Unusual and Tunable One-Photon Nonlinearity in Gold-DyePlexcitonic Fano SystemsFan Nan,† Ya-Fang Zhang,† Xiaoguang Li,‡,§ Xiao-Tian Zhang,§ Hang Li,∥ Xinhui Zhang,∥ Ruibin Jiang,⊥

Jianfang Wang,⊥ Wei Zhang,# Li Zhou,† Jia-Hong Wang,† Qu-Quan Wang,*,†,∇ and Zhenyu Zhang*,§

†Department of Physics, Wuhan University, Wuhan, Hubei 430072, People’s Republic of China‡Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, Guangdong 518055, People’s Republic ofChina§International Center for Quantum Design of Functional Materials (ICQD), University of Science and Technology of China, Hefei,Anhui 230026, People’s Republic of China∥State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing100083, People’s Republic of China⊥Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR People’s Republic of China#Institute of Applied Physics and Computational Mathematics, Beijing 100088, People’s Republic of China∇Institute for Advanced Study, Wuhan University, Wuhan, Hubei 430072, People’s Republic of China

*S Supporting Information

ABSTRACT: Recent studies of the coupling between theplasmonic excitations of metallic nanostructures with theexcitonic excitations of molecular species have revealed a richvariety of emergent phenomena known as plexcitonics. Here,we use a combined experimental and theoretical approach todemonstrate new and intriguing aspects in the ultrafastnonlinear responses of strongly coupled hybrid Fano systemsconsisting of gold nanorods decorated with near-infrared dyemolecules. We show that the severely suppressed linear absorption around the Fano dip significantly enhances the unidirectionalenergy transfer from the plasmons to the excitons and further allows one-photon nonlinearity to be drastically and reversiblytuned. These striking observations are interpreted within a microscopic model stressing on two competing processes: saturatedplasmonic absorption and weakened destructive Fano interference from the bleached excitonic absorption. The unusually strongone-photon nonlinearity revealed here provides a promising strategy in fabricating nanoplasmonic devices with both pronouncednonlinearities and good figures of merit.

KEYWORDS: Gold nanorod, Fano interference, plexciton resonance, one-photon nonlinearity, ultrafast energy transfer

As collective excitations of the conduction electrons, thesurface plasmons of metallic nanostructures can induce

large local electromagnetic fields near the metal surfaces uponresonant excitation, which can be exploited to manage light atthe nanometer scale.1−9 Such enhanced local fields can interactstrongly with adjacent semiconductors or organic mole-cules.10−23 Especially, coupled plasmons and excitons form anew type of optical excitation termed as plexciton.24−27 Theoptical behaviors of such hybrid systems can be distinctlydifferent from those of either of their constituents and are ofsubstantial fundamental and practical importance in under-standing light-matter interactions.The strong coupling of plasmonic and molecular resonance

in hybrid systems is signified by its characteristic absorptionspectra, typically exhibiting plexcitonic Fano resonance25−32 orRabi splitting33,34 induced by the interference of the plasmonicand molecular resonances. Such hybrid systems have beenshown to offer great potential in ultrasensitive sensing and in

vivo molecular imaging.4,35 In addition, the inherent nonlinearnature of the Fano resonance has been theoretically proposedto provide a new strategy for tuning the photon statistics ofquantum emitters,36,37 yet direct experimental studies ofnonlinear responses of hybrid systems have been rare.Here, we use complementary experimental approaches to

study both the linear and nonlinear optical responses ofstrongly coupled hybrid Fano systems consisting of Au nanorod(AuNR) cores decorated with near-infrared (NIR) dyemolecular shells. We also develop a microscopic excitonmodel to describe the optical absorption resonance of thedye molecules, and the plasmonic-molecular resonancecoupling is described accordingly within the plexciton picture.We show that in the weak external field regime the absorption

Received: January 31, 2015Revised: March 1, 2015Published: March 10, 2015

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© 2015 American Chemical Society 2705 DOI: 10.1021/acs.nanolett.5b00413Nano Lett. 2015, 15, 2705−2710

