Electromechanically reconfigurable CdS nanoplate based nonlinear optical device

Here, we report experimental demonstration of dynamic control and enhancement of second harmonic generation and two photon excited photoluminescence in CdS nanoplates via an electromechanically reconfigurable Fabry-Perot (FP) microcavity. Microcavity coupled CdS nanoplates can be configured as a single or dual wavelength nonlinear light source by tuning the pump wavelength while the output intensities can be tuned by the on-chip control voltage. Our work realizes a reconfigurable device platform with insight toward advanced optical devices based on semiconductor nanoplates for next generation on-chip tunable light sources, sensors and optomechanical systems. ©2016 Optical Society of America OCIS codes: (350.4238) Nanophotonics and photonic crystals; (160.4236) Nanomaterials; (190.4360) Nonlinear optics, devices. References and links 1. X. Duan, Y. Huang, R. Agarwal, and C. M. Lieber, “Single-nanowire electrically driven lasers,” Nature 421(6920), 241–245 (2003). 2. R. F. Oulton, V. J. 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Introduction
Nanoscale light sources are the essential building blocks for a wealth of applications such as communication, sensing, imaging, microfabrication and data storage [1][2][3][4][5][6][7][8][9][10][11][12].Single crystalline CdS in its nanoscale form provides a superior material system for miniature light sources in the visible region due to its direct band gap of 2.42 eV at room temperature.One or two photon excited photoluminescence and lasing have been observed in CdS nanowires which are either directly used as a Fabry-Perot cavity or integrated into metal-semiconductor hybrid structures [1,[13][14][15][16].On the other hand, CdS nanowires also possess large second order nonlinear optical coefficient of 78 pm/V [17,18], enabling nonlinear light sources through second harmonic generation [1,[19][20][21].The coexistence of strong photoluminescence and second order nonlinearity make CdS nanowires excellent candidates for reconfigurable nonlinear light generation.However, integration of these nanoscale materials into a functional light source for future optoelectronics applications necessitates advanced device architectures to compensate for the poor light-matter interaction volume caused by the intrinsically small nanoscale dimensions [3,13,17,22].Towards this end, a diversity of optical resonator structures have been explored to enhance the light-matter interaction in nanowires for resonantly enhanced light generation and nonlinear frequency conversion.Meanwhile, efficient on-chip voltage reconfiguration of the light matter interaction in CdS-based nanophotonic components is also highly desirable for advanced optoelectronic devices and systems.Therefore we envision that an adaptive device platform enabling voltage reconfigurable cavity-enhanced light-matter interaction [23][24][25][26][27][28][29][30][31] with CdS nanoplates will be a valuable tool for their integration into future smart devices and systems.
In this paper, we take advantage of the strong second order nonlinearity and two-photon excited photoluminescence in CdS nanoplates to demonstrate an on-chip voltage reconfigurable single/dual wavelength nonlinear optical device via resonantly enhanced second harmonic generation and two-photon pumped narrowband light emission through cavity filtering.This is achieved by embedding the nanoplates in a voltage controlled widely tunable micro-Fabry-Perot (micro-FP) cavity formed by a spectrally selective dielectric distributed Bragg reflector (DBR) mirror and a silver mirror on top of a silicon nitride membrane.A 40 nm transparent electrode made of indium tin oxide (ITO) on top of the DBR mirror forms a capacitor structure together with the silver layer, as shown in Fig. 1(a).With applied voltage, the suspended nitride membrane can be mechanically deflected by electrostatic force across the capacitor, enabling the on-chip reconfiguration of the optical cavity length.For visual identification and experimental characterization of the nanoplates inside the cavity, the DBR fabricated on glass substrate is designed to be highly reflective in the pump wave region (800 nm -900 nm) while transparent in the visible region, as shown by the blue dotted line in Fig. 1(b).The optical resonances of the micro-FP cavity with 0 V of control voltage is also shown in Fig. 1(b) (red solid line).When the pump wavelength is tuned to one of the optical resonances in the micro-FP cavity, the excitation field can be resonantly enhanced, significantly improving the light generation efficiency.This is the underlying mechanism for voltage tunable enhanced nonlinear response from our device) [22].The initial cavity length of the micro-FP is determined by the thickness (Z 0 ) of the SU-8 spacer.However, we can tune the cavity length by the voltage induced mechanical deflection (ΔZ) of the silver mirror, thereby allowing efficient on-chip control of the light generation in CdS nanoplates.

