing the generator and detector sufficiently close together can produce a compact system for spectral analysis without lock-in detection or a water-free sample chamber.107 Transmission. The majority of THz spectroscopy is performed in transmission either as THz-TDS or as TRTS. Transmission typically allows for more straightforward alignment and data analysis. Reviews of THz-TDS data analysis can be found in Jepsen.4 and TRTS fundamentals can be found in earlier work.3,29,108 Many of the recent developments in data analysis have been driven by challenges in analyzing time-domain spectra of strongly absorbing, very thin, or granular samples.
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The dynamic range of THz-TDS is strongly frequency dependent. The dynamic range determines the maximum absorption coefficient that can be measured for a given sample thickness as well as and the low- and high-frequency cut-offs.109,110 In transmission, a strongly absorbing sample can exceed the available dynamic range, so comparison to the maximum measurable absorption ensures accurate parameter determination. If more dynamic range is needed, a reflection measurement should be used. The spectral resolution of a scan is determined by the recorded window of the THz waveform. To achieve spectral resolution below 10 GHz, a scan longer than 100 ps is required. In transmission, reflections arising from the generation and detection crystals, the sample, or optical windows frequently occur within this time period. Reflections will appear in the time domain as multiple waveforms. For example, a 1 mm thick ZnTe generation crystal will have a second pulse that appears about 18 ps after the main pulse. Delay from a reflection will depend on the index of refraction of the material and the thickness. The desired spectral resolution should be considered during design of the optical configuration. An algorithm for suppression of these reflections has been reported,111 although longer time-domain scans that include the reflections can increase the accuracy of the determined parameters. The early work of Duvillaret was a critical step in data analysis for determination of optical parameters when time-domain scans include overlapping reflections, as would be the case for thin crystals or thin samples.112 Recently, the generalization of a transfer function methodology was reported for multilayered samples using a spatially variant moving average filter.113 Thin films do not significantly perturb the THz pulse, and noise and drift in the laser or spectrometer can mask the sample properties if they are determined using two separate measurements of substrate and sample. Sample properties can be determined more accurately by differential spectroscopy, which modulates the sample position at a frequency much lower than the laser repetition rate. The difference between substrate and sample can be accurately recorded by locking into the sample modulation frequency. This difference can be added to the substrate time-domain waveform for more accurate determination of thin film permittivity. Even with double modulation, very thin samples with low permittivity can be difficult to measure due to mechanical noise of linear sample motion as well as uncertainty in sample thickness. Two alternatives have been recently reported. A spinning wheel can modulate the samples with reduced noise, even for liquids.114 Alternatively, a data analysis technique can be used that utilizes an additional Fourier transform with a minimization routine to extract the sample thickness, absorption coefficient, and refractive index simultaneously.115 For all techniques, sample thickness can be optimized for minimum parameter error.116 An early review of some important TRTS considerations was given by Schmuttenmaer.29 Later, additional systematic analysis of 2-D conductivity was reported by Nemec.117,118 More recently, analysis in the thin sample limit was described that also gives reasonable results for thicker samples.119 Interest in nano- and microstructured materials and their analysis with THz spectroscopy has resulted in increased application of effective medium theory. Samples should be treated as effective media when the wavelength of the THz radiation is larger than the characteristic length scale of heterogeneity in the sample. For example, the measurement of a highly absorbing powder can be facilitated by diluting with a polymer like 4351
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Analytical Chemistry polyethylene and forming a pressed pellet. However, the different dielectric constants of the sample and the host can contribute to scattering, which must be accounted for in measurements.120 The role of scattering has also been investigated with modified Kramers Kronig relations to quantify deviations from the extracted complex-valued index of refraction.121 A finite-difference time-domain analysis has also been used to characterize scattering from rough surfaces and grains.122 New effective medium theories have been developed and demonstrated for three polymeric compounds with inclusions of different shape and permittivity.123 Reflection. THz-TDS in reflection mode is very useful for samples that are highly absorbing or opaque. Reflection can be at normal incidence or off-normal with specular reflection. A beam splitter is required for normal incidence, which reduces the signal intensity by at least a factor of 4. Off-normal reflection avoids losses due to beam splitting. Material parameter extraction is more difficult, but appropriate methods for data analysis have been developed. The dynamic range and noise limitations are somewhat different in reflection, compared to transmission. Although the dynamic range of the spectrometer is larger in reflection, the noise floor becomes more strongly dependent on the laser power and phase stability.109 More details and schematics of reflection spectrometers have been published elsewhere.4,124 One of the first reports of reflection THz-TDS measured liquid water contained behind a silicon window.125 In that case, the reflection from the front surface of the silicon window was used as the reference signal. This technique requires choosing a useful window thickness. A thinner window will have smaller signal loss from absorption, but the reflected sample signal will be closer in time to the reference, reducing spectral resolution. Alternatively, the sample can be replaced by a highly conductive metal mirror to collect the reference signal.126 This method is sensitive to placement of the reference mirror because a small variation between reference and sample position leads to errors in the measured phase shift that must be removed numerically. The phase shifts can also be corrected using two different polarizations of the THz pulse.127 For simple analysis, the second derivative of the phase in the frequency domain can be used.128 The most complete method may be the self-referenced reflection configuration and associated generalized analysis. This method can be used without incident angle or polarization limitations, as demonstrated for aqueous salt and alcohol solutions.129 Generalized Kramers Kronig relations may also be used to more rigorously correct for phase shifts in reflection measurements as well as other geometries.130 The high dynamic range of a THz spectrometer in reflection is useful with wide bandwidth sources and detectors for characterization of phonon resonances.75 Alternatively, the reflection measurement can be conducted with the entire THz waveform remaining in a nonlinear ferroelectric crystal, like LiNbO3, which leads to a very compact setup.131 The sample is placed in direct contact with the crystal surface to reflect the THz pulse, which is then detected within the same crystal. Attenuated Total Reflection (ATR). The use of attenuated total reflection (ATR) for spectroscopy has been developed by the Tanaka group as an alternative to transmission or reflection measurements for strongly absorbing samples. The method was first demonstrated using a particularly important highly absorbing material: water.132 The ATR crystal is typically a Dove cut prism of silicon designed for a single reflection. The sample should be placed directly on the surface where the THz pulse is
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totally internally reflected, so that it can interact with the evanescent wave. ATR can be conveniently employed because the crystal and sample can be placed directly in a transmission spectrometer, unlike reflection measurements. The temperature of the prism must be carefully maintained because each degree K results in a 6 fs shift.133 Occasionally, the sample will be held a controlled distance away from the prism, for example, in the investigation of surface plasmons.134 Recently, THz-ATR spectroscopy was used to determine the hydration state of solvated disaccharides.135 THz-ATR should be applicable to measure the role of hydration in biomolecules or biopolymer solutions as well. Sensitivity for a specific application may depend on the total number of water molecules participating in the hydration. The technique can work for a high concentration of the sample with few hydration water molecules, like the disaccharides, or dilute concentration with a large number of hydrating water molecules. The limiting factor appears to be the number of the water molecules directly involved in hydration in the sample region. Hydration of monomeric and miscellar 2-butoxyethanol molecules was studied by THz-ATR in sufficient detail to propose a hydrogen bonded hydration layer that stabilizes the assembled structure.136 The sensitivity of the ATR method has also enabled identification of four component modes of vibration in the far-IR region of water and heavy water, supporting a two component model.137 ATR geometry may also be compatible with TRTS for tracking chemical reactions or biological changes with much higher sensitivity than reflection or transmission measurements. Parallel Plate Waveguides. Parallel plate waveguides (PPWGs) concentrate and confine the THz radiation in one dimension which increases THz sample interaction. The other major advantages of PPWGs are high sensitivity even with small sample volumes, increased THz field strength, and good temperature control. Much of the development of metal waveguides, and particularly PPWGs, for highly efficient coupling of THz radiation can be attributed to the Grishkowsky and Mittleman groups. PPWGs were proposed in 2001138 and were demonstrated for spectroscopy of nanometer thick water layers in 2004139 and thin polycrystalline films in 2006.140 This early work demonstrated the feasibility of the use of the enhanced THz sample interaction to greatly increase the spectral resolution and sensitivity for small sample quantities. A PPWG typically consists of two polished, flat, conductive metal plates that are centimeters in length and are separated by tens of micrometers. High-resistivity silicon plano-cylindrical lenses are used to couple the THz into and out of the waveguide. The metal plates also enable good control over sample temperature. Because PPWGs are centimeters long, any reflections from the lens surfaces are far apart in time, allowing long scans to achieve a few GHz of spectral resolution. The geometry enables drop casting of thin films for measurement, as opposed to forming pellets from pressed powders. The significantly smaller sample mass (tens of micrograms) compared to pellet formation (milligrams) is attractive for dangerous materials like explosives and toxins.141 Recent work has focused on improving coupling into the waveguide, propagation efficiency, field concentration, and better understanding of the internal field distribution. Confinement of a THz pulse within a waveguide will increase the field strength and could improve the signal-to-noise ratio. Theory predicts that adiabatic compression in a waveguide could enhance THz field strength by as much as 10 250, with a minimum spot size of 100 250 nm.142 Despite this great 4352
