Review pubs.acs.org/journal/apchd5
Plasmonic Metasurfaces for Nonlinear Optics and Quantitative SERS Shangjr Gwo,*,†,‡ Chun-Yuan Wang,† Hung-Ying Chen,† Meng-Hsien Lin,† Liuyang Sun,§ Xiaoqin Li,§ Wei-Liang Chen,∥ Yu-Ming Chang,∥ and Hyeyoung Ahn⊥ †
Department of Physics, National Tsing-Hua University, Hsinchu 30013, Taiwan National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan § Department of Physics, The University of Texas at Austin, Austin, Texas 78712, United States ∥ Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan ⊥ Department of Photonics, National Chiao-Tung University, Hsinchu 30010, Taiwan ‡
ABSTRACT: Plasmonic metasurfaces consist of two-dimensional arrays of metallic nanoresonators (plasmonic “metaatoms”), which exhibit collective and tunable resonance properties controlled by electromagnetic near-field coupling. These man-made surfaces can produce a range of unique optical properties unattainable with natural materials. In this review, we focus on the emerging applications of metasurfaces with precisely engineered plasmonic properties for nonlinear optics and surface-enhanced Raman spectroscopy (SERS). In practice, these applications are quite susceptible to material losses and structural imperfections, such as variations in size, shape, periodicity of meta-atoms, and their material states (crystallinity, impurity, and oxidation, etc.). In these aspects, conventional top-down lithographic techniques are facing major challenges due to inherent limitations in intrinsic material properties and material quality introduced during growth, synthesis, and fabrication processes, as well as achievable lithographic resolution. Moreover, they are prohibitively expensive and timeconsuming for fabrication over large areas. Here, we show that colloidal silver crystals (millimeter-sized single-crystalline plates and thiolate-capped nanoparticles) synthesized by solution-based chemical methods are excellent material platforms for the fabrication of high-quality plasmonic metasurfaces. In particular, both top-down (focused ion-beam milling) and bottom-up (centimeter-scale self-assembly) techniques can be exploited to generate uniform and precisely engineered colloidal metasurfaces for broadband tunable (across the full visible range) second-harmonic generation and quantitative SERS at the single-molecule level. KEYWORDS: plasmonics, metasurface, nonlinear optics, surface-enhanced Raman spectroscopy, silver, colloidal crystal, nanoparticle, self-assembly
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tional Raman spectroscopy limits its widespread applicability. The discovery of SERS dramatically changed this situation.4−8 On the other hand, nonlinear optical processes play an important role in photonics, including laser frequency conversion and generation of ultrashort laser pulses, as well as realization of all-optical signal processing and ultrafast switching.20−26 However, material nonlinearities are inherently weak and, as a result, high-power lasers and special nonlinear crystals are typically necessary to observe the nonlinear optical effects. The SPR modes in metallic nanostructures are very efficient in capturing incident light due to a large effective cross-section (a nanoantenna effect). Under resonant illumination, a strong enhancement of the incident optical field can occur from a
urface plasmon resonances (SPRs) associated with metallic nanostructures offer unique possibilities for light concentration beyond the diffraction limit, which can lead to optical field confinement and enhancement in deep subwavelength regions,1,2 also known as plasmonic “hot spots”. In recent years, many exciting applications have emerged in the research field of plasmonics,1−3 taking advantage of tunability of SPR at the nanoscale. For example, there have been significant advances in applications based on plasmonics, including surface-enhanced Raman spectroscopy (SERS),4−8 plasmon-enhanced light harvesting,9−14 photocatalysis,13−15 plasmonic labeling and nanoparticle-based therapies,16 optical sensing,17−19 nonlinear optics,20−27 and optoelectronic devices, such as photodetectors28 and imaging sensors,29 to name just a few. In this review we focus on the plasmonic applications for nonlinear optics and SERS. Raman spectroscopy arises from the inelastic light scattering by molecular vibrations, which can provide “fingerprint” information for molecular diagnostics. However, the inherently low scattering intensity of conven© XXXX American Chemical Society
Special Issue: Nonlinear and Ultrafast Nanophotonics Received: February 14, 2016
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DOI: 10.1021/acsphotonics.6b00104 ACS Photonics XXXX, XXX, XXX−XXX
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This review is organized in the following way. First, we discuss the general advantages of plasmonic nanostructures and metasurfaces for nonlinear plasmonics and SERS. Second, we give the readers some background information about important material issues. Then, we discuss the results for (1) plasmonenhanced SHG using large (millimeter-scale) colloidal Ag crystals with double resonant plasmonic modes matched with the fundamental and second-harmonic frequencies35 and (2) quantitative SERS using centimeter-scale, uniformly hot plasmonic metasurface composed of close-packed Ag nanoparticle arrays with tunable interparticle gaps controlled by the carbon chain length of alkanethiolate molecules.38 Finally, we conclude and provide some future perspectives.
