Research On High-Q Terahertz All-Dielectric BIC Metasurfaces

High-quality factor(Q)all-dielectric metasurfaces have important application prospects in the fields of high-sensitivity sensing,lasers,narrow-band filtering and nonlinear resonance due to the strong local electromagnetic field enhancement effect.The recent progress in the study of Bound states in the contimuum(BIC)shows that the high-Q resonance of metasurfaces in the optical band has been developed rapidly.Optical second and third nonlinear harmonics were also observed.However,in the terahertz band,it is still a challenging task to obtain high-Q resonance in metasurfaces due to the absorption loss of materials,the design and processing of resonance unit structures,and the high-signal-to-noise ratio high-resolution spectral measurement.The highest Q value experimentally obtained internationally is currently only ～ 250.This paper focuses on the high-Q resonant all-dielectric BIC metasurface in the terahertz band.The main research work and results are as follows:(1)Several terahertz all-dielectric metasurfaces are designed based on symmetryprotected BIC-based quartz-substrate silicon structures.The transmission spectrum and near-field electromagnetic field distribution of the metasurface are calculated by electromagnetic wave numerical simulation.Multipole resonance and eigenmode analysis show that the metasurface supports symmetry-protected longitudinal toroidal dipole BIC.The influence of the metasurface structure parameters on the BIC frequency is studied,and the parameter design method of the BIC metasurface working in the large frequency range of terahertz is given.The paper also calculates the refractive index sensing characteristics of the metasurface based on the high-Q quasiBIC resonance.The sensitivity of the sensor is 254 GHz/RIU(Refractive index unit)and the figure of merit(FOM)is 2423.(2)An all-silicon terahertz high-Q BIC metasurface composed of tetrameric air holes was designed and experimentally realized.This metasurface supports two Friedrich–Wintgen(F–W)BICs in the range of 0.2 ～ 1.0 THz.Two methods are proposed to collapse the F–W BIC into a quasi-BIC Fano resonance with finite Q by changing the structural parameters.The designed samples were prepared by deep reactive ion etching method,and measured by a high-resolution terahertz frequencydomain spectrum system.The simulation and experimental measurement results were analyzed and compared.The Q value of the polarization-insensitive quasi-BIC resonance obtained experimentally is as high as 1049,which is more than 4 times the highest Q value reported by the international community.Moreover,using the high-Q metasurface,we demonstrate a terahertz intensity modulation with low optical pump power.The above research results will play a powerful role in promoting the research and application of international high-Q terahertz metamaterials.