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Analytical Chemistry potential, fabrication and application of these structures will be difficult. A simpler configuration was recently demonstrated in which the entrance and exit of the waveguide are machined as a taper to achieve better coupling without the reflective losses and alignment difficulties of the Si lens coupling.143 Open-ended PPWGs also have no THz reflections, enabling very long temporal scan lengths for even higher resolution. A tapered coupling has been produced by fabricating flares out of 100 μm copper shim stock. The coupling efficiency remained high for a wide range of curvatures, allowing for easy application.144,145 Parallel plate waveguides typically operate with a plate separation on the order of tens of micrometers for efficient propagation of the TEM mode. The small separation requires a tightly focused spot that can be difficult to achieve. Propagation of the TE1 mode in a PPWG has been compared to the TEM mode in simulation and experiment.146 Mendis and Mittleman simulated the coupling efficiencies and determined the limit for the TEM mode to be about 89% compared to 99% for the TE1 mode. The TE1 mode also shows a much slower decrease in efficiency for increasing ratio of plate separation to spot size. The plate separation for the TE1 mode can be on the order of millimeters which makes fabrication and operation much easier. This mode also has exciting prospects for sensing with resonance and extremely low losses, on the order of dB/km. Coupling into the TE1 mode has also benefited from metal flares.146 Probing the dynamics of semiconductor thin films upon photoexcitation has received much recent attention. PPWGs are desirable for characterizing static permittivity of thin films, but metal plates do not readily allow photoexcitation. However, a transparent conducting oxide (TCO) PPWG has been demonstrated for photoconductivity measurements of silicon thin films.147 The propagation losses were high due to the relatively low conductivity of the TCO compared to aluminum or copper. However, the same group later demonstrated a full time-resolved optical-pump THz-probe measurement with silicon in an aluminum waveguide with a TCO window for the pump beam.148 Imaging. Two- and even three-dimensional mapping of the THz response of an object has potential applications including security screening, nondestructive evaluation, quality control, and art conservation. While a wealth of information can be obtained from pixel-by-pixel time-domain spectra using standard THz-TDS, this process is slow and has spatial resolution on the order of 100 μm 1 mm. Different imaging applications require different compromises between spatial resolution, spectral resolution, and speed. Speed can be increased at the expense of spectral resolution by monitoring only the peak height or peak position, rather than the full THz waveform, or using a narrowband continuous wave (cw) THz source. Spatial resolution can be improved by moving from far-field imaging conditions to near-field imaging. Far-field imaging is diffraction limited to over 100 μm spatial resolution by the THz wavelength. Far-field imaging uses standard optical elements to control the THz radiation. We will emphasize recent improvements in imaging speed, image reconstruction, and tomography. Near-field imaging enables subwavelength resolution well below the diffraction limit. We will focus on aperture and aperture-less techniques. Near-field imaging often uses a narrowband, cw THz source because high power is needed to overcome the small signals, although recent developments have reduced this constraint. Although this Review focuses on pulsed broadband THz radiation, cw narrowband THz sources and detectors will be
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included here because of their widespread use in imaging. The prevalence of cw THz for imaging arises because cw systems are typically higher power, lower cost, simpler, lighter, and more robust than pulsed systems. Continuous wave systems can therefore be portable, which enables many additional applications. The cw wavelength can be selected to avoid water absorption lines for long-range stand-off detection. Many cw systems are narrowband but have wavelengths that are tunable over finite ranges. For applications requiring broadband coverage and portability, fiber-based pulsed spectrometers could garner significant interest. Most cw generation and detection elements are photomixers that are similar in materials, construction, and electrical bias to photoconductive antennas. In contrast to photoconductive antennas, two collimated and aligned cw lasers with visible or nearIR wavelengths illuminate the photomixer. The resulting beat frequency is in the THz region. Quantum cascade lasers can be used for high powers, but they will not be discussed further. Continuous wave systems can usually be tuned, but the tunable range is typically well below the broadband range of pulsed systems. However, the spectral resolution of cw systems is well into the low MHz region, compared to GHz for broadband measurements. Continuous wave THz radiation is typically detected incoherently with a bolometer. Coherent detection can be accomplished with another photomixer149 or EO detector.150 Further information on cw THz imaging can be found in the book by Y.S. Lee5 and the review by Seedkia and Safavi-Naeini.37 Pulsed and cw imaging systems have been reviewed by the Zhang group.6,151 Here, we describe some recent advances in imaging systems, generally categorized by far-field and near-field configurations. Far-Field Imaging. In the far field, both pulsed and cw THz radiation can be used, depending on the desired information. Increased speed is a fundamental need for further development and application of THz imaging. Faster imaging can be accomplished using a variety of measurement schemes. Imaging speed can be increased through the use of parallel elements. For example, an array of 16 photoconductive antennas with a matched microlens array has been used for parallel multichannel measurements.152 Alternatively, THz generation from a single large-area EO crystal can be combined with CCD detection to increase imaging speed.153 However, the ability to image on a scale much larger than a few centimeters would be difficult due to the large increase in optical pumping power needed for large area THz generation. The development of a cw system with rapid (100 kHz) phase modulation allows for coherent detection with about 3 orders of magnitude faster scanning than mechanical methods.154 Minimizing the number of measurements that need to be performed to construct a complete image could also greatly increase acquisition speed. Compressed sensing techniques are ideal for this because they use a single element detector and a set of masks that can be rapidly changed. A random array of masks, as reported by Chan, will allow an N N element image to be reconstructed with fewer than N-2 measurements.155 If masks are optimized, a 20 20 pixel image can be reconstructed with 40 measurements, ten times less than by raster scanning.156 Sparse arrays in a synthetic aperture arrangement have been reported with adaptive reconstruction for applications requiring a large field of view.157 A four-element detector synthetic aperture array with interferometric narrow band measurement was demonstrated by Liu at a rate of 62.5 frames per second.158 Doubling the effective frame rate by beam multiplexing has also been demonstrated.159 The two beams are offset, 4353
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Analytical Chemistry orthogonally polarized, and delayed by 2 ns to allow easy postprocessing of the single detection channel. This method is also attractive due to a small number of additional components needed to effectively double the measurement speed. The growing interest in high quality THz imaging has led to additional efforts in image processing to improve resolution from low contrast images. Taylor et al. have quantified image quality using optical character recognition.160 It would be useful to further extend this idea in an analogous way to optical imaging metrics. THz imaging is also useful for concealed object identification in stand-off security applications. A two-step image processing method has been developed for broadband THz images that can provide better results than two different segmentation methods, one of which included supervision.161 Removing the need for human image verification would greatly increase measurement throughput. The developed techniques could be run in real time on dedicated hardware. THz tomography, or depth profiling, has been used for quite some time due to the reflecting nature of THz pulses from interfaces buried within an object.162 The reflections from a multi-interface sample can be used for time-of-flight analysis. Reflections contain information on the encoded sample permittivity and thickness. Complete computational methods for volumetric spectral analysis of THz pulses from a 3-D sample was achieved only recently.163 Advances have also been made in algorithms that can successfully account for high refractive losses through multipeak averaging.164 Near-Field Imaging. There are a wide variety of methods being used to increase the spatial resolution of THz imaging in the near-field. Near-field measurements can be considered as either aperture-based or aperture-less. Aperture techniques use a small opening in a larger mask to concentrate the incident THz field to a subwavelength region. Aperture-less techniques use concentrating elements like an atomic force microscopy (AFM) tip or fine pointed wire waveguide to collect and concentrate the incident THz radiation. Recent efforts in aperture-based techniques seek to optimize the aperture size and shape for increased fidelity, with a primary goal of understanding the enhanced transmission that accompanies subwavelength hole arrays. Recently, three-dimensional characterization of the electric field in the presence of these holes and hole arrays has shed much light on their resonant and diffractive characteristics.165 Work is also being done to incorporate microelectromechanical systems for manipulation of single apertures, which could allow easy subwavelength imaging and utilization of compressed imaging techniques for near-field measurements.166 The field of polaritonics,167 which enables visualization of THz field propagation, is also becoming useful for characterizing the performance of strongly subwavelength λ/ 1000 slits.168 Additionally, more macroscale approaches are being used for fabricating THz optics. Superfocusing optics have achieved THz focusing in two directions with dimensions of λ/ 260 by λ/145.169 Instead of apertures, sharp metal tips can be used to concentrate THz radiation. Tip-induced field enhancement for subwavelength THz imaging with resolution on the order of 100 nm was reported as early as 2003 by Chen.170 Typical aperture-less techniques suffer from low imaging speed because the tip must be raster scanned across the sample. Alternatively, an array of fine metal rods can provide both high spatial resolution and high throughput.171 Huber et al. have demonstrated 40 nm resolution for near-field narrowband THz imaging of carrier density in a
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nanoscale transistor.172 The sharpness of the metal-coated AFM tip concentrated the THz field, and the image was created by operating the AFM in tapping mode while detecting the THz signal scattered from the tip. Detecting scattered radiation results in very low signal strengths. This problem can be overcome by higher-harmonic demodulation and high input powers. Further experimental analysis was able to quantify the role of background signals, and a model for quantitative analysis had been developed.173 Increasing the useful bandwidth of near-field measurements is an important step for full utilization of the THz region. Recently, the use of photoconductive tips has been reported with a bandwidth of 2 THz.174
