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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, which reflect a narrowband of incident light while transmitting the rest. At the Army Research Laboratory (ARL), we are developing such spectral filters based on two different thin-film technologies—(i) metasurfaces that utilize the guided-mode resonance (GMR) effect in dielectric materials and (ii) electronically tunable plasmonic graphene metasurfaces with an array of nanoantennas. Both these approaches use nano-engineered subwavelength structures metasurfaces to develop very compact low-cost, rugged, lightweight spectrally tunable LWIR notch filters. Such filters are designed to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest. The optical filter based on the GMR effect consists of a subwavelength dielectric grating and a planar waveguide using high-index dielectric transparent materials, i.e., germanium (Ge) and on top of a zinc selenide (ZnSe) substrate and spectral tuning is achieved by mechanically tilting the filter. In the second approach, we fabricate the graphene plasmonic nanoantennas on a dielectric coated substrate. This approach uses four independent plasmonic metasurfaces to cover the full LWIR range, each individually tunable over one μm and a gating voltage is applied to each metasurface to obtain a spectral notch. Diameter of the nanoantenna determines the resonant wavelength. Each of these two approaches use high-precision nanofabrication technologies. We will present modeling and simulation results for both approaches as well as some fabrication and characterization results.
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