Fano resonance in the SWG-DBR metastructure

We study a metastructure formed by vertically integrating a subwavelength grating (SWG) with a distributed Bragg reflector (DBR). Fano resonance occurs in the reflection spectrum of the SWG-DBR metastructure under normal incidence. The Fano resonance arises from the coupling between a broadband reflection background (continuum state) provided by the top reflector (SWG and top DBR) and a narrowband Fabry-Pérot cavity mode (discrete state) formed by the top reflector and the bottom DBR, corresponding to the continuum and discrete states, respectively. The multipole decomposition verifies that the Fano resonance of the proposed SWG-DBR metastructure is dominated by the toroidal dipole. The electric field intensity of the Fano resonance has a maximum at the SWG surface, making it sensitive to the change of the interfacial refractive index. The SWG-DBR metastructure is fabricated, and the reflection spectra under different ambient refractive-index conditions are measured. The Fano resonance peak shift is observed under different liquids covering the surface of the SWG, consistent with the simulated results. The proposed SWG-DBR metastructure is a promising platform for various applications in sensing, lasing, and polariton condensation.