Skin depth effects in wavelength-selective infrared microbolometers based on lossy frequency selective surfaces

Past work has discussed infrared absorption using a patterned thin resistive sheet as the frequency-selective absorber for use in wavelength-selective long wave infrared (LWIR) microbolometer focal planes arrays. These patterned resistive sheets are essentially slot antennas formed in a lossy resistive ground plane layer placed a quarter-wavelength in front of a mirror. Design studies have shown that for efficient IR absorption cross-shaped slots require a lossy sheet with the optimized sheet resistance. For realistic metal layers, however, the skin effect produces a complex surface impedance that can be quite large in the LWIR band. In this paper we consider metal layers of thickness between one and three skin depths as the absorber layer instead of a thin resistive sheet layer, and show that the thick metal layers can still produce excellent absorption in the LWIR.