Abstract
We study theoretically four-wave parametric amplification arising from the nonlinear optical response of hybrid molecules composed of semiconductor quantum dots and metallic nanoparticles. It is shown that highly efficient four-wave parametric amplification can be achieved by adjusting the frequency and intensity of the pump field and the distance between the quantum dot and the metallic nanoparticle. Specifically, the induced probe-wave gain is tunable in a large range from 1 to 1.43 × 105. This gain reaches its maximum at the position of three-photon resonance. Our findings hold great promise for developing four-wave parametric oscillators.
© 2014 Optical Society of America
1. Introduction
The optical properties of hybrid molecules composed of metallic nanostructures and semiconductor quantum dots (SQDs) have attracted much interest in recent years [1–8]. Based on the exciton-plasmon interaction, such hybrid molecules exhibit many interesting optical phenomena, such as nonlinear Fano effect [9], exciton-plasmon-photon conversion [10], superradiance [11], gain without inversion [12], intensity-dependent enhancement of saturable absorption [13], slow light effect [14,15], plasmon-enhanced terahertz emission and ac-Stark shifts [16,17], and photoinduced diffraction grating [18]. Taking SQD-metallic nanoparticle (MNP) hybrid nanosystem for example, Artuso et al. found the double peaked Fano structure and bistability when the SQD couples strongly to the MNP [19]. Malyshev et al. showed that optical bistability and hysteresis in a hybrid SQD-MNP nanodimer can be revealed by measuring the optical hysteresis of Rayleigh scattering [20]. Then Malyshev derived the analytical bistability condition in such a system [21]. Singh predicted that the second-harmonic generation (SHG) signals from the QD and MNP in the SQD-MNP hybrid system can be switched on and off by applying an external control field [22]. Based on these optical properties, the SQD-MNP hybrid molecules can be used to develop single-photon transistor [23], nanosensor [24], optical modulator [25], and photodetector [26].
As an attractive nonlinear optical phenomenon, four-wave mixing effect is appealing hosts for the investigation of fundamental physics and novel applications. Resulting from four-wave mixing effect, Kerr nonlinearity of the SQD-MNP hybrid molecule can be enhanced via exciton-plasmon interaction [27]. In our previous work, we revealed that the bistable properties of nonlinear absorption and refraction response of four-wave mixing effect in the same system [28]. Recently, Paspalakis et al. studied the four-wave mixing effect in an SQD-MNP system and found that the four-wave mixing spectrum can display a three- or one-peaked structure by adjusting the distance between the SQD and the MNP [29]. However, the parametric amplification by four-wave mixing in these hybrid systems has not been reported yet.
In this paper, we study four-wave parametric amplification that arises in SQD-MNP hybrid molecules. We find that the efficiency of four-wave parametric amplification is strongly modified by the frequency and intensity of the pump field and the interparticle distance. Additionally, the probe-wave gain induced by four-wave parametric amplification is tunable in a large range. It is possible to utilize such high gain to construct a tunable four-wave parametric oscillator.
2. Model and formalism
We consider a hybrid molecule composed of a SQD of radius r and a spherical MNP of radius R in an environment with dielectric constant ε0 as shown in Figs. 1(a) and 1(b). The surface-to-surface distance between the two nanoparticles is denoted as d. The hybrid molecule is subjected to two applied fields: a strong pump field with amplitude E0 and frequency ω0; a weak probe field with amplitude E1 and frequency ω1 (E0 >> E1). The SQD is regarded as a two-level system which consists of the ground state |0> and the first excited state |1>.