Results and discussion
To investigate the light emission properties of the microcavity controlled CdS nanoplates as a function of the pump wavelength, we used transform limited optical pulses with a duration of 140 fs as the pump wave and scanned the pulse center wavelength from 850 nm -1000 nm, corresponding to the region where the micro FP cavity supports fundamental wave resonances, and monitor the evolution of the spectra of emitted light in the 460 nm to 540 nm region.We observed both second harmonic generation (SHG) and two photon excited luminescence (TPL) during the scan and investigated the relative strengths of the SHG and TPL as a function of the pump wavelength.As shown in Fig. 2(a), the peak wavelength of the second harmonic wave λ SHG is a function of the pump wavelength λ pump (λ SHG = λ pump /2) while the wavelength of the two photon excited luminescence λ TPL is fixed at 508 nm.During the scan of the pump wavelength, the light output is initially dominated by the two photon excited luminescence.As λ SHG approaches λ TPL , the second harmonic generation process becomes dominant and the two photon excited luminescence vanishes.In Figs.We then investigate the optomechanical linear spectral tuning response of the device by measuring the voltage controlled spectral reflectance R of the micro-FP at the center point of the membrane, as shown in Fig. 3(a).In Fig. 3(b) we plot the trace of the voltage dependent resonant wavelengths of two micro-FP cavity modes (M 1 and M 2 in Fig. 3(a)).An applied voltage of 50 V causes the resonant wavelengths of M 1 and M 2 to blue-shift by 15 nm.The average voltage tuning efficiency of the resonant wavelength corresponds to 0.3 nm/V.From the blue-shift in the optical resonances caused by the reduction of the cavity length we then calculate the voltage controlled mechanical deflection at the center of the membrane using the transfer matrix method (TMM), as shown by the blue squares in Fig. 3(c).With a tuning voltage of 50 V, the cavity length of our device can be reduced electromechanically by 180 nm.In order to gain more insights into the electrostatic force induced mechanical deflection of the membrane, we use the electromechanical module in COMSOL to simulate a capacitor structure formed by a flexible plate and a fixed plate.As expected, the center of the membrane deflects the most under electrostatic force, as shown by the voltage dependent mechanical deflection along the center line of the membrane in Fig. 3(d).Since the mechanical deflection of the membrane is a function of its initial stress, we can also extract the initial stress of the membrane by matching the numerically simulated mechanical deflection with the experimentally measured values.The blue dotted line in Fig. 3(d  Having characterized the electromechanical tuning capability of the micro-FP cavity, we look into the optomechanically controlled light-matter interaction for the CdS nanoplates in the cavity.Figure 4(a) plots the 2D map of the voltage dependent light generation from a micro-FP controlled CdS nanoplate.The measured nanoplate is located close to the center of the nitride membrane.The optical pump wavelength is chosen to be 950 nm so that both the SHG and the TPL are observed (dual wavelength mode).The peak intensities of SHG (I SHG ) and TPL (I TPL ) as a function of the control voltage are plotted in Fig. 4(b).With 45 V of applied voltage, both the SHG and the TPL can be tuned from on-state to off-state, as shown in Fig. 4(d) and the experimentally obtained ON-OFF ratio of the SHG and TPL are 130x and 14x, respectively.In this case these numbers also correspond to the overall cavity enhancements factors since the OFF-state is equivalent to bare nanoplate nonlinear output signal.In addition to the on-chip voltage control of the output intensities, the peak wavelengths of the SHG (λ SHG ) and TPL (λ TPL ) are also tuned by 4 nm and 6 nm, respectively, within the pump pulse due to the narrow FP resonances as shown in Fig. 4(c).By fitting the measured voltage dependence of the second harmonic wave output with the calculation based on a TMM code, we estimate the thickness of the measured nanoplate to be 125 nm, as shown in Fig. 5.We also theoretically examine the thickness dependence of the light generation from micro-FP controlled CdS nanoplates.Figure 6(a) plots in a 2D map the calculated normalized second harmonic output power as a function of the micro-FP cavity length and the thickness of the CdS nanoplates calculated using a TMM based code [22,32].For a certain thickness t CdS , the SHG output I ω2 can be tuned from maximum (ON -state) to minimum (OFF -state) by scanning the micro-FP cavity length for fixed pump wavelength.This provides us with information on the possible modulation that can be achieved by only electromechanically tuning the cavity length for the nonlinear device for a given nanoplate thickness.We then plot in Fig. 6(b) and 6c the maximum (I ω2_max ) and minimum (I ω2_min ) of the SHG output obtained by varying the micro-FP cavity length for different nanoplate thicknesses t CdS , respectively.From Fig. 6(b) it can be seen that the largest SHG output is obtained when the thickness t CdS is 25 nm. Figure 6(d) plots the ratio between I ω2_max and I ω2_min defined as the figure of merit for the possible nonlinear output modulation depth for a given nanoplate thickness by only varying the applied voltage.Unlike the absolute SHG output (I ω2_max ) which is maximized at t CdS = 25 nm, the voltage tuning range of the light generation is maximized when t CdS approaches zero, which indicates that our micro-FP cavity can achieve the largest voltage tuning range if used with monolayer materials [22].