’ APPLICATIONS Advances in the analytical capabilities, coupled with progress in other areas of science and engineering, have led to an explosion of work in application of THz spectroscopy to a diverse range of new material and biological systems. This section will highlight some of the recent knowledge gained in areas including molecular spectroscopy, solid state physics, biology, pharmaceuticals, and imaging. Although there are many more papers and specific application areas than can be listed here, this section introduces a few of the exciting arenas now impacted by THz spectroscopy. Chemical Analysis and Molecular Spectroscopy. Many molecular vibrational and rotational modes fall within the THz region of the electromagnetic spectrum. THz-TDS has been used for identification of molecules and compounds, with great possibilities for fundamental science as well as potential commercial applications such as stand-off detection of explosives and illegal drugs. Here, we will provide some examples of molecular spectroscopy and chemical analysis using THz-TDS. Gases. The first demonstration of a THz-TDS system, in 1989, reported the absorption spectrum of water vapor.1 Grischkowsky and co-workers employed photoconductive antennas excited at 100 MHz by 70 fs pulses from a colliding-pulse, mode-locked dye laser to generate and detect THz radiation. THz radiation propagated through a path length of 88 cm in an enclosure whose humidity could be carefully controlled. Absorption lines in the gas phase are very sharp, requiring long scans in the time domain to achieve sufficient frequency resolution. Scanning the delay line over 200 ps yields 5 GHz frequency resolution, and signal-to-noise was sufficiently good so that Lorentzian lineshapes could be fit to the data with peak position resolved to 1 GHz. More detailed measurements of water vapor investigated pressure broadening of pure rotational transitions with inclusion of nitrogen and oxygen gases.175 Water vapor is a key component in the Earth’s temperature balance. Precise knowledge of pressure broadening of water absorption features in the range around atmospheric pressure is critical for climate modeling. THz-TDS provides more accurate data in the 1 5 THz region than other methods such as FT-IR. Absorption spectra of other gases have been measured as well. For example, the explosive 2,4-dinitrotoluene (DNT) showed a broad absorption from 0.05 0.6 THz, corresponding to its pure rotational spectrum.176 The spectrum also contains many discrete absorption features that were hypothesized to arise from internal rotor motions of the NO2 and CH3 groups. Identification of gas-phase explosives has obvious value for defense and security applications. However, while measurements of water vapor are fairly straightforward, explosives must be carefully 4354
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Analytical Chemistry heated to generate sufficient vapor pressure without decomposition. Overall, THz-TDS has not been widely used to study gases of any type. Instead, it is much more commonly applied to liquids, and the great majority of work is on solids. Liquids. The THz dielectric properties of liquids depend primarily on interaction and relaxation of dipoles. These dipoles can be either permanent dipoles in polar liquids or induced dipoles in nonpolar liquids. Hydrogen bonding can also have a significant effect on the THz response. Reorientation of dipoles in liquids on picosecond time scales is important for both chemical reaction kinetics and biological function. As expected, water and aqueous solutions are the most widely studied liquids. Water related to biological applications is described in Biology, Medicine, and Pharmaceuticals. Here, we provide a sampling of the literature related to water, mixtures, aqueous salt solutions, and ionic liquids. Water strongly absorbs THz radiation and must be contained within a thin (∼100 μm) sample cell if measured in transmission. Reflection and ATR geometries may also be employed. The dielectric spectrum of pure liquid water was reported in the mid1990s by Thrane et al. using reflection geometry and by Kindt and Schmuttenmaer using transmission geometry.177,178 The dielectric response can be modeled using two Debye relaxation processes with time constants of ∼200 fs and 8 ps. Mixing water with another liquid can be expected to significantly impact its dielectric function. Water ethanol mixtures require a third Debye relaxation to be accurately modeled.124 Figure 5 shows the complex dielectric function for a series of water ethanol volume fractions. The time constant of the slow process increases linearly with ethanol fraction from 8 ps to over 150 ps. However, the intermediate time constant peaks at moderate ethanol fractions, and its shape approximately corresponds to the volume change upon mixing. THz-TDS and femtosecond mid-IR transient absorption were used to understand water dynamics in the presence of different ions.179 The presence of ions slows the relaxation of water molecules, and careful spectroscopy allowed determination of the range of influence of these ions. In some cases, the ions affect not only the water molecules directly surrounding the ion but also molecules well beyond the first solvation shell. This farreaching effect is propagated through water’s hydrogen bonding network. Ionic liquids have received much recent attention for their potential use as electrolytes and solvents. Ionic liquids and ionic liquid water mixtures have been studied by THz-TDS. The THz dielectric function of these solutions could also be modeled with a two-term Debye model.180 Imidazolium ionic liquids with a series of different anions have dielectric values similar to shortchain alcohols.181 However, the ionic liquid spectra also have significant structure that corresponds to interion vibrations. Librational motion of cations also contributes to the dielectric response in the THz region, while intramolecular vibrational modes do not.182 Solids. THz-TDS has been widely applied to many different molecular solids. THz absorption fingerprints have been recorded for a variety of explosives, poisons, and illicit drugs; with potential applications in security screening, defense, and food safety. THz absorption features of biomolecules such as amino acids can provide critical information on chirality and crystallization for pharmaceutical applications. Spectra from many common materials in art conservation, reagent grade chemicals, and other materials can be found in online databases such as
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Figure 5. (a) Real and (b) imaginary part of the complex dielectric function of 10 different ethanol water mixtures at 298 K. Reprinted with permission from ref 124. Copyright 2009 Optical Society of America.
Terahertz Database, http://thzdb.org, operated by RIKEN and NICT in Japan and the “THz Spectral Database” found in the NIST Chemistry Webbook. Solid samples are frequently prepared by diluting in a nonabsorbing powder such as polyethylene, grinding into a fine powder, and then pressing the composite into pellets. The pellet method can provide a fingerprint of the molecules when care is taken to control sample uniformity and properly account for multiple reflections. Multiple reflections, also known as Fabry Perot fringes or etaloning, can be mistaken for absorption features in the frequency domain. Etaloning will not impact response in the THz range if the pellets are sufficiently thin so that interference is beyond the bandwidth limit or sufficiently thick so that fringe spacing is less than the experimental resolution. An alternative approach is to deposit the pure sample on the surface of a parallel plate waveguide (PPWG). As discussed in Parallel Plate Waveguides, the PPWG provides a long interaction length of the THz radiation with the sample, which improves sensitivity and eliminates reflections from the sample. Increased sensitivity enables reduction in sample size, which is particularly important when the sample is expensive, dangerous, or in short supply. Absorption features typically become sharper at lower temperatures, and the PPWG provides superior temperature control compared to a pellet. Assigning resonances in experimental absorption spectra to specific modes typically requires significant simulation and modeling efforts. These simulations are often carried out using commercial software such as Gaussian 03, CHARMM, and various packages for density functional theory. Here, we focus on applications related to pesticides, explosives, and drugs. Amino acids and other biomolecules are studied in a similar fashion but are covered in Biomolecules. Food safety and screening is becoming increasingly important with increased use of pesticides and the threat of bioterrorism. 4355
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Hua and Zhang have reported the THz spectra of four common pesticides using THz-TDS of pressed pellets.183 They also measured the spectra of pesticides mixed with common food items such as rice powder and were able to show a linear increase in peak absorption coefficients with weight ratio over weight fractions of 0 80% pesticide. This result shows promise, although practical safety screening would require detection down to trace amounts of pesticides and chemicals. Stand-off detection of explosives is desirable for national security. Building a library of the THz absorption spectra of different explosives is a key step, which is proceeding in parallel with developments in long-range detection. This area has been reviewed by several groups.22,23,184 In building the library, there are several experimental factors that influence the measured spectra, especially sample temperature and the use of waveguides versus pellets. Significant broadening occurs in pellets because molecules are randomly mixed with other molecules as well as the filler material. Crystallized films deposited on PPWGs produce much sharper features. Broad absorption features can often be resolved into multiple narrow peaks at low temperature. For instance, Figure 6, from Melinger et al., shows the sharp spectral features of RDX and TNT explosives drop-cast as thin films on a waveguide and measured at 13 K. The figure compares the PPWG spectra to spectra of the same explosives in pellet or single-crystal form measured at room temperature and cryogenic temperatures.185 Good quality spectra showing approximately 20 vibrational modes between 0.5 and 3.5 THz with linewidths an order of magnitude sharper than pellets were obtained using less than 300 μg of explosive material. Linewidths down to 7 GHz and center-line frequency precision as good as 1 GHz have been obtained.186 While an experiment can provide the molecular absorption spectra, theory is required to assign the different features to specific modes. Much work in this area has been done by the Korter group. They have applied density functional theory to predict and assign mode structure for molecules such as explosives (RDX),187 chiral solids (serine amino acid),188 crystalline pharmaceuticals (ibuprofen),189 and illegal drugs (MDMA, Ecstasy).190
Figure 6. (Top) THz-TDS absorbance spectra of the explosives (a) RDX and (b) TNT drop-cast on a parallel plate waveguide and measured at 13 K. (Bottom) Absorbance spectra of RDX and TNT in pellet and single-crystal form, at room temperature and low temperature. Use of the waveguide and measurement at low temperatures sharpens absorption features by removing homogeneous and heterogeneous broadening. See original paper for data sources of pellets and single crystals. Reprinted with permission from ref 185. Copyright 2008 American Institute of Physics.
