Research On High Contrast Subwavelength Gratings And Its Application For Photodetectors

As the rapid growth of the communication capacity, the demands for devices in optical communication systems are increasing. On the one hand, conventional photodetector, which is one of the key components for optical communication systems, limited by the materials, structures, and craft, no longer completely satisfy the development of the new generation information technology. On the other hand, the new micro/nano-structural integrated optoelectronics is one of the trends of the new generation information technology. As one of the new nano structure, the subwavelength grating is promising for applications to high-performance optoelectronics. So we decide to use photodetector integrated with the subwavelength grating for better performance. It is necessary for us to make in-depth study of the subwavelength grating for combining it with photodetector together more suitable. This thesis focuses mainly on the high index contrast subwavelength gratings (High Contrast Gratings) for applications to photodetectors. These are gratings with a subwavelength periodic patterning fabricated with materials exhibiting a large refractive index difference. The new features of HCGQ including high reflectivity and steering ability, are just what we need.This thesis makes an analysis around the salient features of HCG such as high reflectivity and steering ability. We investigate the design approach of HCG to meet our demands. The main works can be summarized as follows:1. The two different numerical methods, rigorous coupled wave analysis (RCWA) and modal analysis theory, used to analyze the reflection spectra of HCG structures, have been mastered. The commercial software COMSOL, which implements finite element method (FEM), has also been mastered in order to compute the non-periodic structure.2. An approach of designing high-reflectivity, wideband HCG mirror is proposed for its reflectance property. An iterative optimization algorithm is introduced to quickly determine the optimal grating period and the correponding duty cycle of the two-dimensional HCG mirror based on a blocky structure. The spectra show a bandwidth of 182nm from 1439 to 1621nm with a reflectivity over 99% as well as polarization independence. Finally, this approach can also be applied to any structures which resemble the type of symmetric grating.3. Two approachs of designing different planar dielectric reflectors with focusing ability are proposed for its steering ability. One is based on the concentric circular HCG The reflector with a focal length of 6um is designed, which exhibits high focusing ability at normal incidenc with a radially polarized plane wave. At the reflection focal plane, the full-width-half-maximum (FWHM) of the electric field intensity is 0.89um. Numerical aperture value as high as 0.93 is achieved for the reflector with very high reflectivity of 92%. The other one is based on a blocky HCG The reflector with a focal length of 6um and a grating area of 10.5um*10.5um is designed, which exhibits a focusing ability without polarization sensitivity under normal incident light at wavelength 1.55um.In the reflection focal plane, the FWHM of the electric field intensity is 1.338um. The results show a numerical aperture of 0.66 with a reflectivity over 82%.4. The high-reflectivity, wideband HCG mirror on a standard silicon-on-insulator wafer is fabricated. The sample of the HCG mirror is measured through a test system established by ourselves.