Controlling the spectral and polarization features of absorption in the midwave infrared (MIR) is key for detector applications. We design and fabricate ultrathin metal absorbers with a tunable spectral response. Our approach uses deep subwavelength perforations to tune the metal film's sheet conductivity with no in-plane resonances. When placed over a reflecting cavity, these films allow strong and selective absorption over a wide range of MIR wavelengths while maintaining minimal absorber volume. We further reduce the thermal mass of the device and demonstrate 70% absorption near 3 μm in a suspended, thin-film device suitable for bolometric applications. Finally, we investigate the polarimetric response of symmetry-broken perforated films for polarization-dependent MIR bolometry. We observe in-plane anisotropic responses of the films, which can be explained within our analytical model as arising from varying effective sheet permittivities along different mesh directions. These findings are promising for applications in MIR bolometry.
Guadagnini et al. (2025) studied this question.