Performance characterization of tunable longwave infrared notch filters using quantum cascade lasers

We describe the design and performance characterization of spectrally tunable microengineered notch filters operating in the longwave infrared from 8 to 12 micron using quantum cascade lasers (QCLs) tunable over the full spectral range. The filter design is based on the guided mode resonance phenomenon. The device structure consists of a subwavelength dielectric grating on top of a planar waveguide using high-index transparent dielectric materials, i.e., germanium (Ge) and zinc selenide (ZnSe) with refractive indices of 4.0 and 2.4, respectively. 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. Spectral tuning of the reflected wavelength is achieved by changing the angle of incidence 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 close to polarization independent. Simple two-layer antireflection coatings were applied to minimize reflections from the nonpatterned side of the filter substrate. Our experimental setup consisted of a commercial QCL system operating at room temperature, a microengineered filter, and an uncooled broadband sensor. We present the filter design and detailed characterization experiment, and compare the theoretical and experimental results for 1-D filters.