Development of tunable longwave infrared filters based on guided-mode resonance

Abstract

We describe development of spectrally tunable micro-engineered filters operating in the longwave infrared (LWIR) region from 8 to 12 μm based on using the guided-mode resonance (GMR) effect. The device structure consists of a subwavelength dielectric grating on top of a homogeneous waveguide using high index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) substrate 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. The 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 gratings are polarization dependent and those based on two-dimensional gratings are polarization independent at normal incidence of light. Both 1-D and 2-D gratings were designed and characterized. Anti-reflection coatings were applied on substrate interfaces to improve transmission over the entire spectral region. We carried out transmission measurements of these filters using an automated tunable room temperature quantum cascade laser (QCL) system operating from 7.3 to 12.9 μm, and a modified Fourier Transform Spectrometer with a custom designed chamber for normal incidence of light on the sample. We present detailed characterization experiments and compare the theoretical and experimental results.