Letter pubs.acs.org/NanoLett
Unusual and Tunable One-Photon Nonlinearity in Gold-Dye Plexcitonic Fano Systems Fan 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 of China § 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, Beijing 100083, 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 the plasmonic excitations of metallic nanostructures with the excitonic excitations of molecular species have revealed a rich variety of emergent phenomena known as plexcitonics. Here, we use a combined experimental and theoretical approach to demonstrate new and intriguing aspects in the ultrafast nonlinear responses of strongly coupled hybrid Fano systems consisting of gold nanorods decorated with near-infrared dye molecules. We show that the severely suppressed linear absorption around the Fano dip significantly enhances the unidirectional energy transfer from the plasmons to the excitons and further allows one-photon nonlinearity to be drastically and reversibly tuned. These striking observations are interpreted within a microscopic model stressing on two competing processes: saturated plasmonic absorption and weakened destructive Fano interference from the bleached excitonic absorption. The unusually strong one-photon nonlinearity revealed here provides a promising strategy in fabricating nanoplasmonic devices with both pronounced nonlinearities and good figures of merit. KEYWORDS: Gold nanorod, Fano interference, plexciton resonance, one-photon nonlinearity, ultrafast energy transfer
A
vivo molecular imaging.4,35 In addition, the inherent nonlinear nature of the Fano resonance has been theoretically proposed to provide a new strategy for tuning the photon statistics of quantum emitters,36,37 yet direct experimental studies of nonlinear responses of hybrid systems have been rare. Here, we use complementary experimental approaches to study both the linear and nonlinear optical responses of strongly coupled hybrid Fano systems consisting of Au nanorod (AuNR) cores decorated with near-infrared (NIR) dye molecular shells. We also develop a microscopic exciton model to describe the optical absorption resonance of the dye molecules, and the plasmonic-molecular resonance coupling is described accordingly within the plexciton picture. We show that in the weak external field regime the absorption
s collective excitations of the conduction electrons, the surface plasmons of metallic nanostructures can induce large local electromagnetic fields near the metal surfaces upon resonant excitation, which can be exploited to manage light at the nanometer scale.1−9 Such enhanced local fields can interact strongly with adjacent semiconductors or organic molecules.10−23 Especially, coupled plasmons and excitons form a new type of optical excitation termed as plexciton.24−27 The optical behaviors of such hybrid systems can be distinctly different from those of either of their constituents and are of substantial fundamental and practical importance in understanding light-matter interactions. The strong coupling of plasmonic and molecular resonance in hybrid systems is signified by its characteristic absorption spectra, typically exhibiting plexcitonic Fano resonance25−32 or Rabi splitting33,34 induced by the interference of the plasmonic and molecular resonances. Such hybrid systems have been shown to offer great potential in ultrasensitive sensing and in © XXXX American Chemical Society
Received: January 31, 2015 Revised: March 1, 2015
A
DOI: 10.1021/acs.nanolett.5b00413 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
In a weak external field, we have investigated the absorption of the hybrid systems with different dye concentrations, Cdye. As shown in Figure 1D, a prominent absorption dip is gradually established as Cdye increases from 0 to 1.6 μM. The dip depth becomes saturated when Cdye reaches the transition point (1.2 μM), where the amount of IR-806 adsorbed on the surface of AuNRs is probably one monolayer.12 The dipped spectra are similar to those of J-aggregates on Au nanoshells,24,25 and can be interpreted as the result of destructive Fano interference between the strongly coupled plasmonic and molecular resonances. Compared with previously reported results from AuNRs with HITC molecules,12 the Au@IR-806 hybrids studied here exhibit much more prominent dips due to stronger