On Wave Propagation And Acousto-optical Coupling In Phoxonic Crystal Structures

Phoxonic crystals(PXCs)are periodic structures which possess photonic and phononic bandgaps simultaneously.PXCs can be applied as systematic platforms for manipulating optical and acoustic waves simultaneously,and can be utilized in various fields such as optical,acoustic and acouto-optical(AO)devices,and cavity optomechanics.Based on finite element method,several problems related to PXCs are investigated in this thesis,including modification of dual bandgaps,propagation characteristics of bulk and boundary modes,AO coupling in cavities,and design of devices.The main contents and conclusions include:1.Two-dimensional(2D)PXCs with vein-topology are proposed.This kind of PXCs can generate large photonic and phononic bandgaps simultaneously.The propagation characteristics of optical and acoustic bulk modes,and surface modes are investigated.The surface-mode cavity and the air-slot cavity are proposed,and the AO coupling in such cavities is studied.The results show that square and triangle lattices are favorable for the generation of large simultaneous photonic and phononic bandgaps for bulk waves.According to different structures of the surface,the acoustic surface wave may appear as a coupling-resonant mode or a locally resonant mode.The optical and acoustic surface modes can be modified efficiently by changing the surface structure,besides the optical and acoustic surface modes(or the bandgaps for surface waves)can be easily obtained.If the acoustic cavity mode shows even-symmetry,and the field overlap between the optical cavity mode and acoustic cavity mode is high,the AO coupling strength increases.Due to the high field overlap between the optical cavity mode and acoustic cavity mode,the air-slot cavity exhibits stronger AO coupling than the surface-mode cavity.2.Based on the 2D PXCs with vein-topology,PXC slabs which possess simultaneous photonic and phononic bandgaps are proposed.The propagation characteristics of optical and acoustic bulk modes,and side-surface modes are investigated.The air-slot PXC slab cavity is proposed,and the AO coupling in this cavity is studied.The results show that this kind of PXC slabs can simultaneously generate large photonic even mode(or odd mode)bandgap and phononic bandgap in a wide range of geometrical parameters.The optical and acoustic side-surface modes can be modified efficiently by changing the side-surface structure,besides the optical and acoustic side-surface modes(or the bandgaps for side-surface waves)can be easily obtained.For the air-slot PXC slab cavity,the high field overlap between the optical cavity mode and acoustic cavity mode leads to a strong AO coupling where the moving interface effect plays a dominant role.3.Two kinds of three-dimensional(3D)PXCs with vein-topology are proposed.Both kinds of the PXCs can generate photonic and phononic bandgaps simultaneously.For the first kind the scatterers are located at the lattice points and connected by thin cylinders;the second kind PXC is an open periodic structure formed by drilling sphere holes in silicon matrix.The propagation characteristics of optical and acoustic bulk,surface and edge modes are investigated.The effect of the geometrical parameters of the defects on 3D PXCs with point-defects and line-defects are discussed.The results show that the first kind of 3D PXCs is favorable for generating large simultaneous photonic and phononic bandgaps while the second kind can be easily fabricated in labs.The optical and acoustic surface modes can be modified efficiently by changing the surface structure.The optical and acoustic surface modes can be easily obtained.However,it is difficult to obtain dual bandgaps for surface modes.The optical and acoustic edge modes can be modified efficiently by changing the edge structure.To obtaining dual edge modes,it is better to obtain optical edge modes first,and then obtain acoustic edge modes by changing the edge structure since the optical edge modes are scarcely available.The optical and acoustic fields of the defect states are highly localized,and the defect states can be modified by changing the geometrical parameters of the defects.4.A PXC liquid sensor based on surface modes and a PXC uni-directional transmission structure are designed.The results show that PXC sensor with high sensitivities can simultaneously detect the refractive index and sound velocity of the liquid analyte.Compared with traditional pure photonic or phononic crystal sensors,the advantage of the PXC sensor is the multi-physical-parameter detection.Moreover,the surface waves can be excited and detected easily.For the uni-directional transmission structure,the effect of the grating diffraction and directional bandgap leads to the uni-directional transmission of optical and acoustic waves with the frequency in the directional bandgaps;and the effect of polarization mode transformation and mode bandgap leads to the uni-directional transmission of acoustic waves with the frequency in the mode bandgaps.The present analysis show that in 2D or 3D systems,the PXCs can possess the following structural topology for simultaneously large photonic and phononic bandgaps:dielectric cylinders or spheres are located at the lattice points and connected by thin dielectric cylinders.The bandgaps for bulk waves are the prerequisites for generating surface waves and surface mode bandgaps;and the bandgaps for surface waves are the prerequisites for generating edge waves.Therefore a large bandgap for bulk waves plays an important role in modifying boundary modes and related applications.