Photonic Crystal Thz Devices And Photonic Crystal Fiber Supercontinuum Generation

Photonic crystals (PCs), in which dielectric materials with different dielectric constants are periodically arrayed in one or more dimensions, are formed naturally or artificially. The essential property of photonic crystals is the photonic band gaps (PBGs) where light with certain wavelength cannot propagate through it. The designed defect in PCs is also a very important issue. Most applications of PCs are based on how to introduce the defects into the PCs. The PC products have been growing rapidly.In this thesis, the supercontinuum spectra generation in photonic crystal fiber (PCF) and the propagation properties of THz waves in PCs are studied theoretically and experimentally. Based on the electrically or magnetically birefringence of liquid crystal, several efficient methods for the realization of THz devices by using PCs are proposed. The details are described as follows:1. To provide a supercontinuum light source for the corherent anti stokes Raman scarring (CARS) confocal system, the supercontinuum spectra generation in PCF pumped with femtosecond laser pulses are studied theoretically and experimentally. The nonlinear processes responsible for the generation of supercontinuum spectra, including high order dispersion, self phase modulation, self-steepening and intra-pulse Raman scattering effects, are identified through the simulation of the nonlinear Schrodinger equation by split-step fast Fourier transform method. The experimental results demonstrated the effects of parameters of the femtosecond laser on the generation of supercontinuum.2. We proposed a novel method to achieve highly nonlinear PCF by filling highly nonlinear liquid including carbon disulfide, chloroform, and methylbenzene in the core of hollow-core PCF. The propagation, dispersion properties and mode characteristics are studied by using the finite element method (FEM). Simulation results indicate that the zero-dispersion wavelengths (ZDWL) of the liquid-filled PCFs are adjustable around 800 nm. Hence the femtosecond laser pulses with central wavelength at 800 nm can propagate in the anomalous dispersion regime of PCFs, which leads to inducing the spectra broadening.3. The propagation properties of THz waves in liquid-crystal (LC) -filled PC waveguides are investigated. The effects of magnetic birefringence of LC (E7) on the PBGs and the transmitting properties of two-dimensional PC waveguides with three typical structures are studied by using the plane wave expansion (PWE) and finite-difference time domain (FDTD) methods. The simulation results indicate that the magnetically controlled PC waveguide filled with the LC can serve as not only a THz switch in a wide range of frequencies, but also a acontinuously tunable bandpass filter.4. We proposed a THz switch that has high extinction ratio. We utilized Bragg fiber as the basic structure. The PWE method is used to simulate the PBGs of the one-dimensional PC in the cladding of the fiber. The performance of the switch is analyzed by FEM and the results show that the extinction ratio of this switch can reach to 39.93 dB.5. The defect modes characteristics of a line defect and point defect in two-dimensional PCs are studied by using PWE and FDTD methods. Based on the analysis on the coupling characteristic between them, we proposed a novel tunable THz narrow-band filter by using electrical birefringence of liquid crystal. Simulation results on the defect modes and output spectra show that the PC consisting of two line defects and one point defect can act as a tunable filter with narrow bandwidth of 0.02 THz.6. A wavelength division multiplexer and a narrow-band filter based on two point defects and two waveguides in PCs are proposed. There are two narrow peaks with band width 0.015THz in the transmitting spectrum when the upload and download waveguides are parallel. Thus, it can be used as a wavelength division multiplexer. When the upload and download waveguides are perpendicular, only one peak appears in the transmitting spectrum. Therefore, it can be used as a narrow-band filter.