coupling of plasmonic and molecular resonance. The strength of plasmon-molecular resonance coupling is dependent on the distance between AuNRs and IR-806 molecules (see Supporting Information Figure S2).41 Wavelength-Dependent Nonlinear Transmittance of Au@IR-806 Hybrids around the Fano Dip. The nonlinear absorptions of the bare AuNRs and Au@IR-806 hybrids have been investigated via the open-aperture (OP) Z-scan technique using a wavelength-tunable and picosecond-pulsed Ti:sapphire laser.42 The power-dependent absorption coefficient can be written as α(I) = α0 + βeffI with the linear coefficient α0 and the nonlinear part absorbed in the function βeffI. The normalized OP 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 are the peak irradiance at the focus (z = 0) and effective Rayleigh length of the beam, respectively, and Leff is the effective thickness of the sample. When we consider only the second order nonlinear process with a constant βeff, the measured TOP can 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 concentration Cdye = 1.6 μM) at different incident laser wavelengths. At all the wavelengths, the bare AuNRs exhibit saturated absorption (SA). This is induced by the bleaching of the electronic ground state because the population is very efficiently pumped to the excited state via plasmonic excitation while the electron relaxation rate to the ground state is not fast enough.43 The intensity-dependent absorption of the saturated effect is expressed by αAuNR = α0/(1 + I/IS,p), where IS,p is the saturated intensity. The SA effect of the AuNRs is more prominent at the plasmon resonance wavelength owing to stronger local field enhancement. In contrast, the Au@IR-806 hybrids display three types of nonlinear behaviors: (i) the reversed saturated absorption (RSA) that exhibits a valley in TOP at the wavelength of the Fano dip; (ii) the SA at wavelengths far away from the Fano dip; and (iii) a mixture of at least two nonlinear processes exhibiting “W-shaped” transmittance near the wavelength of the Fano dip. In Figure 2C,D, where we use a constant βeff to fit the Z-scan results, a clear correspondence between the absorption peak (valley) and the negative (positive) βeff can be seen. However, the W-shaped transmittance implies the existence of more than one competing nonlinear process, which cannot be properly described by a single-signed βeff, requiring a further elucidation of the nonlinear behaviors in such strongly coupled systems. Dye-Concentration and Laser-Power Dependences of Nonlinear Transmittance at the Fano Dip. To help elucidate the underlying mechanism of the peculiar nonlinear behaviors, we have carried out Z-scan measurements at
of the hybrids at the Fano dip decreases as the concentration of the dye molecules increases, establishing effective suppression of the linear absorption around the dip and significant enhancement of the unidirectional energy transfer from the plasmons to the excitons. Furthermore, in the strong field regime the severely suppressed linear absorption around the Fano dip allows one-photon nonlinear responses to be readily revealed and reversibly tuned by the photon energy, laser power, and dye concentration. The one-photon nonlinearity involving no multiphoton processes is distinctly different from ordinary nonlinear (or multiphoton) phenomena. The counterintuitive nature of the unusually strong one-photon nonlinearity interpreted within a microscopic phenomenological model is attributed to the saturated-absorption-induced decoherence of the initially coherently coupled Fano system and the competition between the two nonlinear (or saturation) processes in the gold nanorods and dye molecules. The suppressed linear and enhanced nonlinear absorptions around the Fano dip further provide a promising strategy in fabricating nanoplasmonic devices with both pronounced nonlinearities and good figures of merit.38,39 Hybrid Nanostructures and Fano Resonance of AuNRs Decorated with IR-806 Molecules. Figure 1A
Figure 1. Structures and absorption spectra of the Au@IR-806 hybrids. (A) Illustration of the Au@IR-806 hybrid. (B) TEM images of the Au@IR-806 hybrids. (C) Absorption spectra of the bare AuNRs, dye IR-806 molecules, and Au@IR-806 hybrids. (D) Measured absorption spectra of the hybrids with different dye concentrations Cdye.
