Cavity-coupled narrow bandwidth Fano resonance in dielectric metasurface

Dielectric metasurfaces show great potential in light manipulation for applications such as optical filters and semiconductor metasurfaces for generating entangled photons. These applications critically rely on resonant mode properties in micro/nano-structured metasurfaces. Several modes including electric dipole (ED), magnetic dipole (MD), and quasi-bound state in the continuum resonance have been widely explored with the merits of high quality factor and narrow bandwidth. However, these reported metasurface resonances generally do not absorb light due to the transparency of dielectric materials, which limits their usage in some applications such as thermal emission and photon detection etc. In this work, we demonstrate a new class of dielectric metasurfaces, which exhibit the features of both strong light absorption and narrower bandwidth. Our fabricated metasurface consists of a Si cuboid array on top of a SiO2 spacer layer backed with a metallic Cu film. The cavity mode within the SiO2 spacer couples with electrical dipole in the Si cuboid, leading to an asymmetric narrow bandwidth Fano resonance within our metasurface structure. Our spectral measurements show that the Fano resonance occurs at the wavelength of 4.19 μm. It has a strong absorption efficiency of 65.8% and a narrow bandwidth of 32.5nm, corresponding to a quality factor as high as 112. As an example to reveal the potentials of our metasurface, the Fano resonance is applied in refractive index sensing with a sensitivity of 518.75nm/RIU and a high figure-of-merit (FoM) of 14.82 RIU-1. These results indicate that cavity-coupled dielectric metasurface presents an effective way for obtaining both strong light absorption and narrow bandwidth, opening new space for metasurface applications.