Solid State Physics. THz spectroscopy, and especially TRTS, has been applied most often to study solid state electronic materials. This high level of effort has arisen for two main reasons. First, the natural energy and time scales of physical phenomena such as electron transport fall in the THz frequency and picosecond time regimes. Second, electronic materials are used for THz generation and detection, so understanding their properties is necessary to improve spectrometer performance. Here, we highlight recent activities in THz spectroscopy of semiconductors, nanomaterials, and correlated electron materials. Semiconductors. Mobile charge carriers (electrons and holes) in semiconductors can be created by either doping or photoexcitation. Understanding the dynamics of these carriers is essential for applications in microelectronics, optoelectronics, and photovoltaics. The simplest description of charge carriers moving in response to an electric field is given by the Drude model, σ(ω) = (ε0ω2p τ)/(1 iωτ), where σ is the conductivity, ω is the frequency, ωp is the plasma frequency, εo is the free space permittivity, τ is the carrier scattering time, and i is the unit imaginary. Electrons and holes in semiconductors often have scattering rates, τ 1, on the order of 1012 1014 s 1 and thus show strong characteristic response in the THz frequency regime. THz-TDS enables direct determination of the frequency-
dependent, complex conductivity, allowing detailed modeling of carrier behavior. Additionally, THz-TDS is a noncontact probe, which circumvents experimental difficulties with making contacts to semiconductors. Contact-free probing is of particular importance for measuring nanomaterials. THz-TDS adds critical information to what can be obtained from dc conductivity measurements such as Hall effect. dc measurements are dominated by the slowest conduction step as charges are moved over relatively large distances. Conversely, high-frequency THz radiation can be used to probe intrinsic material properties that are less affected by macroscopic disorder or sparse defects. Another significant advantage of THz spectroscopy is that it can be coupled to an ultrafast pump pulse for measurement of nonequilibrium photoexcited carrier dynamics on subpicosecond to nanosecond time scales. The pump pulse is typically in the visible or near- or mid-IR, but recent improvements in THz generation intensities are beginning to enable THz-pump/THz-probe experiments as well.191 In addition to measuring electrons and holes, THz radiation is also a useful probe of other low energy excitations and quasi-particles including excitons, polarons, and phonons. The first demonstration of the utility of THz-TDS to investigate semiconductors and dielectrics was reported in 1990 by 4356
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Figure 7. THz-TDS and TRTS of a ZnO wafer. (a) Real and (b) imaginary conductivity of a ZnO wafer in the THz frequency range for temperatures ranging from 20 to 120 K. Conductivities are measured relative to the conductivity at 10 K. (c) Frequency-dependent real (solid circles) and imaginary (open circles) conductivity of the ZnO wafer at 70 K with fits by the Drude model (solid lines). (d) Temperature-dependent electron densities calculated by fitting the Generalized Drude model to experimental conductivity data. Inset: Arrhenius plot shows activation energy of 27 meV. (e) TRTS pump scans with 0.07 mJ/cm2 fluence of 400 nm photoexcitation as a function of ZnO temperature, normalized by their maximum transmission change. (f) Drude model fits of the plasma frequency (filled circles) and scattering time (open triangles) as a function of pump probe delay time at 10 K. (a d) Reprinted with permission from ref 194. Copyright 2009 American Physical Society. (e,f) Reprinted with permission from ref 195. Copyright 2009 American Physical Society.
Grischkowsky and co-workers.2 By probing frequencies that span above and below the plasma frequency and carrier damping rate, accurate extraction of carrier densities and mobilities was shown for Si wafers moderately doped either n- or p-type.192 Heavily doped semiconductors are optically dense to THz radiation, inhibiting their measurement at room temperature by conventional transmission THz-TDS. However, THz-TDS in reflection geometry enabled characterization of highly conductive samples with precision approaching that of transmission measurements.126 Close examination of the response in moderately doped Si showed that carriers did not precisely follow the Drude model, which assumes a single scattering rate, independent free electrons, and elastic scattering. Instead, a Cole-Davidson distribution was found to describe the nature of conduction more accurately. This was the first demonstration of fractal conductivity in a single-crystal semiconductor rather than in disordered materials.193 More recently, Baxter and Schmuttenmaer evaluated conductivity of bulk ZnO using THz-TDS and TRTS. Instead of
measuring a variety of wafers with different doping densities as in Grischkowsky’s work, electron density was varied by thermal excitation from shallow trap states at temperatures ranging from 10 to 120 K.194 Variable-temperature THz-TDS enabled determination of both donor density and activation energy, while also allowing independent determination of temperature-dependent mobility, Figure 7. In a related work, carriers in ZnO were generated by photoexcitation.195 TRTS of photoexcited carriers in ZnO was used to demonstrate the dependence of carrier lifetime on factors including temperature, excitation wavelength, and excitation fluence. While thermal excitation of ZnO generates only conduction band electrons from shallow donor states, photoexcitation necessarily creates equal numbers of electrons and holes. Interestingly, selection rules dictate that one-photon absorption in ZnO excites the light hole band but two-photon absorption excites heavy holes. As a result, tuning the excitation wavelength enabled specific investigation of hole dynamics.195 4357
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Analytical Chemistry GaAs has also been among the semiconductors most widely studied using THz spectroscopy. GaAs is commonly used in THz emitters and detectors as well as in high-speed electronics. High THz field amplitudes (300 kV/cm) have enabled observation of coherent ballistic electron transport over a large portion of the first Brillouin zone.196 In some cases, carriers can be generated by high-intensity THz pulses rather than optical pulses, allowing THz pump THz probe spectroscopy. If energy greater than the band gap is imparted to carriers, impact ionization can occur. Exciting carriers using a sub-bandgap THz pump rather than an optical pump eliminates complications from direct electron hole generation, allowing observation of dynamics of impact ionization and carrier cooling. For GaAs at temperatures below 200 K, the strong electric field generates additional electron hole pairs by tunneling between valence and conduction bands. Tunneling efficiency is directly related to decoherence times of the band states.196 High fields have also been used to investigate saturable absorption in GaAs, as well as GaP and Ge.197 Saturation occurs because of a decrease in electron conductivity at high electron momentum states that arises due to band nonparabolicity and scattering into satellite valleys which is enhanced by the strong THz fields. THz pump THz probe spectroscopy has been applied to a number of materials beyond GaAs, including InSb, Ge, and Si.166,191,198 In addition to measuring free carriers, THz spectroscopy is also sensitive to excitons, which are bound electron hole pairs. Visible-region optical experiments are only sensitive to excitons with very small center-of-mass momenta, while other excitons are termed “optically dark.” However, THz measurement of exciton polarizability is not subject to this constraint, enabling detailed investigations of transitions between free carrier and excitonic states. Significant investigations of exciton formation dynamics and interactions with THz fields include studies of GaAs quantum wells,199 201 ZnSe quantum wells,202 Cu2O,203 and bulk ZnO.204 The most complete information can be obtained by the various combinations of optical and THz pumps with optical and THz probes. In some materials, charge carriers create associated lattice deformations, and the collective excitations are known as polarons. These low energy excitations have responses in the THz regime and are significant in semiconductors and oxides as well as polymers, molecular electronic materials, and photoexcited insulators. Polarons can significantly affect charge transport. For example, THz-TDS showed that electron phonon scattering limits electron transport in crystalline TiO2.205 High-field THz radiation has been used to excite polarons, as well as to probe them.206 TES with an applied magnetic field has been used to determine light and heavy hole effective masses and electronpolaron resonances in GaAs and InP.207 Semiconducting polymers and small molecules are becoming increasingly important for applications in organic photovoltaics and light emitting diodes. THz spectroscopy can provide significant insight into carrier dynamics in both the neat organic component and blends of the organic with electron acceptors. The most common polymers used in these applications are based on polythiophenes and polyphenylvinylenes. Hendry et al. performed some of the earliest investigations of semiconducting polymers, using complex conductivity to evaluate the distribution and dynamics of both excitons and free carriers in MEHPPV, finding that excitons were the dominant species.208 Conversely, early TRTS studies of pentacene molecular crystals showed that mobile free carriers were the primary photoexcited species and displayed dispersive transport character.209 Recently,
