Research On Dynamic Silicon-based Photonic Devices By Reverse Design

Traditional photonic device design is based on the fundamental physical principles.By selecting the appropriate structure and adjusting the structural parameters to meet the design requirements,one can achieve the corresponding functions.In addition,another design method namely reverse design is different from the traditional principle-based heuristic design method.Reverse design is a method that obtains the structures inversely by determining the clear objective features and optimizing multiple characteristic parameters simultaneously with simulated algorithm optimization.Traditional design method has extremely extensive research prospects in the past decades,and it has almost dominated in many research fields including metasurface devices and microresonators.Most devices have been obtained based on this method.However,with the expansion of application fields,it is usually difficult for designing devices to meet a variety of requirements by traditional design method and also difficult to ensure the optimization of the design.However,these problems faced by traditional method can be solved by reverse design method,which is of great significance in the field of new-generation photonic device design.In our research,we designed some integrated silicon-based photonic devices with adjustable function based on the reverse design method,so as to achieve functions that do not exist on original devices.We combined global optimization algorithm and finite-difference time-domain(FDTD)simulations to design and implement the double-channel switchable wavelength/mode-division multiplexer on silicon on insulator(SOI),and the orbital angular-momentum(OAM)light emitter with switchable topological charge.Among them,the two wavelengths are 1310 nm and 1550 nm respectively,the two modes are transverse electric(TE)mode and transverse magnetic(TM)mode respectively,and the topological charge of the light emitted by the emitter is +1 and-1.The function switching of designed devices is achieved by refractive-index alteration that induced by photo-generated carrier in silicon.The principle of modulation is utilizing high-power femtosecond-pulsed lasers to adjust the refractive index of silicon by changing the carrier concentration of silicon and eventually achieving the function switchable photonic devices.Our research mainly includes structure optimization,micro-nano processing and experimental system construction for device testing.Firstly,we exploited the idea of density topology optimization,that is,the optimized structure is finitely discretized,and the optimized objective function is calculated in order to obtain the optimal pixel structure distribution.Due to the huge solution space,we need to consider the global optimization algorithms,whose advantage is that the obtained solution will not be trapped in the local optimal solution.We used the simulated annealing algorithm combined with FDTD simulation to optimize the structures that are silicon-disc shape.We optimized the spatial position of unit pixels of silicon through binary coding to maximize the transmittance(objective function)of emitted light by utilizing the optical scattering of silicon.The simulated calculation results of the optimized structure show that the devices we designed have corresponding adjustable functions.Among them,the transmission efficiency of the wavelength/mode splitter before and after the channel switching is more than 20%,and the corresponding channel isolation is mostly greater than 10 d B;the emitting efficiency of the OAM light emitter before and after the topological charge switching is more than 25%.Secondly,after the structure is optimized,micro-nano processing can be performed,and we adopted the standard SOI processing flow,including photoresist spin coating,electron beam lithography,reactive ion etching and other processes.Finally we introduce the experimental system to be constructed for detecting the wavelength/mode-division multiplexer,and OAM light emitter before and after laser modulation,as well as experimental data collection.This thesis focuses on the device design method,simulation verification,device processing,and the experimental testing system to be built.The actual experiments of device performance testing are planned to be carried out in subsequent researches.