光子晶体THz器件及光子晶体光纤超连续谱产生的研究

【作者】 张会；

【导师】 常胜江；

【作者基本信息】 南开大学 ， 光学工程， 2009， 博士

【摘要】 光子晶体是具有频率带隙的周期介电材料,自20世纪80年代光子晶体的概念提出以来一直受到广泛的关注。在光子晶体中各种缺陷的引入也为光子晶体的应用和发展提供了广阔的前景。特别是在太赫兹科学及技术这一新兴的前沿学科领域,光子晶体灵活控制电磁波传输的能力使之倍受关注。本文结合理论分析和实验研究,对近红外波段光子晶体光纤(PCF)中超连续谱的产生、THz波在光子晶体中的传输特性进行了系统的研究。利用液晶材料的双折射效应,探索和分析了用光子晶体实现THz功能器件的有效技术途径。论文主要研究内容和创新点包括以下几个方面:1、为了将超连续谱应用于相干反斯托克斯拉曼散射共焦成像系统,我们在理论和实验上分析和研究了飞秒激光脉冲与PCF相互作用的多种非线性效应。利用分步傅立叶法对广义的非线性薛定谔方程进行了数值求解,在考虑高阶色散和非线性效应的情况下,对飞秒激光脉冲在PCF中的传输特性进行了模拟分析。研究了高阶色散、自陡峭、脉冲内拉曼散射效应对PCF中超短脉冲的传输以及最后超连续谱产生的影响。并在实验上分析了外界条件如入射激光的功率和中心波长对超连续谱产生的影响。2、利用有限元法分析了在空芯PCF纤芯中填充二硫化碳、三氯甲烷、甲苯等高非线性液体后PCF的传输特性、模式及色散特点。在此基础上,系统分析了在液芯高非线性PCF中超连续谱产生的过程及特点。研究结果表明,该类PCF的零色散波长可在800nm左右调节,因此中心波长为800nm的钛蓝宝石飞秒激光可以在较短的距离(几个毫米)和较小的能量下产生比较平坦的超连续谱。3、理论分析了THz波段填充液晶的光子晶体波导在磁场调控下的传输特性,在此基础上,提出了一种基于二维光子晶体波导的THz多功能器件的设计方案。利用平面波展开法和时域有限差分法对这种液晶调谐的THz光子晶体开关和滤波器的带隙结构、磁场控制下带隙的变化特点以及传输特性等进行了系统的分析。结果表明:在磁场控制的液晶双折射作用下,该结构的光子晶体波导同时具有开关和可调谐滤波的功能。4、提出了一种基于布拉格光纤的高消光比THz开关结构。通过在包层中采用高阻硅和向列相液晶E7构成的周期性一维光子晶体结构,并施加外加磁场来控制液晶的取向,实现了对THz波的开关控制功能。数值模拟结果表明该结构的太赫兹开关消光比可达39.93dB。5、通过分析二维正方形光子晶体中点缺陷和线缺陷的缺陷模特性以及缺陷模之间的耦合特性,提出了一种基于电场控制的液晶双折射THz可调谐窄带滤波器。此滤波器可实现中心频率从0.94THz到0.996THz的连续可调谐窄带(0.02THz)滤波。6、提出了一种由两个点缺陷和上下载波导构成的波分复用和窄带滤波器的设计方案。当输入波导和输出波导平行时,这种光子晶体结构的透射谱中有两个窄带峰,其最小宽度可到0.015THz,可作为一种很好的波分复用系统。而当输入波导和输出波导垂直时,其中一个峰消失,此时可做为一种较好的窄带滤波器。更多还原

【Abstract】 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.更多还原