Plasmon-Controlled Fluorescence: Beyond the Intensity Enhancement

Perspective pubs.acs.org/JPCL

Plasmon-Controlled Fluorescence: Beyond the Intensity Enhancement Tian Ming, Huanjun Chen, Ruibin Jiang, Qian Li, and Jianfang Wang* Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China ABSTRACT: Control over light absorption and emission using plasmonic nanostructures is an enabling technology, which can dramatically enhance the performances of existing optical and optoelectronic devices, such as solar cells, light-emitting devices, biosensors, and high-resolution fluorescence microscopes. This Perspective takes fluorescence as an example, illustrating how plasmonic nanostructures can control the light absorption and emission of nanoscale optical species. The origins of fluorescence intensity enhancements will be first discussed. Different parameters that can largely affect the interactions between plasmonic nanostructures and fluorophore molecules will be examined, including the distance between the fluorophore molecule and the metal nanostructure and the wavelengths of their respective optical responses. The role of plasmonic nanostructures on fluorescence will then be reconsidered from the perspective of optical nanoantennas. We expect that more functionalities of plasmonic nanostructures as optical nanoantennas will further be discovered in analogy with the radio frequency antenna counterparts.

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distributed within the subwavelength regions close to the surface of the nanostructures, bridge the size mismatch between the visible light and much smaller optical species, and therefore pave the way for finely tailoring the interactions between propagating radiation and nanoscale optical species.21−31 The most well known examples in plasmon-enhanced light−matter interactions are surface-enhanced Raman scattering (SERS)32−44 and plasmon-enhanced fluorescence (PEF).45−67 In this Perspective, we attempt to provide a brief overview of the flourishing field where plasmonic nanostructures are employed to control the fluorescence process. We will first summarize the state-of-the-art developments and then discuss the related open questions and challenges. The optical properties of a number of optical species have been investigated so far by researchers, in the hope to effectively control their light absorption and emission. A very useful parameter for describing the interaction strength between optical species and light is the optical extinction cross section, which is the sum of the absorption and scattering cross sections. In general, the term cross section is used in physics to quantify the probability of interactions between particular particles. It can be imagined as a hypothetical area surrounding a target particle. An incoming particle falling in this area will interact with the target particle. A larger extinction cross section means a higher probability of light being absorbed or scattered by an optical species. We summarize in Figure 1 the extinction cross sections of the four most common types of nanoscale optical species. They are atoms/ions,68−70 organic fluorophores,71−73 semiconductor QDs,73−76 and plasmonic nanocrystals.77−83 All of them can exhibit strong responses to the

oble metal nanostructures exhibit an extraordinary capability to manipulate light at the nanometer scale. They derive their unique optical properties from an ability to support the collective oscillations of their conduction-band electrons, known as localized surface plasmon resonances. These plasmonic nanostructures are currently a subject of intensive scientific studies, both fundamentally and technologically. They hold great promise for enhancing the efficiencies of solar energy harvesters,1−8 light-emitting devices,9−12 and optical sensors.13−20 Many optical applications need good control of light absorption and/or emission processes. Solar energy harvesting demands efficient absorption of sunlight and conversion of photon energy to electron−hole pairs or thermal energy. Lightemitting devices call for high out-coupling efficiencies and controllable emission direction/polarization. Optical sensing and biological imaging require high excitation and emission rates for detecting trace amounts of targeted species. These requirements have traditionally relied on controlling light propagation and/or focusing light using elements such as mirrors, lenses, filters, gratings, and photonic crystals. These optical elements provide convenient ways for manipulating light and enabling a range of optical instruments, such as telescopes, microscopes, and spectrometers. However, they have limitations. On the one hand, owing to the diffraction limit, lenses or waveguides cannot focus light to spots less than about a half of the working wavelength. On the other hand, most optical species, such as single atoms, ions, molecules, and quantum dots (QDs), have physical sizes in the range from 0.1 to 10 nm. As a result, the diffraction limit severely hinders the improvement of the optical measurement precision in the visible spectral range. Fortunately, this limitation is overcome by localized plasmon resonances. Plasmonic nanostructures can concentrate optical radiation into strong localized electric fields that are © 2011 American Chemical Society

Received: October 17, 2011 Accepted: December 28, 2011 Published: December 28, 2011 191

dx.doi.org/10.1021/jz201392k | J. Phys. Chem. Lett. 2012, 3, 191−202

The Journal of Physical Chemistry Letters

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Figure 1. Comparison of the optical extinction cross sections of various nanoscale optical species. (a) Plot of the extinction cross section versus the physical cross section. (b) Plot of the ratio between the extinction and physical cross sections versus the physical size of optical species.

