基于石墨烯电容器件的可调控全固态脉冲激光器研究

Controllable All-Solid-State Pulsed Laser Based on Graphene Capacitor Devices

为了改变石墨烯可饱和吸收体恒定的光学吸收效应,通过微电子打印工艺制备了一种吸收特性可控的新型石墨烯电容器件,并以超低电调制功率(<1 nW)实现了全固态脉冲绿光激光器输出特性的灵活调控。采用等离子体增强化学气相沉积法制备了石墨烯薄膜,通过喷墨打印、点胶、烘干等流程制备了电压可控的圆环阵列式石墨烯电容器件,该器件具有较好的光学吸收调控效果,通过改变栅压,可将非线性调制深度从4.0%提升至6.9%。将石墨烯电容器件应用于Nd∶YVO_(4)全固态激光器系统中,实现了波长为532 nm的脉冲激光输出,保持吸收泵浦功率为1.78 W不变,当栅极电压从0 V增加至60 V时,可将输出激光的脉冲宽度从1.1μs压缩至345 ns,对应的重复频率从101 kHz提高到312 kHz。

Objective In recent years,the growth rate of China laser industry is about 18%-22%,which is much higher than the average growth rate of the international laser industry.However,there is a lack of independent innovation in domestic laser device industry,and the key technology is restricted,which leads to the "industrial hollowing-out"of laser industry.The research on high-performance and practical all-solid-state lasers is the key to cultivate new economic growth points and seize the commanding position of the emerging laser industry.The traditional optical-optical tunable all-solid-state pulsed laser is the mainstream of laser devices in the current market,but the bottleneck problems such as high power consumption,slow speed,and low efficiency of optically controlled all-solid-state pulsed lasers seriously restrict its industrialization.The key to solve these problems is to construct a new electro-optical transistor device structure based on two-dimensional materials.The results of this paper provides a new application prospect for all-solid-state pulse lasers:on the basis of ensuring the realization of the traditional laser product functions,the device structure is simplified and the product performance is developed along the trend of miniaturization,low energy consumption,and adjustable control,which enriches the practical application fields of all-solid-state pulse lasers and promotes the development of lasers and the related application fields.Methods In this paper,the graphene capacitor device is fabricated on graphene substrates by micro-electronic printing,and the main fabrication processes are given(Fig.1).In order to improve the modulation efficiency of graphene capacitor devices,a unique ring electrode structure is designed,and the carrier concentration distribution of the devices is simulated by the finite-difference time-domain method.The results show that the modulation effect in the central region of the ring is optimal.The photoelectric performance of the graphene capacitor is studied,and the capacitance characteristic of the device is tested,which shows that the device has a good modulation performance.The results show that the vertical electric field can enhance and control the optical absorption capacity of the graphene device significantly.Finally,a 532 nm all-solid-state pulsed laser system is studied and constructed by combining the solid-state graphene capacitor device with the all-solid-state laser system,and the tunable 532 nm laser based on the graphene device is realized.Results and Discussions In the absorption spectrum of the optimized structure of the graphene capacitor,the optical absorption intensity increases with the increase of gate voltage(Fig.4(a)),which shows that the absorption characteristics of the device are regulated and the feasibility of the experiment is verified.In order to further study the effect of gate voltage on the absorption of graphene devices,a balanced synchronous dual-detector system is used to measure the nonlinear transmission characteristics of graphene devices at the same position.In order to guarantee the modulation effect of the device,the center of the ring electrode is selected in the test area according to the simulation results of carrier concentration distributions.The absorption of the graphene device gradually reaches saturation as the intensity of the coherent femtosecond laser increases(Fig.5(a)).When the gate voltage is 0 V,the modulation depth is about 4.0%and the saturation intensity is 8.14 MW/cm^(2).When the gate voltage is 60 V,the modulation depth is about 6.7%and the saturation intensity is 14.9 MW/cm^(2).These results show that in the effective range(V_(GS)≤60 V),the vertical electric field can modulate and enhance the nonlinear optical absorption characteristics of the graphene field effect devices.It is applied to Nd:YVO_(4),laser system(Fig.6),and the stable 532 nm Q-switched laser is achieved.When the gate voltage is increased from 0 V to 60 V,the pulse duration of the output laser can be compressed from 1.1μs to 345 ns while keeping the power of the absorption pump constant at 1.78 W,and the corresponding repetition rate is increased from 101 kHz to 312 kHz(Fig.9).To verify the stability of the Q-switched laser pulse,the radio frequency spectrum of a 285 kHz pulsed laser is measured using a spectrometer(Fig.11(b)).As can be seen from this figure,the pulse signal-to-noise ratio(PSNR)is 39 dB,which indicates that the Q-switched laser output is stable.Conclusions The paper introduces the fabrication,characterization and analysis of a novel graphene capacitor,which can be used as an effective saturable absorber modulator in a passive Q-switched system.The photoelectric interaction mechanism of the device is studied based on its electrical transmission and spectral absorption properties.Due to the change of Fermi level and carrier density,the absorption properties of graphene in the device can be adjusted by gate voltage without changing the properties of SA material.The output characteristics of all-solid-state pulse laser can be adjusted flexibly under ultra-low electric modulation power(〜13 pA current and 1 nW power).It is applied to the Nd:YVO_(4) all solid state laser system,and the stable pulse output at 532.04 nm wavelength is achieved.The laser absorption pump power is kept constant,and the gate voltage is changed.The pulse duration of the Q-switched output can be compressed from 1.1μs to 345 ns.This structure is expected to further promote the development of tunable pulse lasers from visible light to mid-infrared band.In particular,passive Q-switching/mode-locking devices based on graphene devices will be used in future applications such as infrared measurement,projection display,and optical modulation.