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of the hybrids at the Fano dip decreases as the concentration ofthe dye molecules increases, establishing effective suppressionof the linear absorption around the dip and significantenhancement of the unidirectional energy transfer from theplasmons to the excitons. Furthermore, in the strong fieldregime the severely suppressed linear absorption around theFano dip allows one-photon nonlinear responses to be readilyrevealed and reversibly tuned by the photon energy, laserpower, and dye concentration. The one-photon nonlinearityinvolving no multiphoton processes is distinctly different fromordinary nonlinear (or multiphoton) phenomena. The counter-intuitive nature of the unusually strong one-photon nonlinearityinterpreted within a microscopic phenomenological model isattributed to the saturated-absorption-induced decoherence ofthe initially coherently coupled Fano system and thecompetition between the two nonlinear (or saturation)processes in the gold nanorods and dye molecules. Thesuppressed linear and enhanced nonlinear absorptions aroundthe Fano dip further provide a promising strategy in fabricatingnanoplasmonic devices with both pronounced nonlinearitiesand good figures of merit.38,39

Hybrid Nanostructures and Fano Resonance ofAuNRs Decorated with IR-806 Molecules. Figure 1A

displays a schematic diagram of the metal−molecule hybridsystem (denoted by Au@IR-806) consisting of an assembly ofAuNRs each decorated with dye (IR-806) molecules. Thepolyelectrolyte of poly(allylamine hydrochloride) (PAH) wasused to assemble the dye molecules onto AuNRs.40 The actualsystems necessarily contain disordered molecular distributions(including their density and orientations). Figure 1B shows aTEM image of the Au@IR-806 hybrids, whose density is fixedthroughout the experiment. The AuNRs have an averagediameter of ∼15 nm and length of ∼45 nm, and the dye layerthickness is ∼3 nm. The main molecular absorption peak of thedye molecules is at 805 nm with a narrow width of 48 nm(Figure 1C). The plasmon resonance of the AuNRs is at 788nm with a width of ∼100 nm.

In a weak external field, we have investigated the absorptionof the hybrid systems with different dye concentrations, Cdye. Asshown in Figure 1D, a prominent absorption dip is graduallyestablished as Cdye increases from 0 to 1.6 μM. The dip depthbecomes saturated when Cdye reaches the transition point (1.2μM), where the amount of IR-806 adsorbed on the surface ofAuNRs is probably one monolayer.12 The dipped spectra aresimilar to those of J-aggregates on Au nanoshells,24,25 and canbe interpreted as the result of destructive Fano interferencebetween the strongly coupled plasmonic and molecularresonances. Compared with previously reported results fromAuNRs with HITC molecules,12 the Au@IR-806 hybridsstudied here exhibit much more prominent dips due tostronger coupling of plasmonic and molecular resonance. Thestrength of plasmon-molecular resonance coupling is depend-ent on the distance between AuNRs and IR-806 molecules (seeSupporting Information Figure S2).41

Wavelength-Dependent Nonlinear Transmittance ofAu@IR-806 Hybrids around the Fano Dip. The nonlinearabsorptions of the bare AuNRs and Au@IR-806 hybrids havebeen investigated via the open-aperture (OP) Z-scan techniqueusing a wavelength-tunable and picosecond-pulsed Ti:sapphirelaser.42 The power-dependent absorption coefficient can bewritten as α(I) = α0 + βeffI with the linear coefficient α0 and thenonlinear part absorbed in the function βeffI. The normalizedOP Z-scan transmittance (TOP) follows the relation TOP (z) = 1− I(z)βeffLeff,

42 where I(z) = I0/[1 + (z/z0)2] is the light

intensity of a Gaussian laser beam at the position z, I0 and z0 arethe peak irradiance at the focus (z = 0) and effective Rayleighlength of the beam, respectively, and Leff is the effectivethickness of the sample. When we consider only the secondorder nonlinear process with a constant βeff, the measured TOPcan be fitted with a Lorentz function of z.Figure 2A,B presents the nonlinear transmittance of the bare