of a fluorophore molecule, the observed fluorescence intensity I can be described by I = γex·ηf·εcoll, where γex is the excitation rate of the fluorophore molecule, ηf the emission quantum yield of the fluorophore, and εcoll the light collection efficiency of the optical measurement system. Because the light collection efficiency is system-dependent, below, we will consider only γex and ηf. The excitation rate is determined by Fermi’s golden rule according to γ ex = (4π2/h)|⟨e|E·p|g⟩|2ρ e, where h is Planck’s constant and e and g represent the wave functions of the excited and ground states, respectively. The excitation rate is a function of the local electric field E, the absorption transition dipole momentum p, and the density of the excited state ρ e. The quantum yield of a fluorophore η f can be expressed as η f = γ f,r/(γ f,r + γ f,nr), with γ f,r and γ f,nr denoting the radiative and nonradiative decay rates of the fluorophore, respectively. If a plasmonic nanocrystal is present in the vicinity of the fluorophore molecule, the emission intensity of the fluorophore will be modified. The modification of the intensity is usually realized in two ways. First, the excitation rate can be increased by the local electric field enhancements resulting from the excitation of the localized plasmon resonance. The fluorophore molecule situated in the local field-enhancement regions essentially feels an enhanced excitation light intensity. Second, in the vicinity of the plasmonic nanocrystal, the fluorophore molecule at the excited state can undergo nonradiative decay, emit a photon directly to the far field, or relax rapidly by exciting the localized plasmon resonance of the plasmonic nanocrystal via energy transfer. The plasmon resonance can alter both the radiative and nonradiative decay rates owing to the localized density of photonic states introduced by the plasmon resonance. Once the localized plasmon is excited, it can either decay nonradiatively owing to internal damping or reradiate into the far field. The nonradiative decay and reradiation rates of the plasmonic nanocrystal are basically determined by its absorption (Cabs) and scattering (C scat ) cross sections at the emission wavelength, respectively. As a result, the fluorescence emission intensity is jointly affected by both the excitation enhancement and

visible light. Their sizes span 3 orders of magnitude from 0.1 to 100 nm. Both the absorption and scattering cross sections of optical species decrease with decreasing sizes, but the scattering cross section decreases more quickly than the absorption cross section in the size regime of atoms/ions, organic fluorophores, and semiconductor QDs. Therefore, for these species, the absorption cross sections are orders of magnitude larger than the scattering cross sections. For plasmonic metal nanocrystals, both absorption and scattering cross sections are important. Two aspects are notable from Figure 1. First, the sizes of the four types of optical species are all smaller than the wavelength of the visible light. Second, only plasmonic nanocrystals have extinction cross sections larger than their physical cross sections.

With the ratio between the extinction and physical cross sections being larger than 1, plasmonic nanocrystals can strongly concentrate electromagnetic fields in the vicinity of their physical boundary. This feature enables them to function as an intermediary to enhance the interactions between other optical species and far-field light. A majority of PEF studies have so far focused on fluorescence intensity enhancements. In the emission process 192



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wavelength is 560 nm when the nanorod is immersed in air. Figure 2a depicts the electric field intensity enhancement in a square region of 8000 × 8000 nm2. Wavy features are clearly seen around the nanorod. They result from the scattering radiation field. The region enclosed with a square dashed box is magnified in Figure 2b. The deep blue color indicates the region where the electric field intensity is reduced in comparison to the excitation field intensity. Only in a small region around the nanorod is the field intensity enhanced in comparison to the far-field intensity. Figure 2c is a further enlarged area of 200 × 200 nm2. The electric field intensity enhancement occurs mostly in the regions close to the two ends of the Au nanorod. In these regions, the electric field intensity is enhanced by more than 1 order of magnitude. In other words, the incoming light is squeezed into the near-field regions close to the nanorod. These regions will be very effective for enhancing the excitation rate of fluorophore molecules. In general, the field enhancement decays nearly exponentially away from the metal surface. The sizes of the local field-enhancement regions and the field enhancement factors vary with the geometry and size of plasmonic nanocrystals. For a given nanocrystal, they also change as a function of the wavelength at which the localized plasmon is excited. The sharper a nanocrystal, the larger the field enhancement.81 For individual nanocrystals, the field-enhancement regions can be up to several tens of nanometers, and the maximal enhancement factors can reach up to 3 orders of magnitude. Many efforts have been made on the understanding, fabrication, and fine tailoring of the local field-enhancement regions. Numerical simulations77,84−86 and transformation optics4,87−89 have been used to determine the field enhancement factors. Wet chemical15,90−98 and lithographical methods49,99,100 have been utilized to fabricate real nanostructures. In recent years, great progress has been made in near-field optics. The spatial resolution from near-field optics has been dramatically improved to overcome the diffraction limit. With the help of near-field optics, the local field-enhancement regions can be directly and vividly imaged at the subwavelength scale.101−106 Attention has been paid to the enhancement of fluorescence using plasmonic metal nanostructures since 1980s.107,108 In the early experiments, organic fluorophore molecules are randomly deposited on rough metal surfaces, which are typically composed of a number of closely spaced gaps and sharp tips. These structural features support large local field enhancements and therefore can enhance the fluorescence emission. Although fluorescence enhancements are indeed observed from such rough metal surfaces, the localized plasmon resonance modes associated with the nanoscale irregular geometries cannot be clearly identified. The difficulty in identifying the plasmon modes is due to the very sensitive dependence of localized plasmons on the size, shape, composition, and surrounding environment of metal nanostructures. Only after the size and shape are accurately specified can the plasmonic properties of metal nanostructures be unambiguously determined. In the past decade, enormous progresses have been made in the preparation of metal nanostructures with well-defined, highly uniform sizes and shapes. Such progress allows for flourishing studies on PEF. According to Figure 2, the field-enhancement regions are concentrated in close vicinity to the surfaces of plasmonic metal nanostructures. To realize PEF, we need to bring fluorophore molecules close enough to the surfaces of metal nanostructures.

