Topological Localization And Transport Effects In Photonic Crystals

Localization and transport of optics are the core of modern photon devices Photonic crystals,as a new type of optical functional materials,show great potential in optical manipulation.Utilizing the band-dispersion theory in periodic media,photonic crystals can achieve optical localization effects such as defect states,cavity modes,and novel transport effects such as negative refraction,zero refraction,and total reflectionHowever,due to the inevitable disorder and defects during processing,the manipulation performance of traditional photonic crystals is unstable.In recent years,inspired by the concept of topological phases in condensed matter physics,many topological photonic effects based on topological band theory have been proposed such as optical quantum Hall effects,optical quantum spin Hall effects,optical quantum valley Hall effects,and optical higher-order topological insulators.The topological interface states or local states in these effects carry properties of defect and backscattering immunities,providing a new way for optical manipulation.Combined with the existing semiconductor processing technology,these effects have potential applications in topological waveguides,topological lasers,and topological resonatorsThis thesis mainly studies the topological optical localization and transport effects in photonic crystals,including TE-TM coupled bound states in the continuum,higher-order topological insulator with zero-dimensional corner states,zero-dimension topological states of selective excitation by chiral source in higher-order quantum spin Hall effects,and tunable group velocity achieved by the exceptional concentric ring in non-Hermitian stacked materials.The details can be listed as follows1.We construct TE-TM coupled bound states in the continuum based on bilayer photonic crystal slabs.The mismatch in stacking direction significantly enhances TE-TM coupling,resulting in TE-TM coupled bound states.These states protected by symmetry are located in the center of first Brillouin zone.Different from traditional bound states with one polarization,our study expands the conditions of bound states in the continuum,providing a basis for TE-TM hybrid devices(e.g.lasers).2.We demonstrate all-dielectric higher-order topological insulators by theory and microwave experiments.Our second-order photonic insulators protected by crystalline symmetry do not require negative coupling.By sweeping frequencies,one-dimensional interface states and zero-dimensional corner states locating in the bandgap can be distinguished.Three-order topological insulators can be constructed and experimentally verified in sonic crystals,which have the potential for realization in optical systems3.We propose and experimentally demonstrate zero-dimensional corner states selected by chiral sources in higher-order quantum spin Hall effects of optics.A pair of dipole modes px,y and a pair of quadrupole modes dx2-y2,2xy form the pseudospin-dependent dx2-y2±id2xy topolcal interface transport.The topological highly symmetric index and microwave experiments demonstrate pseudospin-dependent corner states can be selectively excited in spatial positions.4.we propose a gapless topological pnase ciassitication ana exceptional concentric rings in non-Hermitian stacked photonic crystals.Two-layer photonic crystals are coupled site-to-site through silicon pillars,which is analogous to a four-level tight-binding model.Exceptional concentric rings labeled by new global topological invariants provide a versatile platform to control group velocity in stacked materialsIn short,this thesis conducts a comprehensive study in theoretical exploration,model design,numerical simulation,and experimental verification,showing the potential of topological manipulation in optical(quantum)communication(or computing)and sensing.