AuNRs and Au@IR-806 hybrids (with the dye concentrationCdye = 1.6 μM) at different incident laser wavelengths. At all thewavelengths, the bare AuNRs exhibit saturated absorption(SA). This is induced by the bleaching of the electronic groundstate because the population is very efficiently pumped to theexcited state via plasmonic excitation while the electronrelaxation rate to the ground state is not fast enough.43 Theintensity-dependent absorption of the saturated effect isexpressed by αAuNR = α0/(1 + I/IS,p), where IS,p is the saturatedintensity. The SA effect of the AuNRs is more prominent at theplasmon resonance wavelength owing to stronger local fieldenhancement. In contrast, the Au@IR-806 hybrids display threetypes of nonlinear behaviors: (i) the reversed saturatedabsorption (RSA) that exhibits a valley in TOP at thewavelength of the Fano dip; (ii) the SA at wavelengths faraway from the Fano dip; and (iii) a mixture of at least twononlinear processes exhibiting “W-shaped” transmittance nearthe wavelength of the Fano dip. In Figure 2C,D, where we use aconstant βeff to fit the Z-scan results, a clear correspondencebetween the absorption peak (valley) and the negative(positive) βeff can be seen. However, the W-shaped trans-mittance implies the existence of more than one competingnonlinear process, which cannot be properly described by asingle-signed βeff, requiring a further elucidation of thenonlinear behaviors in such strongly coupled systems.

Dye-Concentration and Laser-Power Dependences ofNonlinear Transmittance at the Fano Dip. To helpelucidate the underlying mechanism of the peculiar nonlinearbehaviors, we have carried out Z-scan measurements at

Figure 1. Structures and absorption spectra of the Au@IR-806hybrids. (A) Illustration of the Au@IR-806 hybrid. (B) TEM images ofthe Au@IR-806 hybrids. (C) Absorption spectra of the bare AuNRs,dye IR-806 molecules, and Au@IR-806 hybrids. (D) Measuredabsorption spectra of the hybrids with different dye concentrationsCdye.

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different dye concentrations and laser intensities. Figure 3Ashows TOP of the Au@IR-806 hybrids with different dyeconcentrations at the fixed laser power of I0 = 50 mW andwavelength of 810 nm. As Cdye increases, the trace of the Z-scannonlinear transmittance varies from a typical SA to a RSAtrend. These transitions suggest a close correlation between theRSA and the strength of the Fano interference, which can beeffectively controlled by Cdye (see Figure 1D) as well as theAuNR-dye distance (see Supporting Information Figure S2).On the other hand, Figure 3C displays the nonlinear responseof the hybrids by varying the laser power at the fixed dyeconcentration of Cdye = 1.2 μM and wavelength of the Fano dip.If the nonlinear process involves an additional absorptionchannel, we would expect the nonlinear effect to become morepronounced as the laser power increases. However, as shown inFigure 3C, the depth of the valley is not enlarged at higherpowers. On the contrary, the TOP traces vary from a valley toW-shaped as the laser power increases from 30 to 100 mW.This abnormal nonlinear behavior indicates that the dominantphysical mechanisms at low and high laser powers are verydifferent, and the RSA of the plexcitonic hybrids is not inducedby the opening of a new absorption channel.Microscopic Phenomenological Model. The absorption

spectra of the plexcitonic hybrid Fano systems can be explainedusing a microscopic phenomenological model, which is basedon the method introduced by Manjavacas et al.26 TheHamiltonian of the hybrids includes three terms: H = H0 +Hint + Hdecay. The noninteracting part H0 = εpp

+p +∑i=1,2Cdyeεiei

+ei represents the excitations of one plasmonmode p and two molecular excitation ei. The plasmonic-molecular resonance interaction is described as Hint =−Cdye∑i=1,2Δi(p

+ei + e+p), where Δi is the effective (or average)

coupling strength. The absorption is then evaluated followingthe Fermi Golden rule. Because of the much smaller excitonicabsorption of the molecules, the total absorption of the hybridis approximated by the plasmon absorption as

∑

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where Δp describes the coupling between the plasmon and theexternal field, ni represents the population of the excited statesof the dye molecules, δωp + iΓp/2 and δωi + iΓi/2 represent theenergy level shifts of the AuNR plasmon and molecularresonance caused by the environment, respectively, and ΓS,irepresents the spontaneous decay, which is important as weconsider the occupations of the excited states in the molecules.The first term in eq 1 describes the AuNR plasmon resonanceat εp. The second term is responsible for the Fano dip. Clearly,we could expect a more prominent Fano dip at a higher dyeconcentration because increasing Cdye effectively enhances theplasmon and molecular resonance coupling.We now present a theoretical interpretation of the nonlinear

effects based on eq 1. For simplicity, we have ignored thecontributions from the resonance around 750 nm, because thisenergy is far away from the main Fano dip (∼800 nm). The SAof the dye molecules is reflected by (1 − 2ne) → 0 in eq 1. Totheoretically implement this saturation effect, we assume (1 −2ne) = (1 + I/IS,e)

−1, where IS,e is the saturation intensity for the805 nm resonance. We have assumed that IS,p = 10 I0 and IS,e =

Figure 2. Nonlinear transmittance of the AuNRs and Au@IR-806hybrids at different incident wavelengths. Comparison of the OP Z-scan nonlinear transmittance of (A) the bare AuNRs, and (B) Au@IR-806 hybrids with Cdye = 1.6 μM, both at the laser power of I0 = 50mW. The corresponding effective nonlinear absorption coefficients βeffof the samples are shown in (C,D).

