The feasibility of using complex gratings for mid-infrared wavelength-selective absorbers is investigated. Nano/microscale surface features are employed for tailoring thermal radiative properties, which are much different from those of plain surfaces. High absorptance from heavily doped ( > 1×1020 cm−3) silicon for the transverse magnetic wave incidence can be achieved with one-dimensional periodic gratings by exciting surface plasmon polaritons. For simple binary gratings, the associated absorptance peak is narrowband and directional sensitive. These drawbacks can be remedied by using complex gratings, whose features are a superposition of multiple simple surface-relief gratings. The spectral absorptance displays a peak whose full-width-at-half-maximum (FWHM) exceeds 1.5 μm and is less sensitive to the angle of incidence. Moreover, the peak wavelength can be adjusted by varying the doping concentration and grating geometry. This study demonstrates that the use of complex gratings may significantly enhance the performance of infrared detectors.

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