Optimization of Near-Field Flat Focusing Mirrors with Genetic Algorithms

• Main Authors: CHUANG,YU-CHING, 莊于慶
• Other Authors: CHENG,YU-CHIECH
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/rg9y88

碩士 === 國立臺北科技大學 === 光電工程系 === 107 === In this paper, the genetic algorithm (GA) is used for the optimization of the near-field flat focusing mirror based on subwavelength grating structures. The purpose of optimizing is to achieve the longest focal length for a light beam with a high numerical aperture. The flat focusing mirror consists of a higher refractive index sub-wavelength grating (n = 3.435) on top of a lower refractive index waveguide layer (n = 1.7). The structure is designed for a TM-polarized near-infrared light (l = 1.55 um). To obtain the lateral displacement, we calculate the imaginary part of the electromagnetic field by using the Rigorous Coupled Wave Analysis (RCWA) method and take the first derivative of phase with respect to angles of incidence. The calculation method of the lateral phase displacement can be referenced by the Goos-Hänche shift. The focusing condition is the negative slope of the lateral displacement versus the incident angles. The focal length is proportional to the absolute value of this negative slope. In this thesis, we have three optimal cases with a linear target function to demonstrate unpreceded focusing performance by calculating the field propagation through the Finite Difference Time Domain Method (FDTD). Such a focusing mirror has spectacular features such as transversal invariance, angular invariance due to its periodic modulation unlike those curved lenses and metalens relying on the axially spatial modulation.