Abstract
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.
Highlights
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
A mirror symmetry axis was used on the left boundary to reduce memory requirements, with all other boundaries truncated using perfectly matched layers [22]
Two figures of merit were calculated to describe the performance of the structures for the optimizations: transmitted power normalized to the incident power on the entire structure, and the spectral bandwidth as measured by the filter response Q
Summary
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. 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]. Uncooled microbolometer sensors are commonly used as an alternative to the cooled detectors for thermal imaging, but they show low sensitivity [3]. Vertical Fabry-Perot cavities are especially convenient to use since they can be placed above conventional microbolometer arrays 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 microelectromechanical system (MEMS) element This scheme tends to be vibrationally sensitive and requires a high bias voltage. The vertical Fabry-Perot resonators discussed above have bandwidths of 150 nm or larger [2, 8, 12]
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