Study on tunable photonic crystals using liquid crystals or semiconductors

Abstract

In this thesis, we have investigated tunable photonic crystals (PCs) using liquid crystals (LCs) or semiconductors. The tunability of PCs can be applied to optical filters or optical switches. Liquid crystal is a good candidate material for tunable PCs because its refractive index is changed by an external electric field or temperature variation. The refractive index of a semiconductor can also be externally controlled or tuned by changing the density of free carriers because the dielectric constant of a semiconductor depends on the plasma frequency which is a function of the free carrier density. One can change the free carrier density via impurity doping, current injection, light illumination and temperature variation. We present here tunable one-, two-, and three-dimensional photonic crystals, as well as water wave band gaps of periodic structures. The omnidirectional reflection band (ORB) of a one-dimensional photonic band gap (PBG) structure consisting of alternating isotropic dielectric and nematic liquid crystal layers can be tuned by an external electric field. The width of the ORB becomes wider as the external voltage increases, but the center frequency changes little. The frequency of the defect mode created by inserting a layer of liquid crystal into a one-dimensional dielectric PBG structure is also found to be tunable by the application of external voltage. We have also studied the omnidirectional mirror consisting of a semiconductor and a dielectric. Because we can change the dielectric constant of a semiconductor by varying the free carrier density, the width and the center position of the ORB of the mirror can be changed. When a semiconductor slab is inserted into a mirror consisting of two dielectric materials, a defect mode frequency is created and its frequency can be controlled by varying the free carrier density of the slab. We have investigated the effects of liquid crystal infiltration on the PBGs for the s-polarized wave of two-dimension...