emission modification. It can be described by

⎛ γf,r′ I = γex′⎜⎜ + γf,r′ + ⎝ γf,r′ + γf,nr′ + γf,ET′ ·

γf,ET′ γf,nr′ + γf,ET′

⎞ Cscat ⎟εcoll Cabs + Cscat ⎟⎠

where γex′ is the increased excitation rate, γf,ET′ is the energytransfer rate from the fluorophore molecule to the plasmonic nanocrystal, and γf,r′ and γf,nr′ are the modified radiative and nonradiative decay rates of the fluorophore. Because photons emitted through the radiative decay channel of the fluorophore itself and the energy-transfer channel are difficult to distinguish, in most experiments, the measured emission intensity and decay rates are convoluted values of these two emission channels. The sum of the two terms in the parentheses can be treated as an overall emission quantum yield.

The fluorescence emission intensity is jointly affected by both the excitation enhancement and emission modification. The enhancement of the local electric field intensity is realized through a spatial redistribution of the optical electric field. The redistribution can be illustrated with finite-difference time domain (FDTD) simulations. The electric response of a Au nanorod under the illumination of a plane wave linearly polarized along its length direction is shown in Figure 2. Due to

Figure 2. Spatial redistribution of the electric field intensity by a Au nanorod. (a) The Au nanorod is illuminated by a plane wave at the longitudinal plasmon resonance wavelength. The directions of the light wave vector and the electric field are indicated at the lower left corner. (b) Zoomed-in contour of the region indicated with a dashed box in (a). (c) Zoomed-in contour of the region indicated with a dashed box in (b). The field intensity is normalized against the excitation far-field value and plotted on a logarithmic scale.

the anisotropic geometry, each Au nanorod exhibits a longitudinal and a transverse localized plasmon resonance mode, which are polarized parallel and perpendicular to the length axis, respectively. One attractive feature of the longitudinal plasmon mode is that its resonance wavelength can be synthetically controlled from the visible to the nearinfrared spectral regions. The Au nanorod shown in Figure 2 is 30 nm in diameter and 60 nm in length. Its longitudinal plasmon 193



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exhibit inhomogeneous distributions in size, shape, and surface roughness. Such inhomogeneous distributions make the plasmonic properties vary considerably among different nanocrystals, such as the absorption and scattering cross sections, the plasmon resonance wavelength, and the local field enhancement. For the purpose of better understanding the dependence of PEF on the plasmonic properties, single-particle fluorescence measurements are highly desired. Moreover, such measurements can be correlated with single-particle dark-field scattering measurements and scanning/transmission electron microcopy (SEM/TEM) structural characterizations. Structural characterizations allow for the accurate determination of the sizes and shapes of nanocrystals. The geometrical parameters are necessary for building models to understand the plasmonic properties with numerical simulations. Scattering measurements allow for the determination of the plasmon resonance wavelength and scattering intensity. A combination of correlated fluorescence, scattering, and structural measurements is extremely useful for elucidating the complex interactions between the fluorescence emission process and the localized plasmon resonance. Recent developments in optical imaging and nanomanipulation techniques have enabled PEF studies at the single-particle level, even down to the single-molecule level. One can conduct in-depth studies on PEF with little background interference and well-controlled parameters, such as the localized plasmon mode, fluorophore molecule−metal nanocrystal spacing, and molecular orientation. 50,51 Anger et al. measured the fluorescence intensity of a single molecule in the presence of a gradually approaching Au nanosphere (Figure 3a). A maximal