Figure 3. Nonlinear transmittance of the AuNRs and Au@IR-806hybrids at different dye concentrations and laser intensities. (A)Measured and (B) calculated TOP of the Au@IR-806 hybrids atdifferent Cdye and fixed I0 = 50 mW. (C) Measured and (D) calculatedTOP of the hybrids with different laser powers and fixed Cdye = 1.2 μM.

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0.8 I0 in fitting the Z-scan results. Considering that theirradiance at the position z = 0 is around 0.1 GW/cm2 in our Z-scan measurements, this assumption is consistent with thesaturated intensity measured in similar Au nanostructures44 anddye molecular systems.45 The parameters for the energy andline width of the plasmon and molecular resonance areobtained by fitting the absorption spectra of the baredAuNRs and dye molecules as follows: εp = 1.57 eV, ε1 = 1.55eV; Γp = 0.2 eV, Γs,1 = 0.12 eV. The other parameters related tothe coupling between the AuNRs and dye molecules are at thesame order of magnitude as in ref 26. By considering thesaturated absorption of the molecules, we see that at the highlaser intensity, the effective plasmonic-molecular couplingCdyeΔp(1 − 2ni) will be strongly suppressed. Around theFano dip, this suppression weakens the destructive Fanointerference and leads to the absorption efficiency α(I)increasing with the excitation intensity, providing a clear one-photon nonlinear mechanism for the RSA in the hybridsystems. By considering the two competing processes, namely,the SA of the plasmon of AuNRs and weakened destructiveFano interference from the bleached resonance absorption ofthe molecules, the exotic nonlinearity of the hybrids can be wellreproduced by our theoretical model as shown in Figure 3.Ultrafast Plasmon Energy Transfer and Fano Reso-

nance in the Au@IR-806 Hybids. Next, we investigate theultrafast dynamical processes of both the bare AuNRs and Au@IR-806 hybrids by using the time-resolved optical differentialtransmission (DT) method. Because the width of the laserpulse used to excite the systems is ∼150 fs, we are unable todirectly resolve the transient SA of the plasmons with shorterlifetimes. Instead, we actually monitor the decay of the hotelectrons converted from the plasmons within tens of fs, as alsoinferred recently in similar systems.33 We observe a fast decayprocess of ∼2 ps and a slow decay process of ∼200 ps (inset ofFigure 4A), attributed to the hot electron−phonon couplingwithin the AuNRs and phonon−phonon scattering between theAuNRs and their surrounding liquid matrix, respectively.46,47

We note that even though we could not directly observe thedecay of the plasmons within the AuNRs, their traces are wellmanifested by the decay behaviors of the subsequent hotelectrons. For the bare AuNRs, the transient DT ΔIprobe of theprobe beam increases with the optical pumping pulse(indicating SA) at all the excitation wavelengths, and theoverall magnitude of the SA signal is the highest around theplasmon resonance frequency (Figure 4C).In strong contrast, Figure 4B shows that the transient ΔIprobe

of the hybrids decreases upon weak pumping (indicating RSA),and the maximum magnitude is around the wavelength of theFano dip (Figure 4C) consistent with our Z-scan observations.Intriguingly, the reversal from the RSA to SA in the Au@IR-806 hybrid systems is also clearly demonstrated in the time-resolved DT measurements (see Figure 4D and SupportingInformation Figure S5). The ΔIprobe signal of the RSA reaches amaximum within hundreds of fs for the hybrid system with Cdye= 1.0 μM (inset of Figure 4D). If the pump intensity isextremely low (Ppump < 0.64 μJ/cm2), only the RSA can beobserved. The reversal time decreases as the pump intensityincreases (see Supporting Information Figure S5), and cancomplete within several picoseconds consistent with the Z-scanmeasurements. In addition, we observe that ΔI returns to zeroafter relatively long decay (∼ ns) implying a negligiblephotobleaching effect in our measurements.

