Narrowband mid-infrared transmission filtering of a single layer dielectric grating

We experimentally demonstrate a simple narrowband transmission filter using a silicon/air grating, exhibiting broadband high-reflectance between 8 and 14 μm and narrow transmission peaks at slightly off-normal incidence. We explain the response in terms of guided modes in a dielectric slab waveguide, with numerical modal analysis corroborating an intuitive slab waveguide picture and the experimental results. The filtered wavelength depends predictably on the grating period, making wavelength selection straightforward, while the length-scale independent operation principle shows promise for enabling similar filtering capabilities at higher frequencies.