A straightforward method is to dissolve fluorophore molecules in a colloidal solution of metal nanocrystals and let the molecules adsorb on the nanocrystals through electrostatic and/or van der Waals interactions. However, unlike SERS experiments, the measurements of PEF in colloidal solutions suffer severely from fluorescence quenching. When fluorophore molecules are in direct contact with metal nanocrystals, their excited states can undergo nonradiative decay through energy and/or charge transfer from the molecules to the nanocrystals, even though the electric field enhancements arising from the localized plasmons are very strong at the metal surfaces. To avoid fluorescence quenching, spacing layers, made of either polyelectrolytes or silica, are usually employed to separate the molecules several nanometers away from the metal surfaces.59 On the other hand, if the molecules are not adsorbed on the nanocrystals, the relative positions and the spacing between the two types of species will vary continuously due to Brownian motion. For charged molecules and nanocrystals, the spacing can also be affected by the solution pH or the addition of salts. In addition, because plasmonic metal nanocrystals exhibit large absorption and scattering cross sections, the fluorescence emission from one fluorophore−nanocrystal hybrid nanostructure can be absorbed or scattered by the other hybrid nanostructures in the solution if the fluorescence emission peak overlaps with the plasmon resonance peak. This absorption-and-scattering problem can explain why PEF has rarely been observed in solutions. The difficulties encountered in measuring PEF in solutions can be partially overcome by depositing both fluorophore molecules and metal nanocrystals on solid substrates or embedding them in thin-film matrixes. In these ways, the positions of the two types of optical species are fixed, and the absorption-and-scattering problem is eliminated because usually only a single layer of metal nanostructures is present. The use of supporting substrates and embedding matrixes makes ensemble PEF measurements more robust and reproducible. Considerable plasmon-induced fluorescence enhancements have been observed.48,49,56,57,62 For the ensemble PEF measurements on substrates and thinfilm matrixes, several complexities are worthy of attention. First, fluorophore molecules are about 2 orders of magnitude smaller than metal nanocrystals. The former are usually dispersed uniformly in matrixes, and the latter are either assembled bottomup or fabricated top-down on substrates. If the surface number density of metal nanocrystals is not high enough so that the spacing between neighboring nanocrystals is larger than the size of the local field-enhancement region, a large fraction of fluorophore molecules will not interact with metal nanocrystals. The light emission from the molecules that do not interact with the nanocrystals will produce a large background, which makes the measurements of only plasmon-enhanced emission signals very difficult.57 We have been able to deposit Au nanocrystals from solutions on substrates with the spacing between neighboring nanocrystals reaching down to ∼100 nm. In addition, lithographic methods are also capable of producing metal nanostructure arrays with spacings down to a few tens of nanometers,49,99 although they are expensive, time-consuming, and not widely accessible. Second, there are a number of fluorophore molecules around each metal nanocrystal. The spacings and relative orientations of the molecules to the nanocrystal have wide distributions, which prevents us from performing systematic studies of the effects of the spacing and transition dipole orientation on PEF. Third, even if metal nanocrystals are prepared in the same batch or fabricated in the same run, they

Figure 3. (a) Schematic showing a Au nanosphere attached to the end of a pointed optical fiber and scanned over a single fluorophore molecule. Fluorescent nile blue molecules are dispersed in poly(methyl methacrylate) on a glass slide. (b) Fluorescence intensity (left axis, dark dots) as a function of the spacing z between the Au nanosphere and a vertically oriented molecule. The red curve (right axis) shows the emission rate γem normalized against the value (γem0) in free space. Reprinted with permission.50 Copyright 2006 American Physical Society.

fluorescence intensity was found to occur when the molecule was ∼5 nm away from the surface of the Au nanosphere (Figure 3b).50 This finding indicates that the fluorescence intensity is a synergic action of the excitation rate enhancement and the modification of the overall quantum yield. When moving toward the surface of the metal nanocrystal, the molecule experiences a nearly exponentially increasing local electric field intensity, which leads to a continuous increase in the excitation rate. On the other hand, the emission process has a more complicated dependence on the spacing between the 194



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Figure 4. Dependence of the (a) fluorescence intensity, (b) enhancement factor, (c) emission enhancement factor, (d) average lifetime, and (e) decay rate enhancement on the localized plasmon wavelength. (f) Plots of the calculated quantum yield enhancement factors with varying intrinsic quantum yields versus the calculated scattering intensity of the metal nanoparticle. In (a), the dotted and dashed curves are the absorption and emission spectra of rhodamine red fluorophore. In (b), the red and blue curves are the absorption and emission spectra of oxazine 725 fluorophore. In (c−e), the shaded areas indicate the emission spectrum of the CdSe QDs. The red and blue curves are Gaussian fittings. In (e), the black dots and blue squares are for the radiative and nonradiative decay rates. In (f), QY represents quantum yield. The blue, yellow, and red symbols denote three differently sized Ag nanoprisms. Their calculated scattering intensities are 3.5 × 10−15, 9.9 × 10−15, and 20.5 × 10−15 au, respectively. Reprinted with permission.53,60,66 Copyright 2007, 2009, 2010 American Chemical Society.

