LETTER pubs.acs.org/NanoLett
Plexciton Dynamics: ExcitonPlasmon Coupling in a J-AggregateAu Nanoshell Complex Provides a Mechanism for Nonlinearity Nche T. Fofang,†,§ Nathaniel K. Grady,‡,§ Zhiyuan Fan,|| Alexander O. Govorov,|| and Naomi J. Halas*,†,§ Department of Electrical and Computer Engineering, ‡Department of Physics and Astronomy, and §Laboratory for Nanophotonics, Rice University, 6100 Main Street, Houston, Texas 77005, United States Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, United States
)
†
ABSTRACT: Coherently coupled plasmons and excitons give rise to new optical excitations- plexcitons due to the strong coupling of these two oscillator systems. Time-resolved studies of J-aggregate-Au nanoshell complexes when the nanoshell plasmon and J-aggregate exciton energies are degenerate probe the dynamical behavior of this coupled system. Transient absorption of the interacting plasmonexciton system is observed, in dramatic contrast to the photoinduced transmission of the pristine J-aggregate. An additional, transient Fano-shaped modulation within the Fano dip is also observable. The behavior of the J-aggregate-Au nanoshell complex is described by a combined one-exciton and two-exciton state model coupled to the nanoshell plasmon. KEYWORDS: Plexcitons, J-aggregates, Fano resonance
plasmons,
excitons,
T
nanoshells
he optical nonlinearity of materials is a fundamental property required for the realization of active optical frequency devices such as ultrafast switches,14 bistable devices,58 limiters,9 and modulators.1012 Most materials found in nature, however, have a very small nonlinear optical response. Fortunately, this response can be enhanced by exploiting the electronic or vibrational transitions in materials for resonant enhancement at the corresponding near-resonant energies.13 This limitation can also be addressed by designing new, nanoengineered materials whose resonances are tuned to a frequency region of interest, such as specially designed plasmonic structures. Interactions with directly adjacent media can be enhanced by coupling within the intense evanescent field of the plasmon, resulting in new, nanocomposite hybrid materials whose properties are distinct from the properties of the individual constituent media. Hybrid molecularplasmonic nanostructures, where excitations of the respective components of the hybrid are designed to interact, provide a promising strategy for the development of active, nonlinear materials at optical frequencies.14,15 Ultrafast time-domain optical studies can reveal how these interactions result in new properties, and how processes like excitonplasmon hybridization, energy or charge transfer, may contribute to the new hybrid material response. Interest in the combination of molecular media and plasmonic nanostructures results from the ease in design and fabrication of hybrid media with these constituents, and also from the many opportunities to exploit the numerous interaction mechanisms associated with excitonic and plasmonic systems. Metallic nanoparticles sustain collective electronic excitations, known as localized surface plasmons, when resonantly excited. r 2011 American Chemical Society
The local electromagnetic field associated with these plasmons can be quite intense and well localized near the metallic nanoparticle surface; so much so that it may strongly modify the nonlinear properties of any material directly within this fringing field.13 For example, the surface plasmon may affect absorption saturation, nonlinear absorption line shapes,14 nonlinear refractive index,16 or nonradiative decay channels17 in a directly adjacent medium, modifying the optical nonlinear response through any of these mechanisms. In this Letter, we report the examination of the ultrafast optical dynamics of a hybrid resonant plasmonexciton system consisting of a J-aggregateAu nanoshell complex. Nanoshells are tunable plasmonic coreshell nanoparticles that support a distinct hierarchy of localized plasmon modes with resonant energies dependent on the relative dimensions and composition of their core and shell layers, the absolute size of the nanoparticle, and the presence of other media adjacent to, or surrounding, the nanostructure.1822 J-aggregates are organic semiconductors with localized excitations—excitons—that possess very high oscillator strengths, suitable for interaction with the intense near field of localized surface plasmons.23 Strong interaction of J-aggregate excitons with plasmons24,25 and cavity photons,2628 resulting in the formation of mixed states, has been reported. In these systems, the ultrafast dynamical properties of J-aggregates are strongly modified. Excitonphoton interactions have been extensively studied for J-aggregates in microcavities29,30 and more recently Received: December 14, 2010 Revised: February 19, 2011 Published: March 18, 2011 1556
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Figure 1. (a) Schematic of the hybrid nanocomplex consisting of a gold nanoshell coated with J-aggregate molecular film (2,20 -dimethyl-8phenyl-5,6,50 60 -dibenzothiacarbocyanine chloride). (b) Extinction spectrum of the J-aggregate, obtained from a formulation consisting of a mixture of 6 μL of 117 μM dye solution and 3.5 mL of aqueous poly(vinyl alcohol).(c) Extinction spectra of pristine (black) and J-aggregate coated (red) [r1, r2] = [41, 54] nm nanoshells. The vertical arrows indicate the wavelengths (680, 690, 700, 710 nm) where the J-aggregates and J-aggregate-coated nanoshells were optically interrogated.