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carrier transport in pentacene crystals was shown to be limited by coupling to low frequency molecular motions that had characteristic frequency of 1.1 THz.210 TRTS has also been used to measure the mobilities of photoexcited carriers in spin-cast P3HT films. Mobilities were compared to thin film transistor measurements, and trends were investigated as a function of local ordering and conjugation length.211 When P3HT is blended with PCBM acceptor, photoexcitation generates excitons that dissociate by interfacial charge separation, resulting in a free electron in the PCBM and hole-polaron in the P3HT. Carrier mobilities and photon-to-carrier quantum yields have been measured using TRTS. Inclusion of PCBM led to a significant increase in quantum yield.212 Interestingly, carriers can also be generated with photon energies below the polymer bandgap, indicating the presence of a charge transfer complex. Fluencedependent studies at early times indicate that interfacial charge transfer is the dominant exciton decay pathway, which is a necessary condition for efficient photovoltaic response.213 Nanomaterials. Nanomaterials and nanocomposites have architectures with structure on the nanometer length scale that imparts particular features or presents new physics compared to bulk materials. Contacting nanomaterials can be a great, often impossible, challenge; and contact resistances frequently dominate electrical measurements. Therefore, noncontact THz probes of electrical properties are highly desirable. These materials have characteristic length scales that are much smaller than the wavelengths of THz radiation. TRTS pump scans can be carried out in similar fashion to single-phase materials; however, extraction of frequency-dependent permittivity of the material of interest from the measured permittivity of the composite requires some form of effective medium theory. Dynamics of charge carriers, excitons, polarons, and other physical phenomena described above for bulk or thin film materials can also be extracted, with appropriate care, for nanomaterials. Semiconductor quantum dots are of significant interest for optoelectronic and photovoltaic applications. When quantum dot sizes become smaller than the Bohr radius, quantum confinement causes energy levels to become discrete rather than continuous. Spacing between energy levels depends on carrier effective mass and quantum dot size, and spacings are typically in the range of tens to hundreds of meV. This energy is significantly larger than THz energies. Although THz radiation cannot resonantly excite interlevel transitions in isolated colloidal quantum dots, it does probe exciton polarizability.214 The photoexcited THz susceptibility is purely real, is frequency independent, and arises instantaneously. Similar response was found for InAs colloidal quantum dots.215 Wang found that polarizability increases with quantum dot radius as r4, while Pijpers found an increase as r3.214,215 Pijpers also showed that relaxation from higher excitonic states occurs in InAs by an Auger process involving direct electron hole energy transfer. Changes in dynamics that occur with the presence of multiple excitons were also investigated by increasing the excitation fluence. Visible and near-IR transient absorption was demonstrated as a powerful complement to TRTS. Because of different energy spacings of electron and hole levels, TRTS preferentially probed holes while transient absorption primarily probed electrons in InAs.215 THz radiation can probe carrier transport in quantum dot solids, films of colloidal semiconductor quantum dots that exhibit some confinement but also some interdot coupling. Early work on quantum dot solids focused on materials including CdSe and PbSe.216,217 TRTS allows the strength of the interdot electronic 4358
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Analytical Chemistry coupling and the carrier dynamics to be determined simultaneously. Because of the extremely large surface-to-volume ratios of nanocrystals, coupling and dynamics depend sensitively on the nature of the capping ligands and surface treatments. InGaAs quantum dots grown on GaAs substrates by molecular beam epitaxy were investigated by Porte et al. using TRTS.218 THz radiation is absorbed much more strongly by conducting carriers than by immobile ones, allowing access to probe the carrier density in the GaAs upon transfer to or from the quantum dots. When the pump pulse was tuned to excite the quantum dots resonantly, release of carriers into the conducting GaAs states occurred over ∼30 ps and was dependent on fluence. With a pump pulse tuned to excite the higher lying conduction band of GaAs directly, carriers were captured by the quantum dots within a few picoseconds. In a different study, GaAs nanowires, with and without AlGaAs shells, were evaluated using TRTS to determine the impact of surface states.219 The AlGaAs overlayer passivated 82% of the surface traps, resulting in an increase of photoexcited carrier lifetimes from ∼3 to 20 ps. These lifetimes are still significantly shorted than bulk lifetimes, confirming the importance of surface states in nanomaterials. Photophysics of quantum wells has also been investigated using TES. InAs/GaAs quantum dots were recently shown to significantly enhance THz emission compared to undoped GaAs.220 Increased emission was attributed to strain fields at the InAs/GaAs interface. THz spectroscopy can provide important details regarding oxide nanostructures in addition to II VI and III V semiconductor quantum dots. Early work on mesoporous TiO2 nanoparticle films was carried out independently by the Schmuttenmaer and Bonn groups. Turner et al. measured transient photoconductivity and described the response using the Drude Smith model.221 The Drude model assumes scattering events with complete momentum randomization. However, carriers cannot continue to proceed forward at an oxide air interface, so backscattering is preferred. This localization is accounted for by the Smith modification. Alternatively, Hendry et al. used the Maxwell Garnett effective medium theory with Drude parameters measured on single crystal TiO2 to fit the response of the composite.222 They found that mobility is reduced not only by the porosity of the network but also by screening of the applied field. They note that mobilities measured on short (∼tens of ps) time scales by TRTS should be viewed as upper limits since the measurement is not sensitive to carriers trapped at defects or interfaces during the measurement. Richter found that electron transport in TiO2 nanotubes is limited by exciton-like trap states with resonance near 1.9 THz.223 Baxter and Schmuttenmaer combined both effective medium theory and the Drude Smith model to understand carrier dynamics in porous ZnO, including nanowire arrays and nanoparticle films.224 Effective medium theory accounts for the nonuniform distribution of the electric field within the composite, while the Smith modification of the Drude model is appropriate because the high electron mobility in ZnO leads to mean free paths that are on the order of the particle size. Recently, Monte Carlo simulations have also been used to describe transport in mesoporous oxide films with good success.225 Charges encountering an interface are assigned different probabilities for reflection or transmission to simulate localization or interparticle transport. Reflection and transmission probabilities are treated as fitting parameters in the model. Titova et al. reported a wellcontrolled study of the dependence of dynamics on morphology by annealing insulating SiOx films to create different filling
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Figure 8. THz spectroscopy of silicon nanocrystal films. (a) Example of a THz pulse waveform. The corresponding Fourier amplitude spectrum is shown in the inset. (b) Change in transmission of the main peak of the THz probe pulse as a function of time delay with respect to a 400 nm, 100 fs pump pulse for a silicon nanocrystal film embedded in SiOx matrix with 51% silicon by volume. Inset: schematic diagram of the opticalpump/THz-probe experiment. (c) Real (solid red squares) and imaginary (open blue squares) components of the complex conductivity of a silicon nanocrystal film with volume fraction 51%, measured 50 ps after photoexcitation as indicated by the arrow in (b); pump fluence is 1.3 mJ/ cm2. (d) Complex conductivity of photoexcited bulk silicon; pump fluence is 65 μJ/cm2. Lines in (c) and (d) are fits to the Drude Smith model. The corresponding c-parameter fit values are also indicated. Reprinted with permission from ref 226. Copyright 2011 American Physical Society.