molecule and the surface of the nanocrystal. In general, for fluorophores with intrinsic quantum yields close to 100%, their overall quantum yields will decrease first slowly and then rapidly as the molecule approaches toward the surface of the nanocrystal. For fluorophores with small intrinsic quantum yields, their overall quantum yields will first increase due to the dramatic increase in γf,ET′, reach a maximal value at ∼10 nm, and then decrease rapidly, if the scattering cross section of the metal nanocrystal is not too small compared to the absorption cross section. The rapid decrease in the overall quantum yield arises from the increase in γf,nr′ when the molecule is very close to the metal surface. In this spacing regime, there will be strong interactions, which are mostly electrostatic or van der Waals interactions, between the molecule and the metal nanocrystal. These strong interactions can cause a rapid increase in γf,nr′. For both types of fluorophores, when the decreasing overall quantum yield overwhelms the increasing excitation rate enhancement, the fluorescence intensity will reach a maximum at the corresponding spacing. As the molecule gets even closer, the intensity will decrease. When the emission intensity drops below that in the absence of the metal nanocrystal, fluorescence quenching occurs. The spacing for fluorescence quenching has been found to be roughly less than 5 nm.50,62,109 The experiment by Anger et al. also points out that the relative dipole orientations of the fluorophore and plasmonic nanocrystal are very important for PEF50 because both the excitation and emission enhancements are strongly dependent on the molecular dipole orientation relative to the electric field polarization direction. The example in Figure 3 demonstrates the PEF of a single molecule by a single metal nanosphere with a particular

plasmon resonance wavelength. The fluorescence enhancement factors are also dependent on the plasmon resonance wavelengths of plasmonic nanostructures. The localized plasmon wavelengths of metal nanostructures are strongly determined by the composition, geometry, and surrounding dielectric environment. A number of Ag and Au nanostructures with plasmon wavelengths ranging from the ultraviolet to near-infrared spectral regions have been prepared by controlling their geometries using wet chemical methods.15,90−98,110,111 This has greatly facilitated the studies of the dependence of PEF on the plasmon wavelength. Figure 4a illustrates the measurements of the fluorescence intensities from fluorophore molecules that are attached to Ag nanoprisms with different plasmon wavelengths. The fluorescence intensities were found to reach the maximum when the plasmon wavelength of the Ag nanoprism is in between the absorption and emission peak wavelengths of the fluorophore.53 This result can be understood by taking into account the wavelength dependence of the local electric field enhancement and the emisson modification for a given localized plasmon resonance mode. Localized plasmon resonances have certain peak widths due to damping.92 Only when a plasmon resonance is excited at its peak wavelength can the maximal field enhancement be obtained. When the excitation is shifted away from the peak wavelength from either side, the field enhancement will be reduced. The wavelength dependence of the fluorescence emission for a given plasmon mode is more complicated and has not been fully understood yet. However, the maximal effect is also believed to occur when the emission peak overlaps closely with the plasmon resonance peak.53,66 Because the emission peaks of all fluorophores exhibit Stokes shifts, the peak wavelength of a 195



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length axis, reaches the maximum when the plasmon wavelength is close to the excitation laser wavelength (Figure 4b). To reveal the separate emission modification, Munechika et al. investigated comprehensively the luminescence of CdSe QDs that were situated under Ag nanoprisms. Taking advantage of the wide absorption band and large Stokes shift of the semiconductor QDs, they excited the QDs at a wavelength far away from the plasmon wavelengths of the nanoprisms to exclude the excitation enhancement. Figure 4c−e shows the dependence of the emission parameters, including the emission enhancement factor, average luminescence lifetime, and decay rate enhancement, on the plasmon wavelengths of the nanoprisms.66 The emission enhancement factor and radiative and nonradiative decay rates reach the maximum, and the average luminescence lifetime reaches the minimum when the plasmon wavelength is equal to the emission peak wavelength of the QDs. In particular, Figure 4e indicates that the radiative decay rate enhancement is larger than the nonradiative one, which explains the emission intensity enhancement that comes from the increase in the overall quantum yield. As mentioned above, the modified overall quantum yield is highly dependent on the intrinsic quantum yield of the fluorophore and the scattering-to-absorption ratio of the metal nanocrystal. So far, there have been no experimental measurements of the relationship between the overall quantum yield and the scattering intensity. Such information can be gained from numerical calculations, as shown in Figure 4f, where the enhancement of the overall quantum yield is plotted as a function of the scattering intensity for fluorophores with different intrinsic quantum yields. Clearly, Ag nanoprisms with stronger scattering intensities give higher overall quantum yield enhancements. In general, larger plasmonic nanostructures exhibit stronger scattering and have higher scatteringto-absorption ratios. Therefore, larger plasmonic nanostructures with dominant dipolar plasmon resonances are beneficial for PEF. The results also show that fluorophores with lower intrinsic quantum yields exhibit higher enhancements in the overall quantum yield and faster increase in the enhancement with increasing scattering intensities. For example, for a single fluorophore molecule with a low intrinsic quantum yield of 0.07 located within a Au nanobowtie, the fluorescence enhancement factor has been measured to be ∼1300. This is the largest fluorescence intensity enhancement factor among those ever measured so far.61 With the ability to manipulate the excitation and emission rates of nearby fluorophore molecules, a plasmonic nanostructure can be regarded as a nanoantenna, which functions as a transducer between far- and near-field light signals.112−115 More specifically, a plasmonic nanostructure acts as an analogue of an antenna working at the optical frequency (Figure 5a and b). Signals at certain frequencies can be selectively captured and/or sent out by a nanoantenna. In addition, the polarization states as well as the radiation directions of signals can also be controlled. As shown in Figure 5c and d, a dipolar optical nanoantenna based on the longitudinal plasmon mode of a single Au nanorod can selectively pick up light signals that are polarized along the nanorod length axis.60 The same nanostructures can also be utilized for controlling the emission polarization of the nearby fluorophore molecules. The light emitted to the far field is linearly polarized along the nanorod length axis (Figure 5e and f).116 Moreover, the light emission from nanoscale optical species can be steered to target directions by either using simple anisotropic plasmonic nanostructures115,116