collection area that is much larger than the physical size of metallic nanostructures. Therefore, plasmonic nanostructures are very useful in amplifying intrinsically weak optical processes, such as nonlinear optical processes and SERS. In particular, both nonlinear optical processes and Raman spectroscopy are superlinearly dependent on the electromagnetic field intensity and they can be tremendously strengthened in material environments that provide surface plasmon resonance mechanisms for large field enhancement.7,8,25,26 However, in practical applications, tunability and reproducibility of hot spots in plasmonic nanostructures are crucial, which impose serious challenges in nanofabrication because of limitations in intrinsic material properties and material quality introduced during growth, synthesis, and fabrication processes,30−35 as well as achievable resolution offered by conventional lithographic techniques.36,37 Furthermore, both on-chip nonlinear optical systems and quantitative SERS call for uniformly hot surfaces with precisely engineered plasmonic properties.35,38 The recent developments in plasmonic metasurfaces can fulfill these requirements. In general, plasmonic metasurfaces can be designed and implemented by using arrays of subwavelength-spaced plasmonic “meta-atoms” (building blocks), including onedimensional (1D) metallic nanostructures (e.g., nanoslits, nanogrooves, nanoribs), two-dimensional (2D) metallic nanostructures (nanoholes, nanoprotrusions, nanoantennas, splitring resonators), and colloidal metal nanocrystals supported on solid substrates or embedded in optical films. Plasmonic metasurfaces exhibit collective and tunable resonance properties, which can be controlled by plasmonic coupling of metaatoms. Furthermore, in the case of gradient metasurfaces, spatial variations in phase, amplitude, and polarization responses can be introduced by using an array of meta-atoms with spatially varying geometric parameters and symmetries. For example, continuous control of local phase and polarizability changes can be created by using identical anisotropic (or chiral) meta-atoms with spatially varying orientations. Recently, it has been demonstrated that plasmonic metasurfaces can produce a range of unique optical properties unattainable with naturally occurring materials.39−47 At present, top-down lithographic techniques face major challenges in the fabrication of plasmonic metasurfaces because of requirements of exquisite structural resolution and perfection, as well as fabrication over large areas. In comparison, self-assembly methods offer simple, robust, and scalable alternatives to fabricate large-area plasmonic metasurfaces.38,48−54 Here, we review two materials platforms for the fabrication of plasmonic surfaces based on colloidal silver (Ag) crystals (millimeter-sized single-crystalline plates and thiolate-capped nanoparticles) synthesized by solution-based chemical methods. These colloidal crystals show excellent plasmonic properties in the complete visible spectral range due to the improved material qualities. Moreover, alkanethiolate surface ligands on Ag nanoparticle surfaces and conformal alumina coating on single-crystalline Ag plates by atomic layer deposition (ALD) can be applied to passivate the Ag crystals in ambient environment. To fabricate “uniformly hot” metasurfaces with precisely engineered plasmonic properties, both top-down (focused ion-beam (FIB) milling) and bottom-up (centimeter-scale self-assembly) approaches can be employed to achieve broadband tunable second-harmonic generation (SHG) and quantitative SERS at the single-molecule level.
