A resonant absorption cavity that couples long-wavelength infrared (LWIR) light into a movable plate has been demonstrated for thermal detectors, especially microbolometers. Each device is continuously tunable over 8.7-11.1 microm by using electrostatic actuation with voltages from 0 to 42 V. The width of the resonance is relatively broad, approximately 1.5 microm, to match the large widths of many spectral features in the LWIR. At an actuation voltage of 45 V, the device switches into a broadband mode with an absorption width of 2.83 microm. This latter mode is used to enhance sensitivity in low-light situations in which little spectral information is present.

This paper covers the design, fabrication, and testing of a step-wise tunable long-wavelength infrared filter.The design uses a modified type of microbolometer to filter infrared light in the 7-10 micrometer wavelength range. Fabrication of the device is accomplished using standard microelectronic and microelectromechanical systems (MEMS) fabrication techniques. Germanium and Zinc Sulfide are used as optical layers for good reflectivity in the desired infrared range. Testing of the device shows that it filters light in three absorption bands in the specified wavelength range. The tuning of the device is found to be step-wise in response to electrostatic actuation using voltages up to 100 volts.

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