Conclusion
In summary, our strategy of optomechanical integration with semiconductor nanoplates exhibiting large nonlinear optical response enables a new class of functional nanowire optoelectronic devices such as spectrally selective light sources, voltage controlled nonlinear frequency doublers and optical modulators.This approach can also be utilized to study the structure and dynamics of nanoscale materials especially with very low nonlinearities to enhance their signal.Moreover, it opens up the opportunity for investigating optomechanical control of nonlinear light-matter interaction in semiconductor nanowires.We envision that our strategy of on-chip nanoscale opto-electro-mechanical integration will lead to next generation tunable nonlinear light sources and devices.

Fig. 1 .
Fig. 1.(a) Schematic of a CdS nanoplates embedded in a voltage controlled widely tunable electromechanical micro-FP cavity.I ω1 : pump wave; I ω2 : output wave (SHG and/or TPL).The inset shows the image of the nanoplate taken by the optical microscope in the SHG measurement setup.The typical size of the nanoplate is 60 μm × 20 μm.(b) The spectral reflectance of the DBR (blue dotted line) and the micro-FP cavity (red solid line) measured from the glass side, respectively.Here the control voltage of the micro-FP is 0 V and the corresponding cavity length of the micro-FP is 7.65 μm.

Fig. 2 .
Fig. 2. (a) The output spectra of the micro-FP controlled CdS nanoplate as a function of the pump wavelength scanned from 850 nm to 1000 nm; (b), (c), (d) The selected output spectra of the cavity controlled CdS nanoplate pumped at 910 nm (TPL dominant mode), 945 nm (dual wavelength mode) and 980 nm (SHG dominant mode), respectively.

-Fig. 3 .
Fig. 3. (a) The voltage dependent electromechanical tuning of the spectral reflectance of the micro-FP, measured at the center point of the membrane and (b) the corresponding peak wavelengths for the two modes M1 and M2.(c) The mechanical deflection at the center point of the membrane ΔZ 0 , calculated from the measured peak wavelengths of M 1 and M 2 using the transfer-matrix method and (d) The voltage dependence of the mechanical deflection across the center line of the membrane ΔZ(x).

Fig. 4 .
Fig. 4. (a) The output intensities for SHG and TPL from the micro-cavity coupled CdS nanoplates as a function of the control voltage scanned from 0 V to 50 V.Here the pump wavelength is fixed at 950 nm (b) The voltage dependence of the peak output intensities for SHG (I SHG ) and TPL (I TPL ) and (c) The voltage dependence of the peak wavelengths for SHG (λ SHG ) and TPL (λ TPL ).(d) The output spectra of the micro-cavity coupled CdS nanoplates pumped at the mode with the control voltage of 0 V and 45 V respectively.

Fig. 5 .
Fig.5.Estimation of the CdS nanoplate thickness from the measured voltage dependent SHG.Since the nanoplates inside the micro-FP cavity are located randomly on top of the DBR mirror, it is difficult to directly measure their thicknesses.However, we are able to extract the thickness of the tested nanoplate by fitting the measured SHG output as a function of the applied voltage (V app ) with the TMM calculation.The pump wavelength and initial cavity length used in the simulation are 950 nm and 7.625 μm, respectively.The thickness of the measured nanoplate is determined to be 125 nm.

Fig. 6 .
Fig. 6.(a) Normalized second harmonic wave intensity as a function of the nanoplate thickness and the cavity length calculated using TMM.(b) The calculated maximum and (c) minimum second harmonic output intensity as a function of the nanoplate thickness.(d) The achievable ON/OFF ratio, a measure of the modulation depth, between the maximum (ON-state) and the minimum (OFF-state) for the second harmonic output as a function of the nanoplate thickness.