Mechanism Study On Several Novel Effects In Non-hermitian Phononic Crystals And PT-symmetric Metasurfaces

In quantum mechanics,because the measured energy of the system can only be real,the physical properties of Hermitian systems become one of the mainstreams of researches in quantum mechanics.Since the non-Hermitian Hamiltonian with paritytime(PT)symmetry can also have real energy spectra is proposed,non-Hermitian systems have received great attention.However,limited by the experimental conditions,the application prospect of non-Hermitian system has always been in a controversial state.In recent years,with the development of artificial band gap structure and metasurface,these structures have became good carriers for constructing the nonHermitian system.By making use of the similarity between the paraxial wave equation and the Schrodinger equation,the PT-symmetric non-Hermitian system can be realized in the classical wave system.The successful realization of PT-symmetric nonHermitian system and the continuous exploration of novel effects in non-Hermitian system make it possible for the loss in classical wave system to be effectively utilized and greatly promote the development of non-Hermitian physics.At present,the researches of non-Hermitian systems constructed by artificial band gap structure and metasurface are in the ascendant.This paper conducts in-depth research in the nonHermitian phononic crystasl and PT-symmtric metasurface.The main research content and its innovative value are as follows.(1)Many non-Hermitian novel effects are found by studying the chiral edge states in non-Hermitian phononic crystal.However,the helical edge states in nonHermitian acoustic systems remains to be explored,and exploring more complex systems is easier to enrich the novel effects of non-Hermitian topological systems.Based on the phononic crystal that has realized the acoustic quantum spin Hall effect,non-Hermitian parallel and crossed structures are constructed by introducing gain and loss.The results show that not only the parallel structure can be used to realize the topological transmission with controllable intensity in different directions,but also the crossed structure can be used to realize the flat band of the topological edge state.Furthermore,because the pseudo-spin in flat-band region is insensitive to the band and wave vector,the topological transmission with controllable direction is expected to be realized in flat-band region.These phenomenon can be explained theoretically by constructing the equivalent tight-binding model of non-Hermitian phononic crystals.The above research results will further enrich the novel effects of non-Hermitian topological systems and provide theoretical support for controllable topological sound transmission.(2)The introduction of gain or loss in acoustic metasurfaces to construct PTsymmetric systems has breathed new life into the research of acoustic metasurfaces.However,the solutions of exceptional points(EPs)rely on the transfer matrix method(TMM)or parametric sweep,and these processes greatly increase the difficulty and complexity.Therefore,it is urgent to establish a simple theoretical model for the solution of its EPs.Based on PT-symmetric acoustic metasurfaces,a simple theoretical model is established for solving EPs of PT-symmetric acoustic metasurfaces using acoustic equivalent circuit theory from the physical mechanisms of novel phenomena at EPs.The EPs can be obtained by using these models without TMM and parametric sweep,and these models are proved to be correct by both theory and simulation.The results show that these models can provide universal equations for solving EPs,and can be used to analytically obtain and verify the second inherent prerequisite of the EPs realizations.Meanwhile,under arbitrary surrounding ordinary mediums,states of matter,incident wavelengths,and incident angles when two inherent prerequisites are satisfied,unidirectional antireflection negative refraction and positive refraction can be realized using PT-symmetric acoustic metasurfaces based on these universal equations.Simple theoretical models offer a new way for the solution of EPs in the PTsymmtric system,and the research idea of this work can be directly applied to optics and even other classical wave systems.In addition,the limitation of the two fixed systems in previous researches can be broken basen on the simple theoretical model,which can construct the more novel systems.(3)The realization of the novel effects in the conventional non-Herimitian PTsymmetric system depends on the accurate balance of gain and loss,which greatly limits their realization and application.In order to reduce the difficulty of realizing the PT-symmetric system,the generalized PT-symmetric metasurface and passive PTsymmetric system are proposed.However,the generalized PT-symmetric metasurface is limited to a fixed gain-loss ratio,and it does not break away from the limitation of gain.Although the passive PT-symmetric system achieves the EPs under the condition of pure loss,its construction method is more complex,and it can not achieve the novel effects such as unidirectional antireflection negative refraction.Therefore,using a more convenient method to construct the passive PT-symmetric system and realize the novel effects has become a problem to be solved in the promotion of the passive PTsymmetric system.Based on the simple theoretical model of PT-symmetric acoustic metasurface,the negative refraction medium is introduced into the surrounding areas of the PT-symmetric metasurface,and the influence of the negative refraction medium on the Poynting vector of sound waves in these areas is analyzed,and the novel effects such as the antireflection left-handed positive refraction and antireflection righthanded negative refraction,which can not be realized in previous studies,are obtained.By further extending the concept of generalized PT-symmetric metasurface,a simple theoretical model of the more generalized PT-symmetric metasurface is proposed,and a passive PT-symmetric system is built based on this simple theoretical model.Futher,the negative refraction medium is introduced into the passive PTsymmetry system,and the novel effects such as unidirectional antireflection negative refraction,the antireflection coating and coherent perfect absorption are successfully realized.The realization of novel effects in passive PT-symmetric system will further enhance the realizability and application of PT-symmetric metasurface,and provide greater design flexibility for PT-symmetric system.