fractions of nanocrystalline Si.226 Below 38% Si, photoexcited carriers were localized to Si nanocrystals, while at higher filling fractions percolation networks were formed and extended charge transport was possible. Results were described in the context of both the Drude Smith model and random walk Monte Carlo simulations. Figure 8 shows the transient photoconductivity of the composite and a comparison of THz conductivity for a percolating network of Si nanocrystals versus bulk Si. Both the magnitude and the shape of the conductivity spectra change dramatically in going from bulk to nanocrystalline material. In addition to directly photoexciting the semiconductor or oxide to be probed, one can also photoexcite a sensitizing species and then probe the electron density in the acceptor to measure dynamics of interfacial charge transfer. Such assemblies have important implications in photovoltaics and photocatalysis. For example, oxide nanoparticle films sensitized with organometallic dyes are the basis of dye sensitized solar cells. TRTS has shown biphasic injection rates from ruthenium bipyridal complexes into TiO2, consisting of a sub-500 fs component and a ∼100 ps component.227 The population of mobile injected carriers decays over nanoseconds as carriers are trapped. In solar cells, the 4359
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Analytical Chemistry purpose of interfacial charge transfer is to generate photocurrent. However, photocatalytic systems use photoexcited carriers to perform chemistry. Interfacial charge transfer in oxide-supported oxomangense complexes has been studied by the Yale group for applications in water splitting. The first step of the catalytic process is the generation and separation of photoexcited carriers. Transfer from molecular sensitizers to TiO2 proceeds on picosecond time scales with a number of stable linkers including catechol and hydroxamate.228,229 Quantum-dot sensitized oxides are also receiving much recent attention for photovoltaics and photocatalysis. Injection from photoexcited PbSe quantum dots into SnO2 was reported to take place on ∼100 ps time scales.230 Additionally, TRTS was demonstrated to be a more reliable method to evaluate charge transfer than visible transient absorption and time-resolved photoluminescence. TRTS measures mobile carriers as they appear in the oxide conduction band. However, the techniques that rely on visible signatures are ambiguous because nonradiative recombination at the interface cannot be distinguished from interfacial charge transfer. The review by Nemec et al. describes numerous other examples related to the measurement of charge transport and interfacial charge transfer in nanostructured materials for solar energy conversion.28 The preceding discussion focused primarily on quasi-0-D and 1-D materials and their assemblies. THz spectroscopy has also been used to investigate ultrathin films including single sheets of graphene. Graphene has received great attention recently, including the 2010 Nobel Prize in Physics, for its extremely high electron mobility and thermal conductivity. At this writing, Web of Science finds 72 papers on “THz and graphene”, with all but one published in 2007 or later. Intraband and interband transitions in graphene have significant THz signatures. Carrier dynamics depend on carrier density, with cooling and recombination occurring on picosecond time scales.231,232 Single layers of graphene typically must be supported on a substrate for robust measurement. Graphene properties depend strongly on interactions with the substrate, and noncontact mapping of sheet resistance of graphene on silicon showed variation of 30%.233 Strong optical pumping was found to elicit negative dynamic conductivity in the THz frequency range.234 Bilayer graphene was shown to have especially large nonlinear optical response in the THz regime and was suggested as a prime candidate for nonlinear photonic and optoelectronic devices.235 TES shows that graphite, in forms including highly oriented pyrolytic graphite and even pencil on paper, can generate THz radiation when excited with ultrafast lasers.236 There is similarly large interest in carbon nanotubes.237 240 Correlated Electron Materials. In addition to carrier dynamics in semiconductors, THz spectroscopy can also provide significant insight into other areas of solid state physics such as strongly correlated electron materials. While not as widely studied as semiconductors so far, THz spectroscopy has been used to investigate superconductivity, antiferromagnetism, and metal insulator-phase transitions. Averitt and Kaindl reviewed progress in the field up to 2008,3 and we provide a few more recent examples here as well. Understanding interactions of electrons with the crystal lattice is a critical challenge in research on high-temperature superconductors such as YBa2Cu3O7-δ (YBCO). TRTS has been used to simultaneously probe resonant signatures of the superconducting state, quasiparticle excitations, and multiple phonon modes in YBCO with femtosecond resolution.241 This approach enables measurement of
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phonon occupation and coupling to the superconducting condensate and is also applicable to other strongly correlated materials. Metal insulator phase transitions have also been investigated using THz spectroscopy. The phase transition temperature of VO2 is 68 °C, making it a convenient choice for fundamental investigations. Temperature dependent THz-TDS clearly shows the increase in conductivity above the phase transition temperature, as well as hysteresis showing higher conductivity at the same temperature while cooling rather than while heating.242,243 TRTS was used to probe the photoinduced-phase transition that occurs due to photoexcitation at temperatures below the transition temperature.242 The degree of metallic character depends upon fluence, but the characteristic rise time was always ∼100 ps. This transient is significantly longer than the times for either excitation of electrons into the conduction band or lattice heating by carrier thermalization and indicates more complex phenomena. Later work showed that conductivity arises by transition of individual nanograins from insulating to conducting, as determined by modeling using the Drude Smith theory.243 Electronic spin excitations can also be measured and manipulated using THz radiation. THz thermochromism and magnetochromism were investigated using THz-TDS of the noncollinear spin magnet Ba2Mg2Fe12O22.244 Sharp electric-dipole-active magnetic resonances were found in the ordered conical-spin phase. These effects have potential use in terahertz filters tunable by magnetic fields. Kampfrath et al. used the magnetic component of intense THz pulses to coherently control the spin degree of freedom in the antiferromagnet NiO.245 THz pump optical probe measurements verified that the THz field selectively influences spins through the Zeeman interaction. Biology, Medicine, and Pharmaceuticals. THz spectroscopy has attracted significant interest in biological systems because it can provide information about collective vibrational modes in biomolecules, interaction of biomolecules with water, and different types of tissue. In this section, we will describe some recent examples of THz spectroscopy related to biomolecules, biomedicine, and pharmaceuticals. Examples include identification of amino acid enantiomers, understanding hydration of proteins, differentiation of healthy and diseased tissue, and quality control related to drug crystallization and tablet coatings. Biomolecules. THz-TDS has proven to be a useful tool to understand biologically important molecules. Species such as amino acids, proteins, and DNA have intermolecular and intramolecular modes with THz signatures. However, significant challenges remain in utilizing these modes for biomolecular identification or for correlating features to physical phenomena. First, for even moderately large polypeptides and proteins, vibrational and torsional modes are densely spaced and of similar magnitude. Dense mode structure, combined with homogeneous and heterogeneous broadening of macroscopic samples, typically results in room-temperature spectra that are broad and featureless. Second, the most desirable information relates to biomolecules in their native state of hydration and environment. Unfortunately, water molecules have very strong absorbance features in the 1 3 THz range, which masks the features of the biomolecule. Furthermore, solvated molecules are free to access their conformation space, resulting in additional broadening. While there are many challenges, significant progress has been made in determining the relative contributions of collective vibrational modes and relaxation mechanisms to the THz spectra. The THz response is extremely sensitive to factors such 4360
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Analytical Chemistry as hydration, binding, conformational change, and temperature. The status of this field was reviewed by Siegel in 2004,246 Markelz in 2008,24 and Son in 2009.25 We will highlight some key early findings as well as selected recent progress. Significant progress in the use of THz-TDS to probe biomolecules began around the year 2000 with the groups of Markelz, Heilweil, and Jepsen. Markelz et al. reported broad absorption of THz radiation, increasing with frequency, for pressed pellets of lyophilized DNA, bovine serum albumin, and collagen.247 This broad absorption indicates the large number of IR-active, lowfrequency, collective modes in these biomolecules. Changes in the spectra were evaluated as a function of hydration and denaturing. At about the same time, Walther reported on the dynamics of low-frequency torsional vibration modes of three different isomers of retinal.248 Broad absorption bands became narrow peaks as temperature was decreased from 298 to 10 K. Resonances were modeled with Lortenzian lineshapes, and modes could be assigned to different parts of the molecule. A similar approach was taken to understand the differences in absorption spectra between benzoic acid and its monosubstitutes, including aspirin.249 THz-TDS provided a direct fingerprint of the molecular structure and conformational state of the compounds. Sensitivity is essential for many biological materials where quantities are often small and expensive. One early approach to improve sensitivity employed thin-film microstrip lines to guide the THz radiation within the plane on a chip where the sample is placed.250 This design increases the interaction length of the THz radiation with the sample and can improve analytic sensitivity by orders of magnitude for the same sample size, as demonstrated using DNA. Sensitivity down to femtomole levels was achieved, and the capability to observe single base mutations was demonstrated. More recently, the capabilities of THz-TDS to interrogate biological molecules have been extended in a number of exciting directions. The metal PPWG methodology was developed by the Grischkowsky group and applied to a number of small biomolecules such as amino acids.251 They showed that depositing films with planar order reduces inhomogeneous broadening and that cooling reduces homogeneous broadening. The waveguide approach was also applied to tris(hydroxymethyl)aminomethane, a neuroinhibitor and organic thermal energy-storing molecule, which was drop-case or sublimated as a thin film.252 Cooling the sample from room temperature to 14 K resulted in three broad absorption features transitioning into up to 12 sharp spectral features with fwhm as narrow as 12 GHz. Spectral data can be combined with theoretical modeling to understand the structure and vibrational modes of such molecules. All 20 standard polycrystalline R-amino acids were investigated using THz-TDS and classified according to molecular structure and THz spectra.253 Such comprehensive studies enable identification of amino acids, correlation of spectral features to different internal and intermolecular vibrations, and application in other biological or biomedical fields. Additionally, different enantiomers and racemic compounds can have different absorption bands. For example, L- and D-alanine have very similar absorption spectra with peaks at 74.4 and 85.7 cm 1 on a slowly rising background.254 However, the DL-alanine racemic compound has a peak at 41.8 cm 1 and is missing the higher frequency peaks. At least nine other amino acids also show differences between pure enantiomers and racemic compounds, leading to potential applications in quality control for the pharmaceutical industry. DFT calculations have provided reasonable agreement
