The Physical Properties And Applications Of Bound States In The Continuum In All-dielectric Metasurfaces

Bound states in the continuum（BICs）were first proposed in quantum mechanics,as a kind of the eigenstate where the wave function can propagate to infinity,but the energy remains localized in the continuous spectrum.Subsequently,similar situations have been found in wave systems such as water waves,acoustic waves,and light waves.Among them,optical BICs are the most widely studied.In particular,quasi-BICs in dielectric metasurfaces have great advantages in enhancing the interaction between light and matter due to their significant near-field enhancement,controllable radiation loss,and high thermal damage threshold.In recent years,with the deepening of two-dimensional materials research and the maturity of micro-nano processing technology,the exciton-polaritons formed by the interaction between optical modes in optical microcavities and exciton modes in transition metal chalcogenides（TMDCs）can improve the study temperature of the Bose-Einstein condensation from a low level to the room level,with a far-reaching impact on quantum information researches.Based on this,that by constructing a coupling system consisted of quasi-BICs and TMDCs not only can effectively avoid metal absorption loss,but also can improve the overlap of the two near-field modes.It simplifies the manufacturing process,and provides a good platform for the investigate of exciton polaritons.In this paper,by systematically studying the physical properties of BICs,we explore their applications in enhancing the strong photon-exciton coupling.The main research contents are as follows:（1）We theoretically investigate the strong coupling between excitons in monolayer WS₂and quasi-BIC.By breaking an in-plane inverse symmetric of the Ti O₂nanorods,the magnetic dipole in the z direction can be excited.This mode not only radiats the electric field outside the structure to interact with the monolayer two-dimensional material on its surface,but also can control the radiation loss by changing the asymmetry of the nanobars.Through numerical simulation of the coupling system composed of monlayer WS₂and Ti O₂nanobars metasurface,A remarkable spectral splitting of 33.83 me V and typical anticrossing behavior can be observed in the absorption spectrum.It is found that the balance of line width of the quasi-BIC mode and local electric field enhancement should be considered since both of them affect the strong coupling,which is crucial to the design and optimization of metasurface devices.（2）We theoretically explore the dynamical process of the strong coupling between excitons and quasi-BIC.It is found that in the previous coupled systems,the harmonic oscillator model was used to analyze strong coupling between the eigenmodes,but did not calculate the response function（absorption spectrum）of the coupling system to the incident light.Based on this,we combine the tight-binding method to expand temporal coupled mode theory.More importantly,in our theoretical analysis,the dynamic process between the excitons and the resonant modes is clearly described,and the analytical description of the absorption of the coupled system is explicitly given.As a proof of demonstration,we investigate a hybrid structure composed of monolayer WS₂placed on top of Ti O₂nanodisk metasurfaces supporting magnetic dipole quasi-BIC resonance.The simulated results show that a clear spectral splitting of the Rabi splitting 29.33 me V appeared in the absorption curve,which agrees well with our proposed theory.Moreover,we applied the approach to tailor the absorption spectrum in the various regimes from weak coupling to strong coupling.（3）We study the strong coupling between the excitons and quasi-BIC resonance in a bulk Transition metal chalcogenides（TMDCs）metasurface.Owing to the smaller Rabi splitting of the previous coupling system,we design a self-hybridized exciton-polaritons nanodisk metasurface by employing the bulk WS₂,in which the spatial overlap between the quasi-BIC resonance and exciton mode can be realized,resulting in a growth of the Rabi splitting value from 33 me V to 159me V.Specific practices are as follows:Firstly,we setting the oscillator strength f₀=0 and then perform an eigenmode analysis without substrate to confirm the existence of the BIC in our proposed bulk WS₂nanodisk measurface.In order to facilitate the design of the self-hybridized exciton-polaritons in bulk WS₂nanodisk measurface,we study the optical response of the exciton mode and the quasi-BIC mode with different structure parameters.The results show that the height has a great influence on the absorption of the exciton mode and the transmission,thus it remains the same in the subsequent structure design.Further,we describe the coupling scene by constructing a Hamiltonian model containing the surplus excitons based on the temporal coupled mode theory,enabling a discussion of the coupling state evolution through considering surplus excitons.