Controlling Light Absorption At Critical Coupling Through Quasi-bound States In The Continuum Resonance

Metasurfaces have received increasing attention due to their extraordinary characteristics.They are composed of periodic arrays of subwavelength resonators whose arrangement and interaction can modify the properties of electromagnetic waves,such as amplitude,phase,polarization,and propagation direction.In particular,those metasurfaces made of high-index dielectric materials have recently emerged as essential elements in the field because they show a very high diversity of available functionalities while avoiding the ohmic losses associated with their plasmonic counterparts.Their non-radiative states,with near-infinite lifetime and perfect energy confinement,are desirable.The excitations of guided mode,trapped mode,toroidal mode,anapole mode,and supercavity mode fundamentally linked to the physics of bound states in the continuum(BIC)are nicely observed in dielectric metasurfaces.In recent years,enhancing the light absorption of two-dimensional(2D)materials with photonic structures has been demonstrated for high-efficiency optical and photonic devices.Jessica R.Piper designed the complete controlling of the critical coupling within a hybrid graphene/photonic crystal structure,where graphene absorbs up to 100% of light in the optical regime when the radiation rate of the guided mode in the photonic crystal is equal to the absorption rate in graphene.Afterwards,this mechanism inspired various designs of photonic crystal integrating with the whole family of 2D materials,including graphene,transitional metal dichalcogenides,black phosphorus,and halide perovskites.Actually,the radiation engineering is in principle a general way for controlling light absorption,not limited to a specific mode in a certain resonator,including guided mode in photonic crystal,but also applicable to other resonance modes in seemly different structures.With this consideration,the potential of dielectric metasurfaces for critical coupling is highly desirable in that they can provide unprecedented diverse arrangements and thus achieve full control of light absorption,providing promising strategies in controlling light absorption of 2D materials.In this dissertation,through theoretical analyses and numerical simulations,We establish a direct connection between the radiation engineering and the bound states in the continuum(BIC)by constructing metasurfaces and graphene composite structures,and apply them to light absorption enhancement,absorption bandwidth of light modulation.The main achievements are as follows:We develop a general method to control light absorption at critical coupling through the quasi-BIC resonance,and demonstrate it in a typical single-mode,two-port system composed of graphene coupled with silicon nanodisk metasurfaces.When the radiation rate of the magnetic dipole resonance is equal to the dissipate loss rate of graphene,the critical coupling condition is satisfied and the maximum absorption of 0.5 is obtained.By simultaneously changing the asymmetric parameter of metasurfaces,the Fermi level,and the layer number of graphene,the absorption bandwidth can be flexibly adjusted more than two orders of magnitude from 0.9 nm to 94 nm in the near-infrared.More importantly,the quadratic dependence of the absorption bandwidth on the asymmetry parameter is verified,which reveals out the key role of BIC physics in the radiation engineering.