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ADVANTAGES OF PLASMON-ENHANCED NONLINEAR OPTICAL PROCESSES In general, plasmon-enhanced nonlinear optical processes have the following unique advantages.25 i. Stronger Field and Smaller Size. The coupling of light to surface plasmons can result in strong local electromagnetic fields, significantly enhancing optical processes. A prominent example is SERS, where plasmonic excitations at nanoroughened or nanoengineered metal surfaces can enhance the inherently weak Raman scattering signal by many orders of magnitude, in some cases even allowing single-molecule detection.55−62 In nonlinear optics, plasmonic enhancements can lead to higher effective nonlinearities of the metal or the surrounding dielectric. The field enhancement due to plasmonic excitations near a metal−dielectric interface (i.e., surface plasmon polaritons, SPPs) or in the vicinity of a metal nanostructure (i.e., localized surface plasmons, LSPs) can be described by the frequencydependent local-field factor L(ω) = |E loc(ω)| /|E0(ω)|
where Eloc(ω) is the local electric field at frequency ω associated with the plasmonic excitations, and E0(ω) is the incident electric field. As deduced from the SERS experiments, the local-field factor can be larger 100 for some extremely hot spots (e.g., in the gap between two closely spaced silver nanoparticles). On the material side, the response of materials to an optical field is described by the material polarization P with its magnitude P expressed as the following: P = ε0[χ (1) E + χ (2) E2 + χ (3) E3 + ...]
where ε0 is the vacuum permittivity and χ(n) is the nth-order susceptibility of the material. In general, the field-induced material polarization P consists of harmonics at several highharmonic frequencies. For moderate light intensities, only the first term is important. Such a linear response describes conventional optical effects, including reflection, refraction, absorption, and scattering. For strong fields, the high-order (n > 1) terms become significant, which contain sums and differences of incident light frequencies and give rise to electromagnetic radiation at these new frequencies.20,21 For the second-order processes, we can have SHG, sum-, and difference-frequency generation. For the third-order processes, there are possibilities of third-harmonic generation (THG), four-wave mixing (FWM), and the Kerr effect, where the permittivity (refractive index) depends on the light field intensity. B
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nonparabolicity of the electron dispersion in the metal film. In principle, the modulation speed could be as fast as a few tens of femtoseconds as it is ultimately limited only by the electron relaxation process. Also, the switch energy and the level of direct plasmonic signal modulation could be increased by using interferometric arrangements.64 iii. Increased Controllability. Plasmonic excitations can be extremely sensitive to minute changes in the dielectric properties of the composing metal and the surrounding dielectric materials. In nonlinear optics, this extraordinary sensitivity can be exploited to control light with light (alloptical signal manipulation and processing). For example, one can utilize a control beam to induce a nonlinear change in the dielectric properties of one of the materials, thus, modifying the plasmonic resonances and the propagation of a signal beam by means of light reflection, transmission, or absorption.64−66
Ideally, for high-order nonlinear process, the enhancement factor of nonlinearity scales superlinearly with nth power of the local-field factor, which can be orders of magnitude higher than the linear process. However, considering the funnel effect for photons in the vicinity of a metallic nanostructure, the conservation requirement of the total photon number (neglecting absorption) indicates that the effective area of nonlinear contribution is L2(ω) smaller than that in the absence of a metallic nanostructure since the light intensity (I) increases as I → L2(ω)I. Therefore, for the nth order nonlinear process, the effective nonlinear susceptibility is enhanced as χ(n) → L(n−1) (ω)χ(n) .63 In the case of a multiresonant metal nanostructure at both fundamental and nth harmonic frequencies, the enhancement can be further increased by a factor of L2n(ω), which motivate the design of multiresonant metallic nanostructures for plasmon-enhanced high-harmonic generation. In the case of conventional nonlinear optical generation using macroscopic-sized samples (≫light wavelengths), the nonlinear responses can lead to strong, coherent signals only when individual nonlinear sources add up in phase (phase matching) because the nonlinear effects grow only over the coherence length of