The Hamiltonian of the SQD in a rotating frame reads as follows [28]
where Δ = ω10 - ω0 is the frequency difference between the exciton and the pump field, ω10 is the frequency of exciton. σij = |i><j| (i, j = 0, 1) is the dipole transition operator between |i> and |j> of the SQD, σz = (σ11-σ00)/2. µ10 is the interband dipole moment element. ẼSQD is the total optical field felt by the SQD, ẼSQD = E/εeff + SαPMNP /[ε0εeff (d + r + R)3] with εeff = (2ε0 + εs)/(3ε0) [30,31], E = E0 + E1e-iδt is the total applied field, where δ = ω1 – ω0 is the probe-pump detuning [32,33]. εs is the dielectric constant of the SQD. The polarization parameter Sα = 2 (−1) when the applied field is parallel (perpendicular) to the major axis of the hybrid molecule. The dipole of the MNP is PMNP = αMNPẼMNP where αMNP = ε0R3(εm-ε0)/(2ε0 + εm) and ẼMNP = E + SαPSQD /[ε0(d + r + R)3] [30,31,34], and εm is the dielectric constant of the MNP [35], PSQD = µ10σ10 is the dipole moment of the SQD [32]. For simplicity we define p = µσ10 and w = 2σz. In terms of the Heisenberg equation of motion dO/dt = -i[O, H]/ћ, we obtain the temporal evolutions of the exciton in the SQD as follows where T1 is the exciton lifetime and T2 is the exciton dephasing time. ẼSQD = A(E0 + E1e-iδt) + Bp with A = 1/εeff + SααMNP/[ε0εeff(d + r + R)3] and B = Sα2αMNP/[ε02εeff(d + r + R)6]. To obtain a steady-state solution of Eqs. (2) and (3), we make the ansatz p(t) = p0(ω0) + p1(ω1)e-iδt + p-1 (2ω0 - ω1)eiδt, w(t) = w0(ω0) + w1(ω1)e-iδt + w-1(2ω0- ω1)eiδt. p-1 represents a wave-mixing response, which gives rise to generation of a new optical wave with frequency 2ω0 - ω1. Upon working to the lowest order in E1 but to all orders in E0, one can obtain whereand δpr = δT2, Δpu = ΔT2, Bpu = µ102BT2/ћ, Ωpu = µ10E0T2/ћ is the generalized Rabi frequency of the pump field, N is the number density of SQD-MNP hybrid molecules.Next we use the nonlinear polarization to study the spatial propagation effects in the hybrid system. The polarization amplitudes at frequencies ω0 ± δ can be written as [32,33]
Here χeff(1)(ω0 - δ) and χeff(3)(ω0 + δ) can be obtained by formally replacing δ by -δ in Eqs. (4) and (5). E-1 represents the complex amplitude of the new wave with a frequency ω0 - δ.The wave equations are given by
Now it is assumed that the amplitude of the pump wave A0 is constant, and the probe-field amplitudes A ± 1 are small variables, the wave equations become
Here k ± 1 = n ± 1(ω0 ± δ)/c denote the propagation constants, where n ± 1 = (1 + Reχeff(1)(ω0 ± δ))1/2 are the corresponding refraction indexes, c is the speed of light, k0 is the propagation constant of the pump wave at frequency ω0. According to four-wave mixing effect, the wavevector mismatch along the z direction can be expressed as , as shown in Fig. 1(c). The nonlinear absorption and coupling coefficients are given by [32] From Eqs. (13)–(16), we obtain the expressions of E1 and E-1 where A1(0) and A-1(0) denote the field amplitudes at the boundary z = 0 of nonlinear molecular material. η ± are given byTo clearly reflect the parametric amplification process of four-wave mixing, we define a gain factor for the probe-wave G = |A1(L)/A1(0)| in term of Eq. (17), where L denotes the length of nonlinear molecular material.3. Results and discussion
For numerical calculations, we consider a hybrid system composed of CdSe QD-Au NP molecules. For CdSe QD, we take εs = 6ε0, T1 = 0.8 ns, T2 = 0.3 ns and µ10 = 10−28 C·m, hω10 = 2.5 eV. For Au NP, the broad plasmon frequency is near the exciton resonant frequency and the radius R is taken as 7.5 nm [16]. Other parameters are N = 1020/m3, L = 10μm, Sα = −1. In the following, we assume A1(0) = A-1(0) and consider the case of perfect phase matching (Δk = 0).
We are mainly interested in the probe-wave gain achieved by four-wave parametric amplification in the hybrid system. Before investigating the probe-wave gain, we first explore the four-wave mixing response. Figures 2(a) and 2(b) show the variation of four-wave mixing response |p-1(2ω0-ω1)| as a function of the probe-pump detuning δpr for d = 15 nm and Ωpu2 = 20. In Fig. 2(a), we divide the region of mixing response spectra into three parts corresponding to three-photon resonance (TP), Rayleigh resonance (RL) and ac-Stark resonance (AC) [28,33]. For Δpu ≤ 0, the magnitudes of the peaks attributed to three-photon resonance and ac-Stark resonance decrease as Δpu increases. In Fig. 2(b), for Δpu > 0, we obtain an opposite result with further increase of Δp. Additionally, the magnitude of the peak induced by Rayleigh resonance changes relatively little when Δpu ≤ 0 (Δpu > 0). These results suggest that the role of three resonance mechanisms will change as Δpu varies. Moreover, we find that these mixing response spectra are symmetrical about the axis δpr = 0. Such behavior is further revealed in Fig. 2(c). Similar curves have also been observed by Paspalakis et al. in a metal-semiconductor nanosystem [29].
