Angle- and polarization-independent mid-infrared narrowband optical filters using dense arrays of resonant cavities.

We report design and experimental verification of narrowband mid-infrared optical filters with transmission characteristics that are practically constant over a wide range of incident angles. The filter employs a dense array of dielectric resonant cavities in a metal film, where the transmission of each cavity depends upon localized rather than travelling fields, making the filter fundamentally angle-independent. We show experimentally a transmission around 90% from normal incidence up to 60°. Simulations show that the filter becomes polarization-independent when geometry of the cavities is azimuthally symmetric.