for J-aggregates interacting with propagating surface plasmons on extended metallic structures.31 Until now, however, the dynamical response of J-aggregate excitons due to coupling with the localized plasmons of plasmonic nanoparticles has not been directly examined. The ultrafast properties of metallic nanoparticle-J-aggregate complexes have been reported for the case where the transition energy of the J-aggregate is strongly detuned relative to the resonant plasmon energy;32 however, this is a much weaker interaction regime which is substantially different from the stronger interactions in the doubly resonant regime addressed in the present study. In the weaker, nonresonant regime, the plasmonmolecule interaction typically leads to enhancement of molecular absorption and shifts in plasmon resonance frequencies. These characteristics are replaced by entirely new effects in the stronger interaction regime, where the coupled plasmons and excitons hybridize and form plexcitons, and where the behavior of this new highly coupled system is distinct from either of its component subsystems.23 As has been recently shown in a number of plasmonic systems, strong coupling between metallic nanoparticles can lead to Fano interference and resonances.3340 Such interference can also occur in metallic nanoparticle/semiconductor quantum dot hybrid systems where the coupling between excitons and plasmons can also result in an enhancement of the Fano effect.14 Further increased coupling
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may lead to a transition from asymmetric Fano behavior to that of a doubly peaked Fano resonance and finally to a bistable regime.5 In the present study, we examine these interaction regimes experimentally for a hybrid J-aggregateplasmonic nanostructure and construct a straightforward nonlinear dielectric model that describes the observed interaction between the two subsystems. Our coupled plasmonexciton nanocomplex consists of a nanoshell surrounded by an epilayer of J-aggregates (Figure 1a). The J-aggregates are formed on the surface of the nanoshells when a water/ethanol (50:50) solution of the dye molecules (2,20 -dimethyl-8-phenyl-5,6,50 ,60 -dibenzothiacarbocyanine chloride) is added to an aqueous solution of nanoshells. With the peak of the nanoshell spectrum tuned to the maximum absorption of the J-aggregrate (Figure 1b), the complexes exhibit coherent coupling between the localized plasmons of the nanoshell and excitons of the molecular J-aggregates: a well-defined Fano resonance is observed in the ensemble optical spectrum (Figure 1c).23 The Fano resonance created well-expressed dips in the absorption spectra (∼10%) and the corresponding spectral peak-to-peak distances were measured as ∼100 meV.23 The Fano dip structure (antiresonance) can be understood as a coherent destructive coupling between two oscillations, an exciton and a plasmon. This coupling can be adequately described in terms of a classical dielectric model.23 The ultrafast studies were performed using two-color, timeresolved pumpprobe spectroscopy. A mode locked Ti:sapphire laser system consisting of an oscillator (Coherent MIRA 900), a regenerative amplifier, and an OPA (Coherent) was used as the excitation and probe source. Pulses from the OPA were approximately 150 fs. A motorized stage (Newport) was used to delay the probe beam with respect to the pump beam. The sample consisted of an aqueous solution of J-aggregateAu nanoshell complexes in a 1 mm path length cuvette. Signals were collected in a transmission geometry. In Figure 2, the time-resolved transmission obtained from J-aggregateAu nanoshell complexes (Figure 2ad) is compared with that of J-aggregates in the absence of Au nanoshells (Figure 2eh). The pump and probe fluences are 60 and 0.8 μJ/cm2, respectively. This pump fluence is low enough such that no timeresolved signals were obtainable from nanoshells without J-aggregates under identical pump and probe conditions; in other words, only the transient dynamics of the J-aggregate and the J-aggregate complex are being probed. The fwhm of the laser spectrum was limited to 10 nm using a 10 nm band-pass filter. The time-resolved relaxation dynamics shown in Figure 2 were obtained with pump and probe pulses at the same wavelength, for several discrete wavelengths within the spectral region of interaction, i.e., the Fano dip. These wavelengths (680, 690, 700, and 710 nm) are indicated in the nanoshellJ-aggregate extinction spectrum shown in Figure 1c. For the J-aggregateAu nanoshell complex, a clear