Optical Field Simulation Of Terahertz Quantum Well Photodetectors

Terahertz(THz)technology is called “one of the most important ten technologies that change the world”.It has very important application in physics,chemistry,military,medical and other related fields.The research of high-performance THz detectors plays an important role in the development of terahertz technology.Terahertz quantum well photodetectors(THz QWPs)have attracted more and more attention nowadays because of some unique advantages such as fast intrinsic response speed,small volume and the availability of constructing integrated terahertz application systems.At present,THz QWP is far from mature and the primary goal of THz QWP investigation still remains to be improving the device performance as possible.Optimizing the optical field distribution in the device to improve the optical coupling efficiency is an important way to improve the performance of THz QWP.The impact of optical field distribution tends to be more significant in THz region,where the wavelength is relatively long and has reached the order of magnitude of the device.Coupling through a 45° facet is a widely accepted coupling method and the 45° coupling structure is a commonly accepted structure for measuring and comparing THz QWP performance.The internal optical field distribution is particularly important for THz QWP performance improvement.However,there are still few studies on optical field distribution for 45° edge coupled THz QWP.By finite difference time domain(FDTD)technique,this paper investigated three optical field distribution and coupling affecting factors for 45° edge coupled THz QWP,including electrode geometry,the position of active region and the coupling angle.In addition,grating structure THz QWPs based on a previous high performance THz QWP were designed for further improvement of THz QWP performance.First of all,we investigate the impact of electrode geometry on optical coupling efficiency for conventional 45° edge coupled THz QWP with peak frequencies ranging from 1-12 THz.The simulation result shows that the whole THz range could be divided into three zones,the low frequency region(1 THz-6.7 THz),the medium frequency region(6.7 THz-9.8 THz)and the high frequency region(9.8 THz-12 THz).In the low frequency region,a solid electrode is preferred.In the medium frequency region,two electrodes perform similarly in optical coupling.In the high frequency region,the ring electrode is a greater choice.This provides direct guideline for THz QWP experiment.Secondly,we calculated a reasonable active region position for THz devices with different response frequencies.Finally,we studied the influence of the coupling angle on optical coupling efficiency,obtained the most effective coupling angle and gave the corresponding physical explanation.The performance of 45° edge coupled THz QWP can be greatly improved through these structural designs of the three aspects.In addition,the high performance THz QWP designed by the laboratory in the early stage is carried out in the design of one-dimensional grating device and two-dimensional grating device.For one-dimensional grating device design,the best grating period is equal to the wavelength of incident THz wave in GaAs.The best filling factor is 50% for substrates of infinite thickness.Taking the thickness of substrate into account,the best filling factor would exceed 50%.The grating thickness has little effect on the optical field distribution in the device.On this issue,few considerations are required in practical applications.For two-dimensional grating device design,the best filling factor is different in cases of different grating periods.Generally speaking,the coupling efficiency could reach the highest value when the side length of grating is half the grating period.The optical coupling difference between devices with different grating shapes is negligible.The optical coupling efficiency of grating structure THz QWP is usually higher than that of the same designed 45° edge coupled THz QWP.The design of grating structure THz QWP enables further improvement of THz QWP performance.