given plasmon mode cannot be controlled to be simultaneously equal to both the absorption and emission peak wavelengths. Therefore, the location of the plasmon peak in between the absorption and emisison peaks provides an optimized circumstance for obtaining the largest intensity enhancement, although neither of the excitation and emission enhancements are optimal under this situation. We also would point out that laser wavelengths instead of absorption peak wavelengths need to be considered when laser sources are used for excitation in fluorescence measurements. In these cases, the local electric field enhancement is determined by the relative spectral positions of the excitation laser line and the localized plasmon resonance because laser line widths are usually much smaller than plamson resonance band widths. When the laser wavelength matches with the plasmon peak wavelength, a maximal local field enhancement and thus a maximal excitation enhancement will be obtained. When the laser wavelength is shifted away from the plasmon peak in either direction but still within the plamson resonance band, the field enhancement will be reduced, although how the field enhancement varies with the relative shift of the laser line has not been studied. When the laser wavelength is shifted out of the plamson resonance band, the plasmon will not be excited, and therefore, there will be no field enhancement, even though the laser line is still located within the absorption band of the fluorophore. The fluorescence intensity is a key merit in many applications involving fluorophores, such as biological labeling, sensing and imaging, light-emitting devices, and single-photon sources. There are also many applications, such as solar cells, photochemical reactions, and photothermal cancer therapy, which can benefit by maximizing light absorption while minimizing radiative decay. In this sense, to simply obtain fluorescence intensity enhancements will not be enough. A better understanding and control of light absorption and emission separately by localized plasmons will therefore be very helpful. In addition, such investigations can also help in elucidating light−matter interactions at the nanoscale. We studied the separate contribution of the excitation enhancement to PEF by utilizing anisotropic Au nanorods. In our experiments, the fluorescence intensity was measured from single hybrid nanostructures, each of which contained a single Au nanorod core and a silica shell embedded with several thousand fluorophore molecules.60 In such nanostructures, the modification of the overall quantum yield by the nanorod is fixed because the fluorophore molecules are embedded stationarily in the silica shell. Because the longitudinal plasmon mode of Au nanorods is polarized along the length axis, the excitation of the longitudinal plasmon mode can be switched on and off by varying the excitation polarization direction of a linearly polarized laser light. The electric field intensity within the silica shell can also be systematically varied by rotating the excitation polarization. A comparison of the emission intensities under different excitation polarizations allows for the exclusive contribution of the excitation enhancement to the PEF. The results show that the emission intensity from the individual nanostructures follows a squared-cosine relationship with the excitation polarization angle. This relationship originates solely from the excitation enhancement because the electric field intensity enhancement averaged within the silica shell also shows a squared-cosine dependence.60 Moreover, the intensity enhancement factor, which is determined as the intensity ratio between the parallel and perpendicular excitation polarizations with respect to the 196



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Figure 5. (a, b) Schematic showing the transmitting and receiving antenna, respectively. The arrows indicate the direction of signal flow. The two configurations are related by the principle of reciprocity. (c, d) Excitation polarization-dependent fluorescence intensity from a hybrid nanostructure, which is composed of a Au nanorod core and a silica shell embedded with fluorophore molecules. (e, f) Linearly polarized emission from a similar hybrid nanostructure. (g) SEM image of a Yagi-Uda nanoantenna consisting of a feed element, one reflector, and three directors. A QD is attached to one end of the feed element, as indicated by the red box. (h) Experimental (black) and theoretical (red) angular radiation patterns of the Yagi-Uda nanoantenna. (i) Comparison of the Förster energy-transfer radii between organic fluorophore molecules and those from organic fluorophore molecules to plasmonic Au nanocrystals. The curve shows the dependence of the Fö r ster radius on the extinction cross section of the acceptor. Reprinted with permission.60,114,116,121 Copyright 2009, 2011 American Chemical Society, 2010 Science, 2011 Nature Publishing Group.

