Grating structure design using topology optimization at various wavelengths

다국어 초록 (Multilingual Abstract)

Topology optimization is regarded as great method to find optimal shape by many designers of various fields because of its design freedom. Topology optimization can be applied on many various problems such as compliant, elastic, heat and electromagnetic field problems and it can be extended wave propagation problem. The propagating of electromagnetic waves through a single sub-wavelength aperture has been studied for many years to enhance the transmittance or control the direction of radiated beam. Recently, the grating structure is regarded as one of good ways to handle this problem because of characteristic of grating structure. Grating structure positioned at the outlet of radiated light can generate the surface plasmon effect. Therefore appropriate shape of grating structure can be used as the outlet of radiated light. In this study, waveguide at infrared wavelength with asymmetric grating structures is proposed to guide the direction of radiated beam through an aperture using topology optimization. Contrary to traditional optimization, the optimization method based on the phase field method combined with reaction diffusion equation and double well potential functions calculates the sensitivity only at the boundaries not the sensitivity at all design domain. Because of this characteristic, optimization based on the phase field method is adopted in this study to obtain clear shapes. The design objective is not only maximizing the poynting vector at the measuring domain but also finding a clear and simple shape of grating structure for the manufacturing feasibility. In results, dielectric grating structures asymmetrically arranged on the metal slit surface are proposed to diffract the exit beam to a desired direction and their optimal shapes are obtained through the topology optimization process. For proving effectiveness of topology optimization in nano-scale and accuracy of results, experiment is needed. However waveguide at infrared wavelength is too small to make grating structure with optimal shape. Therefore, newly designing the grating structures at radio frequency wavelength using same optimization method is suggested to make an experiment. The simulation and optimization process has been performed by finite element analysis using the commercial package COMSOL associated with the Matlab programming