Research On Novel Subwavelength Gratings And Its Applications In Photodetectors For Optical Communication

The work focuses on subwavelength grating (SWG) which is a kind of important diffractive optical elements.SWG takes an important role in the family of gratings and gives rise to propagation of only the zero-reflected and zero-transmitted diffraction orders as its period is smaller than the incident wavelength.SWG can be considered as a special film whose refractivity is equal to the average refractivity of the grating layer where the refractivity is periodically modulated in space.By modifying SWGs'structure,the refractivity can be tuned continuously. Besides,SWGs'diffraction field may have sharp change when the incidence light wavelength or the structure has small change.This is due to guide mode resonance (GMR) phenomena.In this dissertation, InP based SWGs that work in the C band of optical communication are designed.SWG period is smaller than 1.55μm. Electron beam lithography is adopted to fabricate surface pattern in nano scale.Reliable analyzing models are set up for different SWGs.Utilize GMR to realize reflection resonance filters in C band. SWGs are integrated into resonant cavity enhanced photodetectors (RCE PD) for optical communication system. The reasons of chosing RCE PDs as an application of SWGs are as following.RCE PDs can realize the full absorption of multi-reflected lightwave in the resonance cavity with thin (several hundreds of nanometer) absorption layer which guarantees the ability of high speed signal processing and high quantum efficiency. Make it a good candidate for wavelength-demultiplex receiving application in optical fiber communication systems and networks. The research work and innovations mainly include the followings.1.Analyze SWGs by rigorous coupled-wave analysis (RCWA), plane wave-expansion(PWE) method and finite difference time domain (FDTD) method. SWG types are including SWG with 1D period distribution(1D-SWG),SWG with 2D period distribution (2D-SWG) and SWG (1D-SWQ 2D-SWG) with DBR stacks. RCWA is an analytical method which is accurate and has high calculation speed. FDTD is a numerical method which can analyze SWG with finite period numbers at the cost of calculation time and memory space.RCWA and FDTD are compensated with each other for analyzing.And PWE can reveal the essence of performance of SWGs by energy band graph.2. Program to analyze SWGs by RCWA, FDTD and PWE respectively and to analyze the performances of RCE PDs with SWG filters.Results are verified by published conclusions.Programs can analyze SWGs'diffraction efficiency and RCE PDs'light field distribution,quantum efficiency, cross-talk attenuation,step response, Gauss pulse response, impulse response and frequency response.3.Design InP based SWGs that work in the C band of WDM system. DBR is integrated with SWGs to realize broadband reflection. The influences of SWGs'period length, thickness,duty cycle on its diffraction feature are studied in detail.With DBR,2D-SWGs'reflectivity is higher than 99% from 1.47μm to 1.59μm. Its peak reflectivity is higher than 99.9% in C band.4.Design InP based RCE PD with SWG filters that works in C band of WDM system. Propose three kinds of RCE PD for WDM system demultiplex receiver with single 1D-SWG, single 2D-SWG and double 1D-SWGs respectively.5.Simulation for RCE PD with single 2D-SWG reveals that 1)light field distribution in the RCE PD shows stand wave effects that can be used to decide where to set the absorption layer; 2) with high cross-talk attenuation coefficient,devices'quantum efficiency is higher than 90%;3) compared with pure DBR in RCE PD,the total numbers of reflector's layers are no more than 10 pairs of InGaAsP/InP DBR; And 3dB bandwidth is 15Gb for 1D-SWG RCE PD whose square top mesa area is 30μm×30μm.6.Fabricate InP based SWGs by Electron Beam Lithography (EBL) and Inductively Coupled Plasma (ICP) etching. SWGs are designed according to the following optimization guidelines.SWG's thickness is 265nm. Period is 1.4μ. Duty cycle is 33%.The average bottom roughness of SWG is reduced to 40nm after optimizing ICP conditions.7.Design and fabricate the lithography pattern mask of RCE PD with SWG. "T" style small location mark and square grating protection mask is designed for SWGs.RCE PD's top mesa is square.8.Take part in the work of GaAs nanowire growth.Test nano morphology of Au-Ga alloy nano-particles and nanowires by Atomic Force Microscopy (AFM) and Scan Electron Microscopy (SEM) respectively.The densities of Au-Ga nano particles are 1.64×1010cm-2, 1.26×1010cm-2,2.2×109cm-2 for different thicknesses of Au film.SEM tests reveal that the length of nanowires are 5.2μm,5.3μm,6.3μm with densities larger than 109cm-2 for different nano-particles.9.GaAs nanowires are tested by Transmission Electron Microscopy (TEM).In the part of TEM sample preparation, the paper calculates the size of nanowire sample as lcm2 by SEM testing results and micro-grid holder parameters.TEM tests reveal that nanowires'diameter are 24nm, 68nm and 168nm. Nanowires grow in the[111]direction and pure zinc blend FCC lattice is discovered.