Researchs On The Metasurface-based Fano Resonance And Its Application

Since its discovery in atomic physics,the asymmetric Fano resonance has been a characteristic feature of interacting quantum systems.Later,people found that the Fano resonance can also be induced in photonic crystals,electromagnetic metasurface and other structures,and they have great potential for applications in sensors,lasers,optical modulators,and so on.In this paper,based on the finite-difference time-domain(FDTD)method,we study on the optimal design of Fano resonance type electromagnetic metasurface and their related optic applications.The main contents include the following aspects:(1)By designing a metasurface which consists of two coplanar perpendicular nanostrips with different dimensions,an amplitude modulation and frequency tuning of Fano resonance is realized.The simulation results show that the Fano resonance is induced by destructive interference between the bright dipole mode of a short nanostrip and the dark quadrupole mode of a long nanostrip.The strength and line width,the resonance frequency of the Fano resonance can be actively modulated by changing the spatial separation of those two graphene nanostrips and the Fermi energy of the graphene nanodimer,respectively,without re-fabricating the nanostructures.The tuning of the strength and resonance frequency can be attributed to the coupling strength and optical properties of graphene,respectively.Importantly,a figure of merit value as high as 39 is achieved in the proposed nanostructures.Our results may provide potential applications in optical switching and bio-chemical sensing.(2)A high performance refractive index sensor based on Fano resona nce with figure of merit(FOM)about 56.6,is proposed by designing a dielectric metasurface with a periodically arranged silicon split-rings.The silicon split-rings have two equal splits dividing them into pairs of arcs of different length.Numerical simulation results show that a Fano resonance with quality factor~133 and spectral contrast ratio~100%arises from destructive interference of two antiphase electric dipoles in the two arcs of the split-ring.We can turn on and/or off the Fano resonance with a modulation depth nearly 100%at the operating wavelength of 1067 nm by rotating the polarization of incident light.We believe that our results will open up avenues for the development of applications using Fano resonance with dynamicall y controllability such as biochemical sensors,optical switching and modulator.(3)A high performance optical modulator based on Fano resonance is realized by designing a hybrid graphene-dielectric matasurface.The unit cell of the dielectric metasurface consists of a planar asymmetric silicon split-ring pairs in the mirrored configuration,which can support strong Fano resonance with narrow line width(~0.77 nm)and high quality(Q)-factor(~1702),contrast ratio(~100%).Numerical simulation results show that the transmission amplitude of the Fano resonance can be efficiently modulated by varying the Fermi energy E_F when the graphene layer is integrated with the dielectric metasurface,and the max transmission coefficient difference up to 78%is achieved indicating that the proposed hybrid graphene/dielectric metasurface have a good performance as an optical modulator.The effects of the asymmetry degree of the ASSRRs on the Fano resonance are studied and the efficiency of transmission amplitude modulation of graphene is also investigated.Our results may also provide potential applications in optical filter and bio-chemical sensing.(4)By designing a Dirac semimetal based metasurface which contains a strip and a pair of square split-ring in its cell unit,an amplitude modulated Fano resonance is realized.The results show that Fano resonance arises from the interaction between the bright mode excited by the stripe and the dark mode excited by the square square split-ring pairs.Subsequently,we study the Fermi energy of the Dirac semi-metal and the effect of the longitudinal offset between the geometric center of the strip and the square split-ring on the optical response of the metasurface.The results show that the transmittance at the frequency of resonance dip can be effectively controlled by changing the Fermi energy,and the absolute value of the maximum transmittance difference is more than 60%.The transmittance of the resonance peak can be effectively controlled by longitudinal offset,and the absolute value of the maximum transmittance difference is more than 60%.Therefore,the proposed Dirac semi-metal based metasurface has great potential application in optical modulators.