the interaction. In contrast, for subwavelength nonlinear photonic sources realized by plasmonic nanostructures, phase matching considerations are not important and the nonlinear signals satisfying the mode matching conditions can be strongly enhanced. Therefore, utilization of plasmonic nanostructures make it possible to scale down nonlinear components in size, which is very important for the development of integrated nonlinear photonic devices. ii. Faster Speed and Reduced Power. Plasmonic excitations are ultrafast processes on the time scale of femtoseconds, allowing in principle ultrafast processing of optical signals on the same time scales. Moreover, by enhancing the effective nonlinearity, plasmonic nanostructures can contribute to nanophotonics by allowing nonlinear effects to be utilized with much reduced incident optical power. In an study reported by MacDonald et al.,64 experimental evidence was provided that femtosecond plasmon pulses can be generated, transmitted, and modulated along a metal (aluminum)/dielectric (silica) plasmonic waveguide, utilizing the nonlinear interaction between propagating SPP waves and an incident laser beam in a pump−probe experimental setup. In this study, a pulsed plasmonic probe signal was generated on an aluminum/silica interface by grating coupling from a pulsed 780 nm laser beam. The optical pump/probe wavelength was chosen by them to match with the absorption peak, corresponding to the interband transitions in aluminum at ∼1.5 eV. The plasmonic probe signal, coupled to and from the plasmonic waveguide by two surface gratings on the alumina/ silica interface, is modulated by optical pump pulses as it travels between the surface gratings. The transient effect of pump (control) pulse excitation on the propagating SPPs was monitored by varying the time delay between the SPP excitation and pump pulses. It was found that ultrafast switching time (∼200 fs) and modest switching energy (∼10 mJ cm−2) can be achieved, which open the door to the exploration of ultrafast nonlinear plasmonics. In this experimental setup, the coherent nonlinear interaction between propagating SPP waves and a control laser beam takes place in the skin layer of the metal film. It was argued that the coherent nonlinearity could be linked to anharmonic components of plasmonic oscillation resulting from the
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MATERIAL ISSUES The full advantages of plasmon-enhanced nonlinear optics and SERS could only be realized with the help of plasmonic materials possessing appropriate intrinsic material properties and high material quality. It has been found that material quality strongly depends on growth, synthesis, and fabrication processes.30,32−35 For example, Ohmic losses are always present in plasmonic metals. They limit both the propagation distance and the achievable local-field factors. Inherent losses in plasmonic systems represents a major material challenge in plasmonics, as they limit the propagation length of SPPs, in the best case (for an “ideal” silver crystal) to the range from 10 μm (visible) to several millimeters (telecommunications), depending on wavelength, geometry, and dielectric environment. Losses are also important in the context of SPRs of metallic nanostructures, where the local-field enhancement must be carefully balanced with Ohmic and radiative losses to optimize the enhancement of nonlinear interactions and Raman scattering. Figure 1a,b shows both the real (ε′) and imaginary (ε″) parts of experimental dielectric functions for four well-known plasmonic metals, including gold (Au),67 silver (Ag),33 copper (Cu),68 and aluminum (Al),68 which have low plasmonic losses due to their low resistivity (high electric conductivity). The dielectric function is a fundamental wavelength-dependent quantity related to the electronic structure of the metal and can be used to describe its intrinsic optical (plasmonic) properties.69−73 For practical applications, choosing a plasmonic material with a small value of ε″ is important to realize low-loss plasmonic systems in a specific spectral range. In comparison, Ag can be considered the best plasmonic metal since the imaginary part of its dielectric function is kept very small over the visible and NIR wavelength regions. However, in the ultraviolet region at wavelengths below 300 nm, Al is a better plasmonic metal than Ag,74,75 and in the red and near-infrared regions, Au and Cu are also reasonably good plasmonic metals. Moreover, in contrast to Au and Ag, Al and Cu are compatible with complementary metal−oxide−semiconductor (CMOS) processes, making them attractive as alternative plasmonic materials. Interestingly, Al is also an excellent material for nonlinear plasmonics76−78 with a high chemical stability due to the spontaneous formation of an ultrathin passivating aluminum oxide layer (2−3 nm) in ambient. According to the experimental results reported by Castrol-Lopez et al. (shown in Figure 1c), plasmonic antennas made of Al have a C
