Novel Effects In Non-Hermitian Photonic Devices

In recent years,research on non-Hermiticity has attracted widespread attention.The study of Parity-Time Symmetry revealed the importance of non-Hermitian Hamiltonian.It can not only describe open systems,gain/loss systems but also generate real eigenvalues corresponding to observable physical quantities.As a result,the research on non-Hermitian nature has greatly expanded the scope of people's research and improved people's understanding of non-Hermitian systems.The non-Hermitian characteristics of electronic systems are difficult to adjust as desired in real experimental systems,while optical systems have gradually become an excellent experimental platform that can be compared to electronic systems due to their advantages of easier modulation,less impurities,easier fabrication,and easier characterization.The band structure of photonic crystals can be directly compared with and inspired by the band structure of electronic materials.Besides,by adjusting the strength and spatial distribution of gain/loss,nonHermitian potential provides the system with an additional degree of freedom,which can modulate the system's eigenvalue and energy band structure.Non-Hermitian degrees of freedom can also bring new optical phenomena and more powerful optical control capabilities.Non-Hermitian modulation is perfectly compatible with many optical devices and optical systems,such as optical waveguides,photonic crystals,optical resonators,and optical superlattices.This enables optical systems and nonHermitian research to complement each other.This thesis mainly focuses on optical non-Hermitian research.By combining optical non-Hermitian modulation with traditional optical devives,new phenomena and novel effects are explored.The details are listed as follows:1.We design a Parity-Time Symmetry grating.This grating is an alternating combination of a silicon waveguide and a silver-coated silicon dioxide waveguide,which introduces optical modulation of the real and imaginary parts respectively.By accurately designing the parameters of the grating,we successfully constructed an asymmetric diffraction order at 1550 nm.Besides,this asymmetric diffraction effect is polarization-dependent and only corresponds to the TM polarization.We successfully construct the exceptional point by tuning the distance between the silicon waveguide and the silicon dioxide/silver composite waveguide.We also demonstrate that the asymmetric diffraction of the grating exists over a wide incident angle range.2.We design an optical multilayer structure based on PT symmetry exceptional point.In the near-infrared regime,optical loss is introduced through the III-V semiconductors with smaller band gaps.The optimal solution of the thickness and distribution of the lossy layer is found by the particle swarm optimization algorithm so that this multilayer structure displays an asymmetric reflectance at the communication band of 1550 nm,that is,the forward reflectance or the backward reflectance of the multilayer are large differences.More importantly,the reflectivity on one side can theoretically reach zero under precise control.Under this condition,an external light source is incident into the structure,and the photo-generated carriers generated by the photoelectric effect of the semiconductor material can cause the change of refractive index.By measuring the change in contrast between the two sides of the reflectance,the purpose of detecting the external light source is achieved.3.Based on the coupled resonant optical waveguide,a two-dimensional honeycomb lattice photonic crystal is designed.The recombination of the hexagonal lattice and the kagome lattice makes the Dirac points of the double degeneracy and the triple degeneracy appear at different high symmetry points in the Brillouin region of the photonic crystal.The special design of the compound lattice causes destructive interference in the structure,which results in flat band dispersion across the whole Brillouin zone in the telecommunication region.The unidirectional coupling of clockwise mode and a counterclockwise mode in the waveguide can form an artificial gauge field,thereby achieving pseudo-spin orbital coupling,which opens the Dirac point degeneration and forms a gapless edge state between the flat band and adjacent bands.Based on this topologically protected edge state,we designed and simulate two optical devices: an irregular shape topological microcavity and a beam splitter.4.A one-dimensional Su-Schrieffer-Heeger model based on optically coupled resonant optical waveguides is designed.By using the unidirectional coupling characteristics of coupled resonant optical waveguides and combining it with nonHermitian modulation,we successfully achieve left-right asymmetric coupling.When the open boundary and periodic boundary conditions are taken,the position of the exceptional point changes,which results in a special non-Hermitian bulkboundary correspondence.The bulk states of the system exhibit a non-Block skin effect,that is,the field distribution of bulk state is mainly confined at the boundary.We construct a tight-binding Hamiltonian model and calculate its phase transition point positions under open boundary conditions and periodic boundary conditions respectively.We also simulate the one-dimension CROW chain and obtain a projection band of the optical structure through the finite element method.