Absorption spectroscopy in the long wave infrared provides an effective method for identification of various hazardous chemicals. We present a theoretical design for plasmonic band-pass filters that can be used to provide wavelength selectivity for uncooled microbolometer sensors. The microfilters consist of a pair of input reflection gratings that couple light into a plasmonic waveguide with a central resonant waveguide cavity. An output transmission grating on the other side of the structure pulls light out of the waveguide where it is detected by a closely spaced sensor. Fabrication of the filters can be performed using standard photolithography procedures. A spectral bandpass with a full-width at half-maximum (FWHM) of 100？nm can be obtained with a center wavelength spanning the entire 8–12？μm atmospheric transmission window by simple geometric scaling of only the lateral dimensions. This allows the simultaneous fabrication of all the wavelength filters needed for a full spectrometer on a chip. 1. Introduction Detection of long-wave infrared (LWIR) light in the 8–12？μm atmospheric transmission band is an important area of research with a high demand for good wavelength resolution, high sensitivity, portability, and affordability. An important application for LWIR detection is absorption spectroscopy for remote sensing of chemical vapors [1]. There have been several approaches for spectral imaging of LWIR light. One approach is to rapidly tune a liquid crystal-filled Fabry-Perot etalon coupled with a cooled LWIR camera to detect light over a wide wavelength range with good spectral resolution [2]. Cryogenic cooling, however, greatly increases the cost of operation and portability of the detector. Uncooled microbolometer sensors are commonly used as an alternative to the cooled detectors for thermal imaging, but they show low sensitivity [3]. It has been shown in recent work [4] that uncooled microbolometers, absorbing only a narrow band of radiation, can have sensitivities approaching that of cooled detectors. There are several ways to narrow the band of absorbed light in a microbolometer detector, including antennas [5–7], metamaterials [8–11], and vertical optical cavities [4, 12–17]. Vertical Fabry-Perot cavities are especially convenient to use since they can be placed above conventional microbolometer arrays. However, this requires the vertical dimensions to be changed for the detection of different wavelengths. The most common way this is achieved is by tuning the resonant wavelength by moving one of the mirrors via an included
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