transition from transient bleaching (pump and probe at 690 nm) to transient absorption (pump and probe at 710 nm) is observed, while no such transition can be observed for the pristine J-aggregate (Figure 2fh). The signal at 700 nm shows a complex, mixed transient bleaching, and absorption behavior with the bleaching occurring at earlier times. At 680 nm, while the J-aggregate displays transient absorption (Figure 2e),41 the J-aggregateAu nanoshell complex shows transient bleaching (Figure 2a). The stark difference between the behavior of the plexcitonic complex and the pristine excitonic medium indicates a strong modification of the excitonic states of the J-aggregate due to coupling with the localized dipolar plasmons of the Au nanoshells.23 1557
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Figure 2. (ad) Time-resolved differential transmission of J-aggregateAu nanoshell complexes (corresponding extinction spectrum shown in Figure 1c). (eh) Time-resolved differential transmission of pristine J-aggregates (corresponding extinction spectrum shown in Figure 1b): (a) and (e), pump and probe at 680 nm; (b) and (f), pump and probe at 690 nm; (c) and (g), pump and probe at 700 nm; (d) and (h), pump and probe at 710 nm. Pulse width of both pump and probe is 150 fs and pump fluence is approximately 60 μJ/cm2. (i) Amplitude plots for J aggregates (red triangles) and J-aggregatenanoshell complexes (black squares).
We can explain the qualitative behavior of the transient signal in the vicinity of the Fano dip by considering both the nonlinear photobleaching of the J-aggregate and the Fano effect. When the plexcitonic system is excited by the pump pulse, the optical transition becomes partially saturated. From this we can infer that the Fano dip itself should become transiently modulated (a transient increase in the absorption intensity within the minimum of the Fano line shape). This is due to the decreased excitonplasmon interaction directly following excitation by the pump pulse. This transient increase within the Fano dip means an increase in transient absorption (and a corresponding decrease in transmission). This behavior should be limited to excitations within the Fano resonance since strong excitonplasmon coupling is not essential for the nonresonant optical response of the J-aggregates. Outside the Fano resonance one would expect the usual transient bleaching of the excitonic system to be observed. The above effects (a negative ΔT at the Fano dip and a positive ΔT on the left-hand side of the Fano minimum) should create a shift of the Fano dip in the nonequilibrium absorption spectrum in the presence of optical pumping. Indeed, this transient shift can clearly be seen by comparing the experimental change in transmission ΔT/T as a function of wavelength for the plexcitonic complex and the J-aggregate shown in Figure 2i. This figure also shows a decrease in magnitude of ΔT/T for the hybrid relative to the pristine J-aggregate (Figure 2i). A plasmon resonance can, in principle, enhance the optical nonlinear response of a directly adjacent nonlinear material (in this case the J-aggregates) due to its highintensity local electromagnetic field. However, in this case the presence of the metallic component simultaneously leads to a faster relaxation of the nonequilibrium exciton population, which is the
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Figure 3. (a) Energy structure and transitions in a theoretical model describing the nonlinear Fano effect in our system. Vertical arrows (black) show optical transitions and the horizontal arrows (blue) depict excitonplasmon coupling. (b) Calculated absorption of the J-aggregateAu nanoshell complex at two times: t = 0 (the instant just after the pump pulse) and t = ¥ (long-term equilibrium response). The nanoshell dimensions are [r1, r2] = [48, 58] nm with a 2 nm thick J-aggregate shell layer. Parameters for the dielectric constants of the materials are taken from ref 28. (c) Normalized transient transmission at t = 0 for the hybrid complex (black) and for the pristine J-aggregates (red). (d) Experimental pump-continuum probe absorbance spectra of the J-aggregateAu nanoshell complex pumped at 690 nm at various delay times: t = 0 overlap between pump pulse and continuum probe beam [black]; 80 ps delay [blue]. Continuum spectrum with no pump pulse present (beam blocked) [red]. Spectra are displaced vertically for clarity. (e) Transient transmission obtained from excited state absorbance spectra at t = 0 shown in (d) by continuum background subtraction.
