A silicon-based longwave-pass filter with a 30 µm cut-on has been successfully tested at the Institut National d’Optique (INO; Quebec City, Que., Canada) in an imager containing an uncooled-microbolometer focal-plane array and blackbody source. The goal is to evaluate the suitability of the INO-fabricated arrays for imaging above 35 µm. The filter, fabricated by Lake Shore Cryotronics (Westerville, OH), is based on a novel porous-silicon design that allows use under wide temperature swings ranging from cryogenic to elevated temperatures (traditional multilayer filters tend to delaminate under such conditions).
The filters are fabricated by electrochemically etching silicon. The random macroporous silicon scatters the shorter wavelengths that lie within the blocking range of the filter. Varying the design and fabrication processes can tailor the filter’s rejection edge from mid- to far-IR (150 µm and longer); the filter has a flat spectral response extending to 1 mm (0.3 THz). Rejection-band blocking reaches down to 100 nm with optical densities of greater than 2. Applications include Fourier-transform IR spectroscopy, astronomy, IR detector research, and terahertz imaging. Contact David Klein at [email protected].