Fano resonances in periodically nano-modulated thin films are known to produce narrow bandgaps in both frequency and angular domains [1,2]. At resonance, the incident wave couples to the planar waveguiding mode of the film, resulting in zero transmission and 100% reflection [3]. The bandwidth of these resonances is proportional to the coupling coefficient between the incident radiation and the waveguiding mode. In this work, we demonstrate that combining two closely spaced resonances can create extremely narrow high-pass bands in transmission, with a theoretically tunable passband width that can approach zero in wavelength-angle space.
Using these principles, we fabricate single-layer gratings and experimentally measure their transmission characteristics. By tilting two identical filter structures at a small angle relative to each other, obtaining a high-pass window in the angle-wavelength domain. This approach enables control over the spectral and angular properties of the transmitted light by changing the tilted angle, offering potential applications in compact optical system.
This work has received funding from the Research Council of Lithuania (LMTLT), agreement no. S-LT-TW-24-9.