into account the geometries of plasmonic nanostructures are needed. From the examples above, we see that plasmonic nanostructures can redistribute the optical electric field, alter the de-excitation pathways, change the emission polarization, and redirect the emission intensity. These functionalities are far beyond fluorescence intensity enhancements. In all of the studies mentioned above, the spectral profiles of fluorescence emission have been treated to be the same as the intrinsic ones. This is based on the assumption that each fluorescent emitter is a two-level system, where only the emission wavelength corresponding exactly to the transition between the two electronic energy levels is considered. This two-level model is applicable for explaining the intensity change, polarization dependence, and emission pattern that are induced by plasmonic nanostructures. However, the real energy states of a fluorescent emitter are actually far more complex than those in the twolevel model.124 For example, organic fluorophores usually have more than one excited electronic state. Both the ground and excited states carry multiple vibrational energy levels. The decay rates of the transitions differ among different pairs of the vibrational energy levels,125,126 which generates a spectral profile of the emission. When a fluorescent emitter is placed in the vicinity of a plasmonic nanostructure, the decay rates associated with the different

or designing more complex, multicomponent plasmonic nanostructures, such as optical Yagi-Uda nanoantennas (Figure 5g and h).117−122 The effective out-coupling of light from nanoscale optical emitters to the far field is realized through efficient energy transfer from the emitters to the plasmonic nanoantenna.62,116,123 Figure 5i shows the comparison of the calculated Förster radii for the energy transfer from oxazine 725 to organic fluorophores with those to Au nanocrystals. The absorption peak wavelengths of organic fluorophores and the plasmon wavelengths of Au nanocrystals are chosen to be nearly equal to the emission peak wavelength of oxazine 725. The Förster radii for typically sized Au nanocrystals range from 30 to 90 nm, while those for organic fluorophores as acceptors are in the range of 4−9 nm. There is an order of magnitude difference. This is because the extinction cross sections of Au nanocrystals are 3−8 orders of magnitude larger than the absorption cross sections of organic fluorophores, as shown in Figure 1. The much larger Förster radii for plasmonic nanoantennas mean that they can efficiently accept energy from emitters over a relatively large distance. However, in the estimation of the Förster radii on the basis of the Förster resonance energy-transfer theory, Au nanorods are treated as point dipoles. Such a treatment is not very rigorous because Au nanorods are much larger than organic fluorophore molecules. More accurate analyses taking 197



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Figure 6. (a, b) Scattering and fluorescence spectra of Cy3 molecules placed in the gaps of Au nanosphere dimers, respectively. The spectra in identical colors were obtained from the same dimer. The inset in (b) shows schematically a Au nanosphere dimer attached with fluorophore molecules. (c) Scattering spectra of a hybrid nanostructure similar to those shown in Figure 5c and e. The orange, pink, and cyan spectra were recorded in the absence of a polarization analyzer and in the presence of a polarization analyzer with its polarization axis aligned parallel and perpendicular to the nanorod length axis, respectively. (d) Corresponding fluorescence spectra recorded on the same nanostructure as that in (c). The inset shows the schematic of a hybrid nanostructure. (e) Fluorescence spectra recorded from a different hybrid nanostructure as a function of the angle between the polarization axis of the analyzer and the nanorod length axis. Reprinted with permission.125,126 Copyright 2008 American Physical Society, 2011 Royal Society of Chemistry.

In summary, localized plasmons of plasmonic nanostructures enable us to engineer the fluorescence emission of nanoscale quantum emitters with enormous capabilities and wide flexibilities. The emission intensities can be enhanced by up to 1340 fold,61 the emission can be steered in targeted directions in space,120−122 and the emission from the same molecule can be forced into different colors.124−126,129 As shown above, significant progress has been made in the understanding of the fundamental interactions between fluorescence and localized plasmons and the exploration of their device applications. To unravel the full potential of localized plasmons, several challenges in this field need to be overcome in the near future. First, the theoretical model for the overall quantum yield of a plasmon−fluorophore coupled system needs to be verified experimentally, and the relationships between the involved rate parameters and the plasmonic properties need to be better understood. Such efforts will be critical for the design of specific plasmonic nanostructures to achieve desired emission properties from quantum emitters. Second, the theoretical description of the energy transfer from fluorophore molecules to plasmonic nanostructures should be reconsidered. Our explanation is based on Förster resonance energy-transfer theory, where the plasmon is approximated as a dipole. This approximation is valid when the fluorophore−nanocrystal spacing is relatively large, but it will oversimplify the interaction between the emission dipole and the plasmon when the spacing becomes comparable to the size of the nanostructure. In this spacing regime, the emission dipole can induce complex redistributions of electron clouds in the nanostructure. In other words, when the spacing is very small, higher-order plasmon modes need to be taken into account.50 Third, for the plasmon-induced reshaping of fluorescence spectra, the slope of the linear dependence of the newly generated emission peak wavelength on the plasmon wavelength varies considerably among different fluorophores. In addition, the intensity ratio between the new emission peak and the intrinsic one differs among different fluorophores. The origins of these variations remain for further exploration. Fourth, although few experiments have demonstrated the simultaneous control of molecule−nanocrystal spacings and measurement of single-molecule fluorescence emission,50,51,61 more efforts are needed along this direction. Fifth, recent efforts on the use of ultrashort laser pulses to