likely origin of the decrease in amplitude of ΔT/T observed (Figure 2i). A straightforward theoretical model can be introduced that describes the observed plasmonexciton coupling in the J-aggregateAu nanoshell complex. Figure 3a shows the energy structure and transitions in our model. The Fano interference appears in this model from the possibility for the photons to create a plasmon via two paths. In the resonance with the one exciton, the paths are: |0æ f |LSPæ and |0æ f |1æ f |LSPæ. In the resonance with the two-exciton: |1æ f |LSPæ and |1æ f |2æ f |LSPæ. Photoinduced transmission in molecules alone is the universal behavior originating from saturation of a two-level (excitonic) system, consistent with the behavior of the pristine J-aggregate. Introducing the plasmonexciton interaction requires that we extend this initial theoretical picture of the J-aggregate beyond a simple two-level system. We begin with a description of the exciton dynamics using rate equations that describe the populations of ground, one-exciton, and twoexciton states, n0(t), nS1(t), and nS2(t), that obey the condition: 1558
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n0(t) þ nS1(t) þ nS2(t) = 1. At δtpulse < t < 0, a short pump pulse of duration δtpulse excites both one-exciton and twoexciton states which rapidly relax. In our J-aggregates, these one-exciton and two-exciton states correspond to the states observed at the energies corresponding to ∼700 and 680 nm, respectively. The two-exciton state can only be excited via the one-exciton state: for the pump intensities considered, nS2(t) < nS1(t). To calculate the temporal evolution of the populations, we use a system of coupled rate equations and assume that the frequency of the pump beam is close to the Fano-dip spectral position (λ = 700 nm). We then calculate the transient absorption of this system as a function of probe delay Z 1 Qprobe ðtÞ ¼ Re Bj ð B r Þ dV ð1Þ r , tÞ 3 E Bprobe ð B 2 V where Bj is the current inside the J-aggregateAu nanoshell complex induced by a short, low-power probe pulse and E Bprobe is the nonuniform electric field inside the nanocomplex. The current is calculated as Bj (r B,t) 1)/4π 3 B,t) = iωprobe(ε(r ,t) is the effective dielectric constant of the E Bprobe, where ε(r B system.23 Inside the Au nanosphere, ε(r B,t) = εAu, and inside the J-aggregate shell layer of the complex, ε(r B,t) = εJ(t). The Au dielectric constant (εAu) remains a constant in this picture since, in this experimental regime, the nonlinearity of the Au subsystem is negligible (not observable at the fluences used). The nonlinear response enters this simple model via the timedependent dielectric constant of the J-aggregate εJ(t) which can be described as εJ ðω, tÞ ¼ 1 fS1 fS2
ωS2
2
ωS1
2
ωS1 2 ðnS0 ðtÞ nS1 ðtÞÞ ω2 þ iγω
ωS2 2 ðnS1 ðtÞ nS2 ðtÞÞ ω2 þ iγω
ð2Þ
where fS1 = 0.03 and fS2 = 0.05 are the reduced oscillator strengths, and γ = 0.052 eV. These material parameters were obtained from the available literature on the J-aggregate system.32,41 The coefficients involving the nonequilibrium state populations in eq 2 describe the usual effect of bleaching of a molecular system. Calculation of the electric current and the actual electric field inside the complex is relatively simple for a small nanocomplex in the quasi-static regime that applies here, i.e., when c , λ, where c and λ are the radius of the complex (c = 60 nm) and the wavelength of probe light (λ = 680710 nm), respectively. In this case, one can use the Poisson equation and obtain a result analytically.32 Figure 3 shows the calculated absorption Qprobe just after the probe pulse (t = 0) and at long delay times (t = ¥) when the system comes to equilibrium. The transient transmission signal at t = 0 is ΔTmax