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with experimental THz absorption spectra of amino acids such as serine, including enantiomeric differences.188 In a different study, amino acids and short-chain peptides were studied to investigate the effect of polymerization on THz dynamics.255 Polymerization primarily affected the intensity of the reduced absorption cross section and its power law exponent. These quantities depend on peptide-chain length. Changes arise from both intermolecular and intrachain motion. Recently, coherent control of amino acid motion has been exercised using intense THz pulses generated by tilted pulse front in LiNbO3.256 The intense THz pulse couples to certain vibrational modes of the molecular crystal, which can be detected using a weaker THz probe pulse. When biological materials using dry pellets or dry crystals are investigated, caution must be taken in extrapolating any conclusions to the native state of the biomolecule because structure and function depend sensitively on the degree of hydration. To avoid this leap, methods have been developed to prepare samples that are sufficiently hydrated without being immersed in water. For example, hydrated protein films can be deposited on water-free substrates from buffer solutions, controllably dried, and then mounted in a humidity-controlled cell.257 Degree of hydration has a profound effect on both THz spectra and biological function. The Havenith group was able to use THz absorption to differentiate hydration water from bulk water using a transmission cell.258 The interactions between water of hydration and the biomolecule were primarily evaluated by focusing on the water, rather than the biomolecule. Differences between water of hydration and bulk water were correlated to the hydration state of sugars. They found that the water network is disturbed by the presence of sugars, and they could precisely define the thickness of the hydration shell, 3.7 Å for glucose monosaccharide compared to 6.5 Å for trehalose disaccharide. In a different study, the group found a dramatic decrease in THz absorption in peptides when the number of water molecules per solute was less than 18 to 20, indicating that significant THz absorption arises from collective modes involving both the peptide and the surrounding water network.259 This number of water molecules is much less than monolayer coverage, and it also corresponds to the number required for biological functionality. Both of these studies used a bolometer to measure the THz radiation emitted from a p-Ge laser integrated over the range 2.1 2.8 THz, where hydration water and bulk water have different absorption coefficients. Even more information can be obtained from broadband frequency-resolved studies. The hydration state of disaccharides was evaluated using broadband THz-TDS in ATR geometry.135 ATR geometry enables probing of the interface without significant absorption by the solvent. The concentration dependence of the hydration state was evaluated from the complex dielectric function. THz-TDS has also been used to investigate the effect of hydration on low-frequency conformational dynamics in proteins.260 Degree of hydration strongly influenced the arrangement of hydrogen bonds in the protein secondary structure, with solvation giving rise to some modes that are not present at low hydration. Protein conformation also plays an important role in THz response. Figure 9 shows the THz transmittance spectra of the protein bovine serum albumin (BSA) in its native state and after thermal denaturing.261 Differences in molecular conformation affect the complex dielectric function of the protein. Proteins were fixed to a membrane and dried at 4 °C to maintain some degree of hydration. Proteins can also be studied in aqueous buffers if the sample cell is sufficiently thin. Photoactive yellow 4361
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Figure 9. Amplitude transmittance spectra of native-conformation and thermally denatured bovine serum albumin (BSA) protein measured using a THz-TDS system. The transmittance curve with open circles is for the thermally denatured BSA, and the curve with filled circles is for the native-conformation BSA. The solid curve shows THz transmittance of a standard sample with no BSA. Reprinted with permission from ref 261. Copyright 2008 IOPP Publishing.
protein was investigated in its natural physiological state using a 25 μm thick cell.262 Partial unfolding was induced by illumination with blue light emitting diodes. Unfolding of the protein leads to an increase in THz absorption because of an increase in the occupation of delocalized vibrational modes. Biomedicine. Beyond biomolecules, biological tissues can also be investigated using THz-TDS, either in vivo or ex vivo. Ex vivo studies using transmission geometry were reported by Png et al.263 Freshly harvested tissue most closely mimics in vivo conditions. However, such measurements are often prohibited by high water content that causes high THz absorption and also by challenges in rapid handling and testing. Any storage or drying technique can influence the biological properties and THz response of the tissue. Lyophilization (freeze-drying) is proposed as a viable solution to obtain meaningful data on stored biological tissues. Png describes the details of sample handling and preparation and compares the aging of fresh and necrotic tissues. THz reflection spectroscopy is an alternative to transmission THz-TDS that allows in vivo evaluation of tissue and measurement of tissues with high water content. Reflection spectroscopy was used to detect differences between healthy and cirrhotic liver tissues.264 Diseased tissue had higher absorption coefficient than healthy tissue due to both higher water content and differences in tissue structure. Such findings have obvious importance to biological imaging for cancer detection, described in Terahertz Imaging. Pharmaceuticals. THz spectroscopy is also of great value to the pharmaceutical industry for product screening and quality control. It enables nondestructive chemical analysis of the internal contents of tablets, capsules, and other dosage forms. Crystallinity and polymorphism are particularly important. THzTDS also allows quantification of the thickness of tablet coatings, which is critical to the controlled release of drugs to the body. THz imaging provides additional critical data on the uniformity of the coating. Much of the early work in pharmaceutical application of THz-TDS and THz imaging has been done by the Taday and Zeitler groups, with contribution from many others as well. Several reviews of these topics have been published since 2007.21,26,27,34 A few highlights of more recent spectroscopic studies will be briefly discussed here, with imaging studies presented in Terahertz Imaging.
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THz-TDS was used to identify the polymorphs of mannitol, which is commonly used as an excipient in the freeze-drying industry.265 Different freeze-drying techniques resulted in crystallization into different polymorphs. Results are in agreement with conventional X-ray diffraction measurements, but THz-TDS can be more easily integrated for rapid in-line or online screening. Crystallization from different enantiomers and their mixtures in a pharmaceutical dosage form can result in very different physiological effects. Fast and reliable quality control to screen for undesirable crystal forms is essential. Absorption spectra of ibuprofen differed substantially for the pure S enantiomer and the RS racemic compound.189 DFT calculations were used to assign measured absorption features to specific vibrational modes. Pharmaceutical tablets are porous, so it is appropriate to use effective medium theory to extract the permittivity of the active ingredients from the measurement of the composite.266 Estimation of complex permittivity using Wiener bounds was described for the example of starch acetate tablets. The utility of THz-TDS in combination with other chemometric tools was demonstrated for 4-component mixtures relevant to the pharmaceutical industry for tablet formation.267 THz absorption was found to be useful for identifying chemical composition, while refractive index was appropriate to model tablet density or thickness. THz tomography was proposed to obtain depth-profiled data. Terahertz Imaging. The ability to acquire 2D 3D maps of the THz response of objects has potential applications including pharmaceuticals, biomedicine, security screening, and art conservation. THz radiation is nonionizing, and imaging is noncontact and nondestructive. THz radiation can penetrate plastics, wood, paper, cardboard, and other packaging materials opaque to visible light, rendering it useful for security purposes. THz radiation is strongly absorbed by water, so it does not penetrate significantly into human tissue. However, THz reflectometry has been used to differentiate healthy from diseased tissue in topical applications such as skin cancer.268 Two of the most important parameters in imaging are spatial resolution and speed. In some cases, the full terahertz waveform is necessary to identify chemical compounds with distinct absorption fingerprints in the THz spectral region. In other cases, only the broadband transmission change or time delay is required to provide contrast. The latter case is clearly much faster since it requires measuring only a very small time window or even a single point in the time domain. Spatial resolution in far-field mapping is controlled by the diffraction-limited spot size at the focus of the THz radiation. Because terahertz wavelengths are long, the spatial resolution in the far-field is on the order of 100 μm 1 mm. Multiple companies now offer THz imaging systems, including Picometrix, Aispec, and Teraview. Several recent reviews of THz imaging have been published. PickwellMacPherson reviewed its potential for medical imaging.20 Zeitler described imaging with pharmaceutical applications,34 Federici reviewed its applications in security screening,184 and Wang described tomography.269 Recent improvements in experimental design to improve upon imaging speed and spatial resolution were detailed in Imaging. Many THz imaging systems use continuous wave radiation, as produced by beating two cw lasers of similar wavelengths in a photomixer, rather than THz pulses emitted from a Ti:Sapphire or fiber laser. Each imaging system has advantages and disadvantages.6 cw THz imaging systems are less expensive, lighter 4362
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Figure 10. (a) THz near-field imaging setup and (b) a conventional far-field imaging system. (c) THz transmission image (plotted at 1.36 THz) of a small section of a plant leaf measured with a near-field setup (left) and the same section measured with a far-field setup (right) for comparison. Black corresponds to low transmission. Reprinted with permission from ref 18. Copyright 2010 Springer. Original data from ref 271.
