Research On Subwavelength High Contrast Gratings’ Features And Its Applications In Photodetectors

Subwavelength high contrast grating (HCG) is a new type of subwavelength grating in which the high-index grating is fully surrounded by low-index materials and whose period is smaller than the incident wavelength. Thus, by a simple structural design of HCG, many unexpected, extraordinary features are obtained including ultrabroadband high reflectivity, ideal polarization control, and wave front control of the transmitted and reflected beam. This thesis focuses on HCGs and mainly researches their characteristics and applications in high-performance integrated photoelectric detectors.The thesis mainly discusses three different numerical methods and first order approximation of effective medium theory for HCG, sets up a reliable FEM-based analyzing model of HCG, and designs an ultrabroadband mirror, a polarizer and a flat reflector with focusing ability by using HCG. In the end, the designed HCG structure is integrated to novel long-wavelength photoelectric detectors and how the HCG effects the performance of these optoelectronic devices is analyzed.The main research work and innovations include the followings.1. Three numerical techniques to investigate HCG, namely rigorous coupled-wave analysis (RCWA), finite element method (FEM), and finite difference time domain method (FDTD), are analyzed and their calculation characteristics are summed up. In addition, we investigate some important aspects related to the numerical simulations with FEM performed, set up an analyzing model of one-dimensional HCG with FEM, demonstrate the validity of the modeling method through two practical calculation cases. And the influences of HCG’s period, duty cycle, thickness, angle of incidence, wavelength of incidence on its diffraction feature are investigated with the model. Finally, how to use first order approximation of effective medium theory to analyze HCG are discussed systematically.2. A SOI-based ultrabroadband mirror with very high reflectivity is designed by using HCG. The HCG’s reflectivity for TM light is higher than90%from1318nm to2161nm, thus the designed mirror obtains an ideal broad reflection spectrum (AA/A>39%).3. A SOI-based HCG is designed to result in a good-performance polarizer by utilizing HCG’s high reflectivity over a wide range of wavelengths. The HCG polarizer who has a feature of ultrabroadband, obtains a extinction ratio larger than20dB for both the reflected light and transmitted light over the range of wavelengths from1486nm to1603nm, and both its reflectivity for TE light and transmittivity for TM light are larger than99%at the same wavelengths. Especially when wavelength of the incidence is1550nm, the HCG has a wide tolerance in duty cycle, thickness, angle of incidence for ease of fabrication.4. The thesis analyzes the HCG’s phase front control and shows numerically how a non-periodic design of HCG allows focusing the reflected beam and transmitted beam. Besides, the guidelines for designing HCG focusing reflectors and lens are briefly summarized, and two-dimensional simulations for a HCG focusing reflector reported in the reference are performed with FEM to verify its high focusing power for TM reflected light.5. We test reflection spectrum of the ultrabroadband mirror with very high reflectivity using HCG experimentally. The experimental results show that the reflectance of the HCG mirror for TM light is in the range of77.1%-87.8%over the1460nm～1610nm, and excellent agreement is obtained between the experimental solutions and the simulation results to prove the validity of designing the HCG.6. SOI-based HCGs’applications in long-wavelength photoelectric detectors are investigated, thus based on the designed HCG polarizer, a novel HCG-based drift-enhanced dual-junction PIN photodetector structure is designed after optimization of the structural parameters. For TE-polarized incidence, the maximized bandwidth-efficiency product (BEP) and3dB bandwidth are33.37GHz and39.25GHz respectively, and the quantum efficiency is larger than85%increased by more than38%because of the introduction of HCG.