Qprobe(t=¥) Qprobe(t=0) and is shown in Figure 3c. In Figure 3c, we see the formation of transient absorption (negative ΔTmax) in the spectral region of the Fano dip, as already discussed. Another predicted result from this model is the creation of a second, transient Fano dip in the vicinity of the linear Fano dip (Figure 3b) due to involvement of the two-exciton states (the transition |1æ f |2æ in the energy diagram of Figure 3a). Compared to the nonlinear Fano model used in previous publications,15,42 a novel feature of the present model is the inclusion of the two-exciton states that adds a second nonlinear Fano minimum. This theoretical model is meant to provide a qualitative picture of the coupled system:
detailed properties of the excitonic states, their various relaxation channels, and other effects such as spectral hole burning are not described quantitatively by this simple picture. Probing this coupled system experimentally with a short pump pulse and a continuum probe pulse whose spectral content spans the plexciton Fano resonance allows us to experimentally verify the theoretical predictions of our simple model (Figure 3d). For the case of overlapping pump and probe pulses (t = 0), we are able to observe a significant modulation within the Fano dip. This additional spectral feature follows the modulation predicted from our theoretical model remarkably well (Figure 3c). Here the pump pulse partially saturates the one-exciton transition in the complex. When this occurs we would anticipate that the depth of the Fano minimum would decrease with a transient absorption appearing within the spectral region of the Fano dip. Simultaneously, we would anticipate transient bleaching signals on the sides of this transient absorption region. These modulations are clearly observed (Figure 3e). Interestingly, the above results mean that, with the pump pulse, we are able to selectively modulate the strength of excitonplasmon coupling in the system. The pump pulse diminishes the one-exciton-plasmon coupling (negative ΔT for the dip region in Figures 2i and 3e) and, simultaneously, activates the two-exciton-plasmon interaction (positive ΔT to the blue from the Fano dip in Figure 3e). In conclusion, we have studied the ultrafast optical dynamics of excitons in nanoshellJ-aggregate complexes where localized plasmons and excitons are interacting strongly. The ultrafast characteristics indicate that these hybrid nanostructures might possess enhanced, tunable, on- and off-resonance nonlinear optical properties. We believe that these results may stimulate further interest in the properties of nanoengineered plasmon exciton systems that may ultimately serve as active media in new classes of nonlinear optical devices or in active, functional metamaterials.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge the Department of Defense National Security Science and Engineering Faculty Fellowship (N.J.H.) via Grant N00244-09-0067, the Air Force Office of Scientific Research (F49620-03-C-0068), Robert A. Welch Foundation (C1220), and the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, for financial support. A. O.G. and Z.F. thank the NSF for support (project number CBET0933415). ’ REFERENCES (1) Haraguchi, M.; Fukui, M.; Tamaki, Y.; Okamoto, T. J. Microsc. 2003, 210, 229–233. (2) Min, C.; Wang, P.; Chen, C.; Deng, Y.; Lu, Y.; Ming, H.; Ning, T.; Zhou, Y.; Yang, G. Opt. Lett. 2008, 33 (8), 869–871. (3) Sato, Y.; Furuki, M.; Minquan, T.; Iwasa, I.; Pu, L. S.; Tatsuura, S. Appl. Phys. Lett. 2002, 80 (13), 2254–2256. (4) Dintinger, J.; Klein, S.; Ebbesen, T. W. Adv. Mater. 2006, 18 1267–1270. (5) Artuso, R. D.; Bryant, G. W. Nano Lett. 2008, 8 (7), 2106–2111. 1559
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