To estimate the impact of the frequencies of the pump and probe fields on four-wave parametric amplification, Fig. 3 shows how the probe-wave gain G changes with δpr at perfect phase matching Δk = 0. When the system is excited with a constant pumping (Ωpu2 = 20), the peak of G, depicted in Figs. 3(a) and 3(b), reaches its largest value at the position of three-photon resonance, so this gain can be assigned to the three-photon gain. The gain peak moves to higher δpr with the increasing of Δpu. For Δpu ≤ 0, the maximal gain Gmax is 137121, 110012, 77750.9, 42667.9 when Δpu = −6, 4, 2, 0, respectively. While for Δpu > 0, Gmax becomes relatively small, and Gmax is equal to 19.2, 67.6, 425.1 when Δpu = 2.8, 4, 6, respectively. It is obvious that the parametric amplification process becomes rather significant when Δpu = −6, and the corresponding Gmax is 322 times larger than that in the case of Δpu = 6.
In the following we mainly consider the case of Δpu = −6. To better visualize the dependence of mixing response |p-1(2ω0-ω1)| on the interparticle distance d, we plot |p-1(2ω0-ω1)| as a function of d for different probe-pump detuning δpr in Fig. 4(a). In our calculations we find that the probe-wave gain G becomes infinite when δpr = 0 and Δk = 0. As δpr = −18.75, 17.5, |p-1(2ω0-ω1)| reaches a maximum at d = 15.1, 15.4 nm, respectively. When δpr = −5, 1 and δpr ≈0, |p-1(2ω0-ω1)| increases with the increase of d. The results imply that the hybrid system may undergo an amplification process by four-wave mixing for some specific values of d. In order to clarify it, in Fig. 4(b) we show how the probe-wave gain G changes with d under the same conditions as those in Fig. 4(a). Note that in Fig. 4(b), for δpr = −18.75, G almost keeps invariant until d approaches a certain amount, then attains its maximum at d = 15.1 nm (Gmax = 137125). As we expected, |p-1(2ω0-ω1)| and G both reach their maximums at the same positions. For δpr = −17.5, the gain curve exhibits a sharp peak similar to that in the case of δpr = −18.75 and the peak position has a visible shift. While for other four cases, d almost has no effect on the nature of the gain curves which suggests that the parametric amplification process of four-wave mixing is significantly suppressed despite d changes.
Figure 5(a) presents the impact of the pump-field intensity Ωpu2 on the mixing response |p-1(2ω0-ω1)|. For δpr = −18.75 and −17.5, |p-1(2ω0-ω1)| displays a rather wide peak; when δpr = −5, 1 and δpr ≈0, |p-1(2ω0-ω1)| slightly increases monotonically as Ωpu2 increases. These results suggest that for the case of G versus Ωpu2, we can expect to obtain similar gain curves shown in Fig. 4(b). In Fig. 5(b), for δpr = −18.75, Gmax is equal to 142585 when Ωpu2 = 18.3; for δpr = −17.5, Gmax reduces to 25% that for δpr = −18.75 and the gain peak position moves to larger pump-field intensity. While for δpr = −5, 1, 18.75 and δpr ≈0, these features of the gain peaks are all gone and replaced with four smooth lines (G ≈1). Therefore, we can obtain high efficient four-wave parametric amplification in the hybrid system by purposefully selecting the values of Ωpu2 and δpr.
The results presented in this paper are based on SQD-MNP hybrid molecules. Such system exhibits a large probe-wave gain by four-wave parametric amplification, which can be used to develop four-wave parametric oscillators. We have also discussed the dependence of the mixing response spectrum on the interparticle distance and obtained some results similar with that in [29]. These calculated results could be explained by the molecular-like resonances of Sadeghi [12,16]. As far as we know, in a two-level atomic system, the probe-wave gain can reach a maximum 104 [33]. However, our results show that the maximal probe-wave gain in the SQD-MNP hybrid system is approximately 14 times larger than that in the atomic system. As we expected, it is easily to achieve 1.43 × 105 gain under certain experimental conditions in the realistic CdSe QD-Au NP hybrid molecules. Here Δ = −20 ns−1, δ = −62.5 ns−1, and the pump-field intensity I0 is equal to 2.06 × 102 W/cm2.
4. Conclusion
In summary, four-wave parametric amplification in hybrid molecules composed of semiconductor quantum dots and metallic nanoparticles was theoretically studied. We found that the efficiency of four-wave parametric amplification depends strongly on the frequency, intensity of the pump field and the interparticle distance. In particular, the induced probe-wave gain attributed to three-photon effect is tunable in a large range from 1 to 1.43 × 105. Such high gain allows one to utilize the hybrid system to construct four-wave parametric oscillators. We hope that our calculated results in this work can be experimentally demonstrated in the near future.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 11404410 and 11174372), the Hunan Provincial Natural Science Foundation (Grant No. 14JJ3116) and the Foundation of Talent Introduction of Central South University of Forestry and Technology (Grant No. 104-0260).
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