Research On Subwavelength Gratings Used For Photodetectors

Subwavelength grating (SWG), in which only zero-level reflection and diffraction wave can be transmitted for its operating period is less than the incident wavelength, is one kind of great performance mirror as the high reflectivity. Furthermore, SWG has good prospect of application in the field of optical communications, integrated optics, sensor detection and forgery science for its striking advantages such as small size, simple fabrication, low cost and so on.Through the theory analysis, researches on the reflectivity of SWG and application of SWG in resonant cavity enhanced photodetector (RCE PD) are demonstrated. The main innovation is that the structure of concentric circular SWG is designed. Compared with the traditional SWGs, the concentric circular SWG has a good deal advantages such as polarization characteristics independent of the incident light direction, a large inception area and good 3D integration due to the strict two dimentional axisymmetic structure. The main contents of this paper and research work include the followings.1. The theories for calculating the reflectivity of one-dimensional strip SWG and concentric circular SWG are analyzed. The rigorous coupled wave analyses method and transfer matrix method are combined to study the reflection characteristic of one-dimensional strip SWG. The finite element method and transfer matrix method are combined in order to analyze one-dimensional strip SWG. The rigorous coupled-wave analyses method and transfer matrix method are used for the study of concentric circular SWG.2. The reflection spectrum of concentric circular SWG is simulated and calculated. The solving procedures, principles and boundary conditions of Simulation are studied using finite element method. After verifying the reliability of the software, the reflection spectrums of one dimension SWG and the concentric circular SWG are simulated. The influence of parameters of one-dimensional SWG and concentric circular SWG on the reflectance spectrum is also investigated.3. The concentric circular SWGs are fabricated. The nanofabrication technology is investigated. Two groups of concentric circular SWG mirrors (with different sizes) are fabricated, using the technology of electron beam lithography (EBL) and inductive coupled plasma (ICP) dry etching. The reflection spectrum of the SWGs are also tested.4. Two kinds of RCE PDs are proposed, in which concentric circular SWGs are applied as the top and bottom reflector mirrors of RCE-PD or only for bottom reflector mirror of RCE-PD. The reflectivity of the bottom mirror is higher than 90% from 1370nm to 1630nm, with the peak reflectivity is higher than 99% at 1550nm. The reflectivity of the top mirror is 90% at 1550nm. The difficult problem for the manufacture of Inp/InGaAsP DBR mirror can be solved for the replacement of concentric circular SWG mirrors. The quantum efficiency of RCE PD based on CC-SWG is higher than 90% when the thickness of absorption layer is less than 100nm.