Tunable guided-mode resonance filters operating in the longwave infrared spectral region

The longwave infrared (LWIR) spectral region from 8 to 12 µm is widely used for day/night sensing and imaging applications as it corresponds to an atmospheric window as well as the peak region of the terrestrial blackbody emission. Some of these applications require use of compact spectrally tunable notch or bandstop filters. We are developing such spectral filters based on dielectric metasurfaces that utilize the guided-mode resonance (GMR) effect to provide a resonant wavelength that can be tuned by either changing the incidence angle or the grating period. We describe development of spectrally tunable micro-engineered filters with the device structure consisting of a subwavelength dielectric grating on top of a planar homogeneous layer using high-index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) with refractive index of 2.4. The filters are designed to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. Filters based on one-dimensional (1-d) gratings are polarization dependent and those based on two-dimensional (2-d) gratings are polarization independent for normal incidence of light while polarization sensitive at non-normal incidence. The filter designs were carried out using the rigorous coupled-wave analysis (RCWA) algorithm. We designed, fabricated and characterized a number of filters by carrying out direct transmission measurements using a tunable quantum cascade laser (QCL) system. We will present the simulation and experimental results for both the 1-d and 2-d grating GMR filters.