transitions are modulated to varying degrees. As a result, the spectral profile of the fluorescence emission will be reshaped. We note that the molecular absorption−plasmon resonance coupling can also alter the energy states of quantum emitters, which are manifested in the extinction and scattering spectra.127,128 In general, the occurrence of the molecular absorption−plasmon resonance coupling requires a strong overlap between the absorption and plasmon bands and a high number density of dye molecules that are positioned within ∼5 nm of the metal surface. On the other hand, plasmon-induced spectral modulation does not require many fluorophore molecules. It can take place even when the distance between fluorophore molecules and the metal surface is up to ∼30 nm and there is no overlap between the absorption and plasmon bands. To date, plasmon-induced spectral modulation has been reported only in a few experiments. 60,124−126,129 Ringler et al. examined this phenomenon using Au nanosphere dimers. The spacing between the two nanospheres within a dimer determines the resonance wavelength of the coupled plasmons. They found that fluorophore molecules placed within the gap of the dimers produce plasmon-wavelengthdependent emission spectra (Figure 6a and b). 125 Besides the intrinsic emission peak, a new peak is observed. The new peak red shifts as the plasmon wavelength becomes longer. Moreover, the polarization of the new emission peak follows that of the plasmon mode. When Au nanorods are used, the new emission peak caused by the longitudinal plasmon resonance is found to be linearly polarized along the nanorod length axis. In contrast, the intrinsic emission peak remains unpolarized (Figure 6c−e) 129 because the fluorophore molecules in the silica shell are randomly oriented. This polarization-dependent emission result suggests that the plasmon-induced new emission peak arises from the energy transfer from the fluorophore molecules to the Au nanorod and that the remaining intrinsic one comes from the radiative decay of the molecules. This spectral reshaping not only further enriches the exploration of the interactions between localized plasmons and nanoscale emitters but also offers a potential means for tailoring the emission colors of light-emitting devices with localized plasmons. 198



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excite fluorophore−plasmon coupled systems have led to many interesting phenomena, such as surface plasmon amplification

Jianfang Wang (http://www.phy.cuhk.edu.hk/~jfwang/) is currently an Associate Professor in the Department of Physics of The Chinese University of Hong Kong. He obtained his B.S. in chemistry and software design in 1993 from the University of Science and Technology of China and his M.S. in chemistry in 1996 from Peking University. He obtained his Ph.D. in physical chemistry in 2002 from Harvard University. He was a postdoctoral researcher at the University of California, Santa Barbara, from February 2002 to July 2005. His current research interests are the synthesis and plasmonic and catalytic properties of metal nanocrystals, plasmon-enhanced energy utilization, and multifunctional nanostructured materials.

Ideally, one needs to be able to position a single quantum emitter at an arbitrarily desired spacing, location, and orientation relative to a plasmonic nanocrystal of an arbitrarily given size and shape and measure the emission properties, including the intensity, spectrum, polarization, and spatial intensity distribution.

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ACKNOWLEDGMENTS This work was supported by the RGC GRF Grant (Ref. No.: CUHK403409, Project Code: 2160391) and Direct Allocation (Project Code: 2060417).

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by stimulated emission of radiation (SPASER),26 optical Starks effect, and high-harmonic generation.99 Understanding and controlling these phenomena requires new theoretical approaches, which can describe the fluorophore−plasmon interactions in strong coupling regimes. We believe that overcoming these experimental and theoretical challenges will undoubtedly expedite the progress of this field and lead to many exciting discoveries in science and technology.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].. Biographies Tian Ming is currently a Research Assistant in Professor Jianfang Wang’s group. He received his B.S. in physics in 2007 from Fudan University and his Ph.D. in materials science and engineering from The Chinese University of Hong Kong in 2011. His research interests focus on the design, synthesis, and assembly of noble metal nanocrystals and their applications in plasmon-enhanced spectroscopy and energy conversion. Huanjun Chen is currently a Research Associate in Professor Jianfang Wang’s group. He received his B.S. in 2004 and M.S. in 2007 in physics from Sun Yat-Sen University, China. He got his Ph.D. in physics from The Chinese University of Hong Kong in 2010. His current research interests include the localized plasmon resonances of noble metal nanocrystals and their related applications, especially the sensing and photothermal therapeutic applications of Au nanocrystals. Ruibin Jiang is currently a Ph.D. student in physics at The Chinese University of Hong Kong. He obtained his B.S. in materials physics in 2007 and M. S. in materials science and engineering in 2010 from China University of Petroleum. His current research interests include the localized plasmon resonances of noble metal nanocrystals and their applications in nanophotonics. Qian Li is currently a Ph.D. student in Professor Jianfang Wang’s group at The Chinese University of Hong Kong. She obtained her B.S. in optical information science and technology in 2010 from Northwest University, China. Her current research focuses on the design and preparation of noble metal nanocrystals and their applications in plasmon-enhanced spectroscopy. 199



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