displays a schematic diagram of the metal−molecule hybrid system (denoted by Au@IR-806) consisting of an assembly of AuNRs each decorated with dye (IR-806) molecules. The polyelectrolyte of poly(allylamine hydrochloride) (PAH) was used to assemble the dye molecules onto AuNRs.40 The actual systems necessarily contain disordered molecular distributions (including their density and orientations). Figure 1B shows a TEM image of the Au@IR-806 hybrids, whose density is fixed throughout the experiment. The AuNRs have an average diameter of ∼15 nm and length of ∼45 nm, and the dye layer thickness is ∼3 nm. The main molecular absorption peak of the dye molecules is at 805 nm with a narrow width of 48 nm (Figure 1C). The plasmon resonance of the AuNRs is at 788 nm with a width of ∼100 nm. B
DOI: 10.1021/acs.nanolett.5b00413 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 2. Nonlinear transmittance of the AuNRs and Au@IR-806 hybrids at different incident wavelengths. Comparison of the OP Zscan nonlinear transmittance of (A) the bare AuNRs, and (B) Au@IR806 hybrids with Cdye = 1.6 μM, both at the laser power of I0 = 50 mW. The corresponding effective nonlinear absorption coefficients βeff of the samples are shown in (C,D).
Figure 3. Nonlinear transmittance of the AuNRs and Au@IR-806 hybrids at different dye concentrations and laser intensities. (A) Measured and (B) calculated TOP of the Au@IR-806 hybrids at different Cdye and fixed I0 = 50 mW. (C) Measured and (D) calculated TOP of the hybrids with different laser powers and fixed Cdye = 1.2 μM.
different dye concentrations and laser intensities. Figure 3A shows TOP of the Au@IR-806 hybrids with different dye concentrations at the fixed laser power of I0 = 50 mW and wavelength of 810 nm. As Cdye increases, the trace of the Z-scan nonlinear transmittance varies from a typical SA to a RSA trend. These transitions suggest a close correlation between the RSA and the strength of the Fano interference, which can be effectively controlled by Cdye (see Figure 1D) as well as the AuNR-dye distance (see Supporting Information Figure S2). On the other hand, Figure 3C displays the nonlinear response of the hybrids by varying the laser power at the fixed dye concentration of Cdye = 1.2 μM and wavelength of the Fano dip. If the nonlinear process involves an additional absorption channel, we would expect the nonlinear effect to become more pronounced as the laser power increases. However, as shown in Figure 3C, the depth of the valley is not enlarged at higher powers. On the contrary, the TOP traces vary from a valley to W-shaped as the laser power increases from 30 to 100 mW. This abnormal nonlinear behavior indicates that the dominant physical mechanisms at low and high laser powers are very different, and the RSA of the plexcitonic hybrids is not induced by the opening of a new absorption channel. Microscopic Phenomenological Model. The absorption spectra of the plexcitonic hybrid Fano systems can be explained using a microscopic phenomenological model, which is based on the method introduced by Manjavacas et al.26 The Hamiltonian 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 plasmon mode p and two molecular excitation ei. The plasmonicmolecular 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 following the Fermi Golden rule. Because of the much smaller excitonic absorption of the molecules, the total absorption of the hybrid is approximated by the plasmon absorption as ⎧ Γp ⎞ ⎪⎛ σp ∝ Δ2p Im⎨⎜ℏω − εp + δωp + i ⎟ 2⎠ ⎝ ⎪ ⎩ −
CdyeΔi2(1 − 2ni)
∑ i = 1,2
ℏω − εi +
ΓS, i 2
(
Γ
+ (1 − 2ni) δωi + i 2i
)
⎫−1 ⎪ ⎬ ⎪ ⎭
(1)
where Δp describes the coupling between the plasmon and the external field, ni represents the population of the excited states of the dye molecules, δωp + iΓp/2 and δωi + iΓi/2 represent the energy level shifts of the AuNR plasmon and molecular resonance caused by the environment, respectively, and ΓS,i represents the spontaneous decay, which is important as we consider the occupations of the excited states in the molecules. The first term in eq 1 describes the AuNR plasmon resonance at εp. The second term is responsible for the Fano dip. Clearly, we could expect a more prominent Fano dip at a higher dye concentration because increasing Cdye effectively enhances the plasmon and molecular resonance coupling. We now present a theoretical interpretation of the nonlinear effects based on eq 1. For simplicity, we have ignored the contributions from the resonance around 750 nm, because this energy is far away from the main Fano dip (∼800 nm). The SA of the dye molecules is reflected by (1 − 2ne) → 0 in eq 1. To theoretically implement this saturation effect, we assume (1 − 2ne) = (1 + I/IS,e)−1, where IS,e is the saturation intensity for the 805 nm resonance. We have assumed that IS,p = 10 I0 and IS,e = C
