Theoretical Research Of Mode Characteristic And Optimum Design Of Optical Whispering Gallery Mode Microcavity

This thesis focus on the topic "Theoretical research of mode characteristic and optimum design of optical WGM microcavity", mainly discussing the problem of directional emission and mode selection and suppression in WGM (whispering gallery mode) microcavity, instability phenomena and its explanation in staircased FDTD simulation of high Q-factor circular cavity, scattering loss estimation in microcavity, and eigenfield refraction-reflection model solution of scattering and resonance problem of circular cavity and multi-layer circular cavity, and interference modulation in multi-layer circular cavity. Our primary work includes: 1) Mode selection, suppression, directional emission, and Q-factor stability analysis in microflower cavityLike the microgear cavity, the Bragg grating grafted on the circumference of microflower cavity can effectively split the double-generate WGM with azimuthal mode number equating to the half of grating frequency, and improve the Q-factor of one mode and suppress another. Besides, the Bragg grating also suppresses all other WGM with different azimuthal mode number. The local shape deformation (including lengthening and shortening of the greting gear and deepening and shoaling of grating gap) of microflower cavity can spoil the Q-factor and lead to directional emission, and with optimal control of the deformation, high-Q unidirectional emission can be obtained.2) Artificial scattering loss in orthogonal FDTD microcavity simulation and its accuracy improvementThe staircase interface approximation in many FDTD applications leads to inaccuracy and instability in FDTD simulation results, especially for high-Q calculation in microcavity simulation. The VCM-like (volume current method) which rigorously takes account of influence of cavity to scattering source can well explains the Q-factor results and its instability in high-Q FDTD circular cavity simulation.The effective permittivity FDTD (such as Volume-average Effective Permittivity (V-EP) and Contour-Path Effective Permittivity (CP-EP), etc.) which based on the Cartesian grid with appropriate chosen effective permittivity can improve the numerical accuracy while keeping the simplicity of initial Cartesian grid FDTD and a minimal increase in calculation resource consumption. But they still can not present an accurate enough result for ultrahigh-Q calculation, a simple FDTD algorithm which has more accuracy remain attractive and necessary for ultrahigh Q-factor calculation.3) Self-consistent field description of WGM formation in circular cavity and interference modulation phenomena in multi-layer circular cavityA novel eigenfield reflection-refraction model is developed to handle the scattering or resonance problem of circular and layered circular cavity. The eigenfield self-consistent picture on the formation of whispering-gallery modes (WGMs) is more direct and visual for circular and layered circular cavity, the eigen-wavenumber equation deduced from the self-consistent picture is more compact and is more suitable for analyzing the mode spectral characteristic of WGMs. Besides, the solution of the scattering problem of circular and multi-layer circular cavity based on the eigenfield reflection-refraction model is physically meaningful.The interference modulation phenomena can be found by solving the eigen-wavenumber equation. And the modulation phenomena can be understood based on coupling of WGMs of inner circular cavity and WGMs of outer various cavities. The WGM spectral of total cavity is formed by alternant arranging the uncoupling WGM spectral of various cavities. In case of two-layer circular cavity coated by a thin dielectric layer on the inner circular boundary, The coupled WGMs form into symmetry and antisymmetry WGMs under condition of resonance between inner circular cavity and outer ring cavity. Due to coupling with outer WGMs, the inner WGMs can exhibit modulation phenomena by itself, out of matching condition, the inner WGMs distribute its field mainly in inner cavity, and hence having lower loss, under matching condition, the coupled WGMs have larger field distribution in outer ring cavity, and hence suffering larger loss. If the pump is applied only into the innner circular cavity, the gain of the inner WGMs also can take on an interference modulation characteristic. Based on this interference modulation of loss (Q-factor) and gain, a novel mode selection mechanism can be found for layered microcavity.