Development of tunable longwave infrared metamaterial notch filters

Abstract

We describe development of spectrally tunable micro-engineered filters operating in the longwave infrared (LWIR) from 8 to 12 micron based on the guided mode resonance (GMR) phenomenon. The device structure consists of a subwavelength dielectric grating on top of a planar waveguide 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 incident broadband light at one (or more) narrow spectral band while fully transmitting the other wavelengths. Tuning of the reflection wavelength is achieved by changing the angle of incidence of light by mechanically tilting the filter. Filters based on one dimensional (1-D) gratings are polarization dependent and those based on two dimensional (2-D) gratings are less sensitive to incident polarization of light. Both filters were fabricated using standard dielectric deposition and photolithographic techniques and characterized. Simple 2-layer anti-reflection coatings were applied to improve transmission over the entire spectral region. Our experimental setup consists of a modified commercial Fourier Transform Infrared Spectrometer (FTIR) system with a separate chamber for a collimated incident beam of light, focusing optics, a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector, a wire grid polarizer and a micro-engineered filter. We will present detailed characterization experiment and compare the theoretical and experimental results.