Discussion. The nonlinear transmittance of the RSA in thepresent hybrid Fano system is a one-photon process, which isdistinctly different from the ordinary nonlinearity induced bymultiphoton processes. Both the transient DT and Z-scannonlinear transmittance reveal that the Au@IR-806 hybridshave a large and tunable one-photon nonlinearity at thewavelength of the Fano dip, providing its prominent advantagestoward a low-power (<1 μJ/cm2) excitation and an ultrafastresponse time (<1 ps). Therefore, this one-photon nonlinearityemphasized here could in principle be used in future ultrafastsingle-photon switching and information processing.32,36,37

Before closing, it is worthwhile to emphasize that the Fanoresonance and Rabi splitting represent different yet inherentlyconnected manifestations of the intrinsic interaction betweenthe plasmons and other resonances.46,48 In particular, bothenergy and charge distributions of the hybrids are significantlydifferent at the Fano dip and Rabi peaks, as vividlydemonstrated through FDTD simulations of a model Au@IR-806 system (see Supporting Information Figure S6 fordetails). Furthermore, the Rabi oscillations at the wavelengthsof the two hybridized peaks typically take place within the timescale of ∼100 fs,33,34 while the energy transfer from the AuNRsto molecules around the Fano dip could last much longer,thereby enabling the energy transfer from the “hot electrons” tothe molecules and other degrees of freedom.In summary, we have investigated the ultrafast one-photon

nonlinear responses of strongly coupled Au@IR-806 hybridplexcitonic Fano systems. The unusually strong and reversableone-photon nonlinearity has been interpreted with thedevelopment of a microscopic phenomenological model.These novel findings provide a promising strategy to thefabrication of nanoplasmonic devices with both pronouncednonlinearities and good figures of merit.

■ ASSOCIATED CONTENT*S Supporting InformationTheoretical modeling, simulations of linear absorption andnonlinear absorption, tuning coupling strength of Au@IR-806,

Figure 4. Dynamics of the differential transmissions (ΔI) of theAuNRs and Au@IR-806 hybrids. (A) ΔI of the bare AuNRs atdifferent wavelengths and fixed pump intensity of 6.04 μJ/cm2 (Inset:high-resolution ΔI at 810 nm). (B) ΔI of the Au@IR-806 at differentwavelengths. (C) ΔI of the AuNRs and Au@IR-806 as a function ofthe laser wavelength. (D) ΔI of the Au@IR-806 as a function of thepump power. Inset: ΔI of the Au@IR-806 with high resolution.

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time-resolved differential transmissions of AuNRs with differentwavelengths and Au@IR-806 with different pump powers, andFDTD simulations of Au@IR806 hybrids. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: qqwang@whu.edu.cn (Q.Q.W.).*E-mail: zhangzy@ustc.edu.cn (Z.Z.).Author ContributionsF.N., Y.F.Z., and X.L. contributed equally to this work.F.N. performed optical nonlinear measurements and data

analysis. Y.F.Z. and J.H.W. prepared the samples and recordedabsorption spectra. J.W. and R.J. performed FDTD numericalsimulations and analysis. X.L., X.T.Z., Z.Z., and W.Z. wereresponsible for theoretical modeling and analysis. H.L. and X.Z.were responsible for transient DT measurements. L.Z. helpedwith some of the experimental data analysis and manuscriptpreparation and revision. Q.Q.W. was responsible for theexperimental design and data analysis. Q.Q.W. and Z.Z. wereresponsible for the interpretation as well as writing, revision,and finalization of the manuscript.FundingNational Program on Key Science Research of China(2011CB922201) and the Natural Science Foundation ofChina (11204221, 11174042, 11174229, and 11374039).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank H. Q. Lin for helpful discussions and X. Yu, Y. L.Wang, and S. J. Ding for the assistance in sample preparation.This work was supported by the National Program on KeyScience Research of China (2011CB922201) and the NaturalScience Foundation of China (11204221, 11174042, 11174229,and 11374039).