weight, ∼25 faster than pulsed systems, less complicated to use, and with more straightforward data interpretation. However, pulsed systems contain broadband spectral information, phase as well as amplitude contrast, and capability for depth profiling in reflection mode. In summary, pulsed THz imaging is much slower but has significantly more information content than cw THz imaging. As such, the two are suited for different applications. For example, when many items with presumably similar properties must be screened, as is the case for in-line quality control, cw THz imaging is preferred. In contrast, pulsed THz is necessary for identification of unknown objects such as concealed explosives. Hoshina et al. have suggested a two-stage noninvasive mail inspection system that employs both cw and pulsed THz screening. The first stage measures intensity of cw THz radiation scattered from the mail sample, where scattering intensity is related to the presence of powders inside the envelope. Envelopes suspected to contain powder are then moved to the second stage for inspection using broadband pulsed THz radiation. Spectral signature of the envelope’s contents can be compared to a library of standards for powder identification.270 Far-field imaging resolution is limited by diffraction and is on the order of the wavelength, ∼100 μm 1 mm. Spatial resolution can be improved by confining the detection scheme to be in the near field. One approach to near-field imaging brings the detector to the immediate vicinity of the sample. This approach improves the resolution to ∼25 μm, limited by the probe beam focus and photoconductive antenna configuration. Resolution of 25 μm corresponds to λ/24 at 0.5 THz.18,271 The difference between near-field and far-field imaging is striking, as shown in the image
of a leaf in Figure 10. Other approaches to near-field imaging such as interaction with a metal tip were discussed in Imaging. Security screening is an important potential market for THz imaging.272 Many forms of clothing and packaging are semitransparent to THz radiation, allowing one to search for items such as weapons, explosives, or drugs hidden within packages or under clothing. Metals are strongly absorbing or reflecting of THz radiation and can easily be identified among a collection of nonmetal objects, although metallic packaging would thwart such scanning. A simple amplitude measurement can be used in reflection or transmission to map the metal versus nonmetal areas. In other cases, the need for spectral information requires full time-domain scans. For example, many explosives and illegal drugs have distinct fingerprints in the THz region.23 As mentioned earlier, THz spectroscopy can be used to distinguish between diseased and healthy tissue. Because biological tissue has high water content and hence low THz penetration depth, in vivo imaging requires reflection rather than transmission geometry.273 THz reflectometry can thus be used for applications such as mapping proliferation of skin cancer or caries in teeth. THz imaging of skin showed good correlation of cancerous and healthy tissue distribution when compared with histology.274 Different types of tissue have different water content and spectral response, providing image contrast. For example, muscle tissue can easily be distinguished form adipose (fat) tissue.275 Because sectioning is not required for depth-profiling studies, 3D-THz tomography can be done in situ, although depth is limited to 100 μm 1 mm for watercontaining tissue. However, in situ imaging and tomography are particularly useful for studying teeth, which can be 4363
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Analytical Chemistry re-examined later to track demineralization or remineralization upon treatment.276 THz imaging is useful for both process development and quality control. For example, it is used in the pharmaceutical industry to investigate dissolution of tablet coatings to understand drug delivery. In one study, imaging circular biconvex tablets showed that they typically have the thinnest coating around their central band, which is then the primary weak spot for dissolution.277,278 This information is critical to the design of coating equipment and processes. THz imaging can also be used for quality control of manufactured tablets. In another example, corks for wine bottles were imaged to determine whether internal cracks and defects were present that may allow faster diffusion of oxygen and compromise long-term storage of wine.279 Typically only external surfaces of wine corks are examined, but no information on internal structure is available. THz imaging also has important uses in art conservation and cultural heritage. Depth profiling using THz time domain reflectometry can be used to evaluate subsurface layers of artworks nondestructively and noninvasively.280 282 Different pigments, binders, and mixtures can be mapped in three dimensions to give insight into the history of the artwork and guidance regarding its conservation. Janssens et al. reviewed the benefits of THz-TDS to complement infrared and X-ray techniques.283 The ability of THz-TDS to highlight interfaces between layers in a stratographic buildup is a particular strength.
’ CONCLUSIONS AND FUTURE OUTLOOK The past two decades have seen remarkable progress in THz sources, detectors, spectroscopy systems, and imaging. Progress has continued to accelerate over the past few years. Advances in instrumentation and methodology have led to THz pulses with ultrabroad bandwidth beyond 100 THz, field strengths greater than 1 MV/cm, spectral resolution of 1 GHz, spatial resolution for broadband imaging below 5 μm, increased scanning speed, and remote generation and detection, all with standard benchtop Ti:Sapphire pulsed lasers. These advances in instrumentation and spectroscopy methods have eliminated what has long been known as the “terahertz gap.” Advances in instrumentation and methods, coupled with an increasingly large user base, have led to many exciting new fundamental scientific discoveries and application areas. THz spectroscopy has been used for identification of explosives and drugs. It has led to more profound understanding of solvent dynamics in aqueous solutions of salts, sugars, and proteins. It has enabled new insight into charge carrier and quasi-particle dynamics in semiconductors and correlated electron materials. It has enabled identification of crystalline polymorphs and quantification of tablet coatings in pharmaceutical applications, and it has provided new imaging modalities for biomedicine. Improvements in scanning speed have opened new opportunities for THz imaging for a variety of applications including nondestructive evaluation, in-line monitoring, quality control, art conservation, stand-off detection, and security screening. Several companies now sell complete terahertz spectroscopy and imaging systems. New systems based on fiber lasers or photomixed semiconductor lasers offer good stability, smaller packages, and increased portability at lower cost, although bandwidth is necessarily sacrificed. As prices fall and awareness rises, we can expect to see increased implementation of THz tools in manufacturing facilities, medical buildings, and security checkpoints.
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All of these applications, and more, will continue to be driven by activities in the research laboratory to further advance instrumentation and spectroscopy methods and to improve our fundamental understanding of molecular, material, and biological systems.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ BIOGRAPHIES Jason B. Baxter is an Assistant Professor in the Department of Chemical and Biological Engineering at Drexel University in Philadelphia, PA, where he began in 2007. He received his B.Ch.E. from the University of Delaware in 2000 and his Ph.D. from the University of California Santa Barbara in 2005, both in chemical engineering. From 2005 to 2007, he was an ACS Alternative Energy Postdoctoral Fellow in the lab of Charles Schmuttenmaer at Yale University. His research interests are in semiconductor nanomaterials for solar energy conversion, with particular emphasis on understanding ultrafast photophysics using time-resolved terahertz spectroscopy and transient absorption spectroscopy. He received the NSF CAREER Award in 2009. Glenn W. Guglietta is a Ph.D. candidate working with Jason Baxter in the Department of Chemical and Biological Engineering at Drexel University. He received his B.S. in Chemical Engineering from the University of Maryland College Park in 2009. His research interests include ultrafast spectroscopy to study and apply nanostructured materials for energy conversion. His focus is on investigation of photophysics in nanoscale semiconductors using time-resolved terahertz spectroscopy and transient absorption spectroscopy. ’ ACKNOWLEDGMENT The authors are grateful for support for this work provided by NSF CAREER CBET-0846464 (J.B.B.) and NSF IGERT DGE0654313 (G.W.G.) We also thank X.-C. Zhang, Joseph Melinger, Peter Uhd Jepsen, Lyubov Titova, Frank Hegmann, Toshihiko Ouchi, Markus Walther, and Robin Nicholas for sharing their figures and images. ’ REFERENCES (1) Vanexter, M.; Fattinger, C.; Grischkowsky, D. Opt. Lett. 1989, 14, 1128. (2) Grischkowsky, D.; Keiding, S.; Vanexter, M.; Fattinger, C. J. Opt. Soc. Am. B: Opt. Phys. 1990, 7, 2006. (3) Dexheimer, S. L. Terahertz Spectroscopy: Principles and Applications; CRC Press: Boca Raton, FL, 2008. (4) Jepsen, P. U.; Cooke, D. G.; Koch, M. Laser Photon. Rev. 2011, 5, 124. (5) Lee, Y.-S. Principles of Terahertz Science and Technology; Springer: New York, 2010. (6) Zhang, X. C.; Xu, J. Z. Introduction to THz Wave Photonics; Springer: New York, 2009. (7) Mittleman, D. M. Sensing with Terahertz Radiation; Springer: New York, 2002. (8) Sakai, K. Terahertz Optoelectronics; Springer: New York, 2005. (9) Pereira, M. F.; Shulika, O., Terahertz and Mid Infrared Radiation: Generation, Detection, and Applications. Springer: New York, 2011. 4364
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