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prepared by conventional film deposition techniques. To overcome the limitations of polycrystalline Ag prepared by conventional methods, researchers have devoted extensive efforts to develop new chemical synthesis and thin film growth techniques to obtain single-crystalline Ag and Au nanostructures79−82 and thin films.32,33 For example, colloidal crystals (Au) and epitaxial films (Ag) have been demonstrated to greatly enhance the performance of plasmonic nanolasers.83−85 The growth process of atomically smooth epitaxial Ag films is, however, quite tedious owing to the slow iterative two-step process consisting of low temperature deposition and room temperature annealing in ultrahigh vacuum. As a result, only films with limited thicknesses (typically, tens of nm) can be prepared.33 A template stripping technique has also been applied to produce Ag films with atomically smooth top surfaces, which can lead to a long SPP propagation length at the film/air interface (∼30 μm in the red wavelength region).30 However, a large number of grain boundaries still exist inside the stripped films, which can cause additional losses and reduce the SPP propagation length. In comparison to film growth techniques, colloidal Ag crystals can also be grown using chemical synthetic methods.82 Unfortunatelly, the typically small crystal sizes (up to tens of μm in lateral dimension) prevent them from full characterization of fundamental plasmonic properties and limit their practical applications. Recently, we reported the synthesis of giant colloidal Ag single crystals with millimeter lateral size and tens of microns in thickness.35 Using these crystals with an ALD-deposited alumina surface passivation layer, it is possible to achieve SPP propagation lengths beyond 100 μm in the red wavelength region (see Figure 2). These values even exceed the predicted
Figure 1. Intrinsic optical properties of four well-known plasmonic metals (gold, silver, copper, aluminum). Both the real (a) and imaginary (b) parts of the dielectric functions are shown as functions of incident light wavelength in free space. Silver can be considered as the preferred plasmonic metal in the full visible wavelength range due to its minimal plasmonic damping (small imaginary part). These plots are based on experimental data reported in the literature.33,67,68 (c) Two-photon photoluminescence (TPPL) measured from gold, silver, and aluminum resonant plasmonic antennas. Aluminum shows a surprisingly high TPPL efficiency than silver and gold.77 Panel (c) is adapted with permission from ref 77. Copyright ACS 2011.
Figure 2. SPP propagation lengths measured on giant colloidal silver single crystals. Two methods were used for these measurements: (1) white light interference (black dots) using a planar Fabry-Pérot double-slit interferometry setup and (2) direct scattered intensity method at fixed laser wavelengths (blue squares). This plot shows SPP propagation lengths measured with both methods, in comparison with those reported for different Ag samples in the visible wavelength range (small black square, ref 35; blue square, ref 35; blue open triangle, ref 30; red circles, ref 32; black star, ref 33; pink inverted triangles, ref 67; black circle, ref 82). The error bars shown for the method 1 represent standard deviations of measurement values from several double-slit structures. The predicted propagation length using the JC Ag data were calculated by the eigenmode method for an air/5 nm Al2O3/Ag planar structure, and the error bars are from the Johnson-Christy data.67 This figure is adapted with permission from ref 35. Copyright NPG 2015.
surprisingly high two-photon photoluminescence (TPPL) efficiency.77 At optical frequencies, Ag is considered as the preferred plasmonic material owing to its lowest intrinsic loss among all well-known plasmonic metals. However, its poor chemical stability, in comparison to Au, and additional scattering losses originated from grain boundaries and surface roughness limit the performance of polycrystalline Ag plasmonic structures D
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Figure 3. Literature results of gold bowtie nanoantenna arrays. (a) Gold bowtie nanoantenna array with a nominal 20 nm gap size fabricated by electron-beam lithography on a polycrystalline gold film.86 The TPPL emission nonuniformity is due to the variation of actual gaps present in this array. (b,c) Gold bowtie antenna array with uniform 20 nm (1.2 × 107) and spatially uniform (