■ REFERENCES(1) Zhang, S.; Genov, D. A.; Wang, Y.; Liu, M.; Zhang, X. Phys. Rev.Lett. 2008, 101, 047401.(2) Tassin, P.; Zhang, L.; Koschny, T.; Economou, E. N.; Soukoulis,C. M. Phys. Rev. Lett. 2009, 102, 053901.(3) Liu, N.; Langguth, L.; Weiss, T.; Kastel, J.; Fleischhauer, M.; Pfau,T.; Giessen, H. Nat. Mater. 2009, 8, 758−762.(4) Liu, N.; Weiss, T.; Mesch, M.; Langguth, L.; Eigenthaler, U.;Hirscher, M.; Sonnichsen, C.; Giessen, H. Nano Lett. 2010, 10, 1103−1107.(5) Fan, J. A.; Wu, C.; Bao, K.; Bao, J.; Bardhan, R.; Halas, N. J.;Manoharan, V. N.; Nordlander, P.; Shvets, G.; Capasso, F. Science2010, 328, 1135−1138.(6) Chang, W. S.; Lassiter, J. B.; Swanglap, P.; Sobhani, H.; Khatua,S.; Nordlander, P.; Halas, N. J.; Link, S. Nano Lett. 2012, 12, 4977−4982.(7) Shafiei, F.; Monticone, F.; Le, K. Q.; Liu, X. X.; Hartsfield, T.;Alu, A.; Li, X. Nat. Nanotechnol. 2013, 8, 95−99.(8) Belotelov, V. I.; Kreilkamp, L. E.; Akimov, I. A.; Kalish, A. N.;Bykov, D. A.; Kasture, S.; Yallapragada, V. J.; VenuGopal, A.; Grishin,A. M.; Khartsev, S. I.; Nur-E-Alam, M.; Vasiliev, M.; Doskolovich, L.L.; Yakovlev, D. R.; Alameh, K.; Zvezdin, A. K.; Bayer, M. Nat.Commun. 2013, 4, 2128.(9) Dyer, G. C.; Aizin, G. R.; Allen, S. J.; Grine, A. D.; Bethke, D.;Reno, J. L.; Shaner, E. A. Nat. Photonics 2013, 7, 925−930.(10) Bellessa, J.; Bonnand, C.; Plenet, J. C.; Mugnier, J. Phys. Rev.Lett. 2004, 93, 036404.

