Novel Electromagnetic Phenomena And Applications Based On Planar Metasurfaces And Photonic Crystals

Artificial micro-nanostructures are the man-made micro-and nanoscale structures introduced by artificial design and fabrication.As the analogue of atoms and molecules in natural materials,they can manipulate macroscopic electromagnetic characteristics of materials significantly.The representative ones are planar metasurfaces and photonic crystals.Metasurfaces can change electromagnetic responses of bulk materials by introducing subwavelength meta-atoms on the surface with fine arrangement,leading to the manipulation towards multiple features of electromagnetic waves such as amplitude,phase,and polarization.Photonic crystals,in which periodic dielectric functions play the role of periodic potentials in electronic materials,introduce the concept of photonic bands and bandgaps that can control the behavior of photons.Both of them can effectively modulate electromagnetic waves in free space,bringing important applications in optical communication,sensing,imaging and so on.In this thesis,several emerging physics phenomena are explored based on the platform of metasurfaces and photonic crystals,such as electromagnetically induced transparency,polaritons,photonic topological states,and bound states in the continuum.On this basis,a series of optical micro-and nanodevices are proposed by electromagnetic simulations,realizing a variety of functions and applications for the propagation and localization of electromagnetic waves.The main contents are as follows:1.Tunable electromagnetically induced transparency in active metasurfaces is studied.Firstly,an all-dielectric active metasurface composed of phase change material Ge₂Sb₂Te₅ and silicon is proposed,and realize tunable electromagnetically induced transparency in near-infrared region.The proposed metasurface can be applied to all-optical switches and logic gates.Then,another active metasurface composed of phase change material vanadium dioxide and metal is introduced to achieve tunable multi-peak electromagnetically induced transparency in terahertz region,which has potential applications in future wireless 6G communication devices.2.Polaritonic resonances based on metasurfaces made of two-dimensional materials are studied.At first,an atom-thin metasurface based on the monolayer hexagonal boron nitride is proposed,which can support surface phonon-polaritonic resonances with high quality factor and ultra-deep subwavelength feature in mid-infrared region.The accompanied interesting effects,such as enhanced spontaneous emission,phonon hybridization,and local field enhancement,are further revealed.Secondly,surface plasmon-polaritonic resonances in visible and infrared regions are proposed in nanopatterned borophene,whose capability of refractive index sensing are also investigated in detail.Finally,based on the hybrid two-dimensional material hetero-nanostructure consisting of multilayer hexagonal boron nitride and graphene,a novel concept,polariton induced transparency,is introduced in mid-infrared region.Its multiple functions,including slow light,refractive index sensing,gas sensing,molecular vibration sensing,and dual-band absorption,are further explored.3.Novel photonic topological states called collective topological corner states based on photonic crystal slab supercell arrays are proposed.Utilizing the two-dimensional Su-Schrieffer-Heeger model,coupled topological corner states that are extended to the whole arrays are realized.These collective corner states possess distinct physics features from conventional isolated corner states,such as dispersive bands,nondegenerate eigenfrequencies,and coupling-related behaviors.They can be sufficiently excited by the far-field source where they show high quality factors,and polarization angle of the source can selectively excite these states or their superposed states.Especially,they exhibit a collective response,which means their quality factors will increase with the extension of the array size.The multipole expansion reveals their multipolar nature:they are dominated by the toroidal dipole and magnetic quadrupole.Further investigation also reveals that they still exist in topological nontrivial defects.Finally,by introducing metal reflectors,topologically protected electromagnetic absorption is realized.4.Based on the collective topological corner states,their interactions with two-dimensional materials are further explored.By assembling hexagonal boron nitride onto the photonic crystal array,the localized topological phonon polaritons are realized by the strong coupling between collective corner states and optical phonons,and the corresponding topological protection is also verified.Besides,by covering graphene onto the photonic crystal array,collective corner states can be actively tuned by the Fermi level of graphene.Finally,utilizing the hybrid two-dimensional material integrated heterostructure,active control of strong coupling is achieved.This work has potential applications in large-area active topological devices and topological polaritonic devices.5.Polarization-independent bound states in the continuum atΓpoint in symmetric photonic crystal slabs are investigated.As the analogue of the two-dimensional Su-Schrieffer-Heeger model,several schemes are proposed to realize the transition from bound states in the continuum to their quasi-states without breaking the in-plane structural symmetry,thus maintain their insensitivity to the polarization of the incident source.Meanwhile,these schemes endow them with different frequency dispersive behaviors,and further studies show that they are the lowest-order odd and even modes that can match the symmetry of plane waves.Quality factors of these quasi-states follow the inverse quadratic law,and the multipole expansion reveals the strong correlation between the mode parity and multipole dominance,as well as anomalous behaviors of quality factors.Finally,cases under the anisotropic coupling modulation,as well as influences from material losses and substrates,are discussed in detail.This work proposes new approaches to tailor the bound states in the continuum,and can be applied as microcavities with high quality factors in planar photonic devices,leading to feasible applications in nonlinear enhancement,multispectral sensing,and low threshold lasers.