DOI: 10.1021/acs.nanolett.5b00413 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters 0.8 I0 in fitting the Z-scan results. Considering that the irradiance at the position z = 0 is around 0.1 GW/cm2 in our Zscan measurements, this assumption is consistent with the saturated intensity measured in similar Au nanostructures44 and dye molecular systems.45 The parameters for the energy and line width of the plasmon and molecular resonance are obtained by fitting the absorption spectra of the bared AuNRs and dye molecules as follows: εp = 1.57 eV, ε1 = 1.55 eV; Γp = 0.2 eV, Γs,1 = 0.12 eV. The other parameters related to the coupling between the AuNRs and dye molecules are at the same order of magnitude as in ref 26. By considering the saturated absorption of the molecules, we see that at the high laser intensity, the effective plasmonic-molecular coupling CdyeΔp(1 − 2ni) will be strongly suppressed. Around the Fano dip, this suppression weakens the destructive Fano interference and leads to the absorption efficiency α(I) increasing with the excitation intensity, providing a clear onephoton nonlinear mechanism for the RSA in the hybrid systems. By considering the two competing processes, namely, the SA of the plasmon of AuNRs and weakened destructive Fano interference from the bleached resonance absorption of the molecules, the exotic nonlinearity of the hybrids can be well reproduced by our theoretical model as shown in Figure 3. Ultrafast Plasmon Energy Transfer and Fano Resonance in the Au@IR-806 Hybids. Next, we investigate the ultrafast dynamical processes of both the bare AuNRs and Au@ IR-806 hybrids by using the time-resolved optical differential transmission (DT) method. Because the width of the laser pulse used to excite the systems is ∼150 fs, we are unable to directly resolve the transient SA of the plasmons with shorter lifetimes. Instead, we actually monitor the decay of the hot electrons converted from the plasmons within tens of fs, as also inferred recently in similar systems.33 We observe a fast decay process of ∼2 ps and a slow decay process of ∼200 ps (inset of Figure 4A), attributed to the hot electron−phonon coupling within the AuNRs and phonon−phonon scattering between the AuNRs and their surrounding liquid matrix, respectively.46,47
We note that even though we could not directly observe the decay of the plasmons within the AuNRs, their traces are well manifested by the decay behaviors of the subsequent hot electrons. For the bare AuNRs, the transient DT ΔIprobe of the probe beam increases with the optical pumping pulse (indicating SA) at all the excitation wavelengths, and the overall magnitude of the SA signal is the highest around the plasmon 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 the Fano dip (Figure 4C) consistent with our Z-scan observations. Intriguingly, the reversal from the RSA to SA in the Au@IR806 hybrid systems is also clearly demonstrated in the timeresolved DT measurements (see Figure 4D and Supporting Information Figure S5). The ΔIprobe signal of the RSA reaches a maximum within hundreds of fs for the hybrid system with Cdye = 1.0 μM (inset of Figure 4D). If the pump intensity is extremely low (Ppump < 0.64 μJ/cm2), only the RSA can be observed. The reversal time decreases as the pump intensity increases (see Supporting Information Figure S5), and can complete within several picoseconds consistent with the Z-scan measurements. In addition, we observe that ΔI returns to zero after relatively long decay (∼ ns) implying a negligible photobleaching effect in our measurements. Discussion. The nonlinear transmittance of the RSA in the present hybrid Fano system is a one-photon process, which is distinctly different from the ordinary nonlinearity induced by multiphoton processes. Both the transient DT and Z-scan nonlinear transmittance reveal that the Au@IR-806 hybrids have a large and tunable one-photon nonlinearity at the wavelength of the Fano dip, providing its prominent advantages toward a low-power (