(11) Wurtz, G. A.; Evans, P. R.; Hendren, W.; Atkinson, R.; Dickson,W.; Pollard, R. J.; Zayats, A. V.; Harrison, W.; Bower, C. Nano Lett.2007, 7, 1297−1303.(12) Ni, W.; Yang, Z.; Chen, H.; Li, L.; Wang, J. J. Am. Chem. Soc.2008, 130, 6692−6693.(13) Cade, N. I.; Ritman-Meer, T.; Richards, D. Phys. Rev. B 2009,79, 241404.(14) Bellessa, J.; Symonds, C.; Vynck, K.; Lemaitre, A.; Brioude, A.;Beaur, L.; Plenet, J. C.; Viste, P.; Felbacq, D.; Cambril, E.; Valvin, P.Phys. Rev. B 2009, 80, 033303.(15) Juluri, B. K.; Lu, M.; Zheng, Y. B.; Huang, T. J.; Jensen, L. J.Phys. Chem. C 2009, 113, 18499−18503.(16) Ni, W.; Ambjoornsson, T.; Apell, S. P.; Chen, H.; Wang, J. NanoLett. 2010, 10, 77−84.(17) Chen, H.; Shao, L.; Woo, K. C.; Wang, J.; Lin, H. Q. J. Phys.Chem. C 2012, 116, 14088−14095.(18) Choi, Y.; Kang, T.; Lee, L. P. Nano Lett. 2009, 9, 85−90.(19) Yoshida, A.; Uchida, N.; Kometani, N. Langmuir 2009, 25,11802−11807.(20) Yoshida, A.; Yonezawa, Y.; Kometani, N. Langmuir 2009, 25,6683−6689.(21) Yoshida, A.; Kometani, N. J. Phys. Chem. C 2010, 114, 2867−2872.(22) Manjavacas, A.; Marchesin, F.; Thongrattanasiri, S.; Koval, P.;Nordlander, P.; Sanchez-Portal, D.; García de Abajo, F. J. ACS Nano2013, 7, 3635−3643.(23) Kuhn, S.; Hakanson, U.; Rogobete, L.; Sandoghdar, V. Phys. Rev.Lett. 2006, 97, 017402.(24) Fofang, N. T.; Park, T. H.; Neumann, O.; Mirin, N. A.;Nordlander, P.; Halas, N. J. Nano Lett. 2008, 8, 3481−3487.(25) Fofang, N. T.; Grady, N. K.; Fan, Z.; Govorov, A. O.; Halas, N.J. Nano Lett. 2011, 11, 1556−1560.(26) Manjavacas, A.; García de Abajo, F. J.; Nordlander, P. Nano Lett.2011, 11, 2318−2323.(27) Schlather, A. E.; Large, N.; Urban, A. S.; Nordlander, P.; Halas,N. J. Nano Lett. 2013, 13, 3281−3286.(28) Zhang, W.; Govorov, A. O.; Bryant, G. W. Phys. Rev. Lett. 2006,97, 146804.(29) Artuso, R. D.; Bryant, G. W. Nano Lett. 2008, 8, 2106−2111.(30) Zhang, W.; Govorov, A. O. Phys. Rev. B 2011, 84, 081405.(31) Zengin, G.; Johansson, G.; Johansson, P.; Antosiewicz, T. J.;Kall, M.; Shegai, T. Sci. Rep. 2013, 3, 3074.(32) Chen, X. W.; Sandoghdar, V.; Agio, M. Phys. Rev. Lett. 2013,110, 153605.(33) Hao, Y. W.; Wang, H. Y.; Jiang, Y.; Chen, Q. D.; Ueno, K.;Wang, W. Q.; Misawa, H.; Sun, H. B. Angew. Chem. 2011, 123, 7970−7974.(34) Vasa, P.; Wang, W.; Pomraenke, R.; Lammers, M.; Maiuri, M.;Manzoni, C.; Cerullo, G.; Lienau, C. Nat. Photonics 2013, 7, 128−132.(35) Choi, Y.; Park, Y.; Kang, T.; Lee, L. P. Nat. Nanotechnol. 2009,4, 742−746.(36) Ridolfo, A.; Di Stefano, O.; Fina, N.; Saija, R.; Savasta, S. Phys.Rev. Lett. 2010, 105, 263601.(37) Manjavacas, A.; Nordlander, P.; García de Abajo, F. J. ACS Nano2012, 6, 1724−1731.(38) Kroner, M.; Govorov, A. O.; Remi, S.; Biedermann, B.; Seidl, S.;Badolato, A.; Petroff, P. M.; Zhang, W.; Barbour, R.; Gerardot, B. D.;Warburton, R. J.; Karrai, K. Nature 2008, 451, 311−314.(39) Wang, Q. Q.; Han, J. B.; Gong, H. M.; Chen, D. J.; Zhao, X. J.;Feng, J. Y.; Ren, J. J. Adv. Funct. Mater. 2006, 16, 2405−2408.(40) Murphy, C. J.; Thompson, L. B.; Alkilany, A. M.; Sisco, P. N.;Boulos, S. P.; Sivapalan, S. T.; Yang, J. A.; Chernak, D. J.; Huang, J. J.Phys. Chem. Lett. 2010, 1, 2867−2875.(41) Peng, B.; Zhang, Q.; Liu, X.; Ji, Y.; Demir, H. V.; Huan, C. H.A.; Sum, T. C.; Xiong, Q. ACS Nano 2012, 6, 6250−6259.(42) Sheik-Bahae, M.; Said, A. A.; Wei, T. H.; Hagan, D. J.; VanStryland, E. W. IEEE J. Quantum Electron. 1990, 26, 760−769.(43) Philip, R.; Kumar, G. R.; Sandhyarani, N.; Pradeep, T. Phys. Rev.B 2000, 62, 13160−13166.

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(44) Philip, R.; Chantharasupawong, P.; Qian, H.; Jin, R.; Thomas, J.Nano Lett. 2012, 12, 4661−4667.(45) Yuce, M. Y.; Kiraz, A. Chem. Phys. Lett. 2012, 547, 47−51.(46) Hu, M.; Hartlandet, G. V. J. Phys. Chem. B 2002, 106, 7029−7033.(47) Jiang, Y.; Wang, H. Y.; Xie, L. P.; Gao, B. R.; Wang, L.; Zhang,X. L.; Chen, Q. D.; Yang, H.; Song, H. W.; Sun, H. B. J. Phys. Chem. C2010, 114, 2913−2917.(48) Faucheaux, J. A.; Fu, J.; Jain, P. K. J. Phys. Chem. C 2014, 118,2710−2717.

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