Novel specular-reflection bandpass filter based on composite metagratings

Spectral bandpass filters serving as crucial signal processing components are widely applied in various electromagnetic/optical systems. However, the filtering of reflection signal always faces problems of notably accompanied transmission or strict angular limit. Here, we propose novel transmissionless metagratings for specular-reflection bandpass filtering under wide oblique incidence angles. The metagratings are composed of metallic groove gratings and high-refractive-index dielectric particles, which of them respectively provide continuum of localized cavity mode and discrete magnetic dipole resonance under oblique transverse magnetic waves, and produce broadband and narrowband anomalous reflection. Their destructive interference gives rise to a Fano-type narrow bandpass filter in the specular reflection. By integrating CaTiO 3 ceramic particles with metallic groove gratings, we experimentally demonstrate a single-bandpass metagrating filter at microwave frequency which exhibits a low insertion loss and narrow bandwidth under wide-angle oblique incidence, and agrees with simulations well. Furthermore, by arranging two different sized CaTiO 3 ceramic particles or changing the temperature, the metagrating filter is endowed with dual-bandpass or thermally tunable feature, respectively. Finally, the design is extended to terahertz frequencies by improving the structure to a double-groove configuration. The proposed composite metagratings with novel bandpass filtering function and angular insensitivity not only enrich the category of spatial filter but have promising applications in microwave communication systems and optical devices.