A Linear‐to‐Linear Cross‐Polarization Conversion q‐BIC Metasurface for Transmission Mode Mid‐Infrared Spectral Filtering

Abstract Metasurfaces supporting high‐quality‐factor quasi‐bound states in the continuum (q‐BIC) modes typically operate in reflection mode. By using the chiral circular polarization conversion property, q‐BIC can be applied to transmission‐mode wavefront control. However, in applications involving broadband mid‐infrared spectral filtering, it is challenging to generate broadband circularly polarized light owing to the absence of achromatic waveplates in MIR. In this work, a metasurface that supports linear‐to‐linear cross‐polarization conversion using dual symmetry‐breaking perturbations is experimentally demonstrated. The new device is experimentally shown to have a polarization conversion efficiency of 30% that exceeds the theoretical limit of 25% for a single‐layer, single‐resonance, thin metasurface. The improved efficiency is achieved via inter‐mode coupling that is supported by the dual perturbation metasurface. A Q‐factor of ≈60 is obtained, enabling narrowband spectral filtering using q‐BICs in transmission mode. The narrowband filtering and low angular dispersion of the incident light of the new metasurface are exploited to demonstrate imaging‐based spectroscopy, including the reconstruction and real‐time monitoring of butane gas absorption spectra and polarization‐encrypted spectrally selective imaging.