Research On Phoxonic Bandgaps And Dirac-cone Dispersions

Phoxonic crystals(PxCs)are artificial structures with acoustic and optical parameters arranged periodically in the same lattice.In addition to simultaneous photonic and phononic bandgaps,PxCs also inherit the abundant bulk band characteristics of photonic and phononic crystals.Therefore,PxCs provide a systematic platform for the simultaneous manipulation of light and sound,and show a very broad application prospect in the fields of optics,acoustics,cavity-optomechanics,and phoxonic multifunctional devices and so on.Herein,based on the finite element method,by designing phoxonic band structures properly,we achieved a single-mode PxC waveguide,a topological PxC cavity,a polarization-independent second-order topological photonic insulator and a PxC with simultaneous optical and acoustic zero refractive-indices(ZRIs).The main contents and results are as follows:1.A PxC waveguide with the glide symmetry was designed,in which both electromagnetic and elastic waves can propagate along the glide interface at the same time.Due to the band-sticking effect,super-cell bands of the waveguide degenerate in pairs at the boundary of the first Brillouin zone(FBZ),causing the appearance of gapless guided-modes in the supercell bandgap.The gapless guided-modes are single-modes over a relatively large frequency range.By adjusting the magnitude of the glide dislocation,the dispersion of guided-modes and edge bandgaps can be further adjusted,so as to achieve photonic and phononic single-mode guided-bands with relatively flat dispersion.The proposed waveguide has potential applications in enhancing stimulated Brillouin scattering and designing multifunctional devices,like acousto-optical sensors,optomechanical filters and so on.2.A second-order topological PxC was proposed,in which the topology of bands is characterized by the 2D Zak phase.The topological invariant is characterized by the 2D bulk polarization.Corner boundaries formed by topologically distinct PxCs can host the coexistence of photonic and phononic topological corner states.Numerical simulations show the corner states and their interaction strength are robust to geometric perturbations.The work opens a venue to realize the topological confinement of photons and phonons simultaneously,and provides a platform for studying the interaction between the second-order photonic and phononic topological states.The proposed PxC has potential application value in topological optomechanical devices.3.On the basis of the last work,a polarization-independent second-order photonic topological insulator(SPTI)was proposed,which is made of anisotropic dielectric materials.It was found that the location of in-gap corner states of TM and TE polarizations in the frequency spectrum can be independently controlled by the out-of-plane permittivity ε⊥ and in-plane permittivity ε‖,respectively.From this,we achieved topological corner states for the two polarizations with the same frequency by adjusting ε⊥ and ε‖ and their robustness against disorders and defects are numerically demonstrated.The proposed SPTI provides a potential application scenario for polarization-independent topological photonic devices.4.A PxC with simultaneous acoustic and optical effective ZRIs was designed by subtly constructing low-frequency phononic and photonic Dirac-cone dispersions in a triangular lattice.For phononic modes,we use the band folding mechanism to fold a symmetry-protected Dirac cone from the corner to the center of the FBZ to construct a low-frequency double-Dirac cone that can be used to achieve the acoustic ZRI.For the photonic mode,due to the high contrast permittivity between scatterers and air,a low-frequency photonic Dirac-like cone can be obtained by adjusting geometric parameters.Since the phononic double-Dirac cone is not sensitive to geometric parameters,the properties of acoustic ZRI will not be destroyed in the process of realizing photonic Dirac-like cone.When the low-frequency phononic and photonic Dirac-cone dispersions coexist,the PxC can be mapped into a kind of ZIM.The new mechanism for simultaneous control of sound and light helps to achieve some new applications,such as synchronous cloaking and unidirectional transmission for both sound and light,which are further demonstrated by numerical simulations.