Polarization-independent angle-tolerant mid-infrared spectral resonance using amorphous germanium high contrast gratings for notch filtering application

Abstract

Spectral resonances in the mid-infrared region with polarization independence and angle tolerance are useful for filtering applications in infrared spectroscopy and imaging systems, when used with unpolarized light and across a wide field-of-view. Guided mode resonances are particularly attractive for this purpose due to the simple fabrication procedure to realize grating structures and the robust filter characteristics achievable through design. In this paper, the electromagnetic design, fabrication, and experimental characterization of polarization-independent, angle-tolerant mid-infrared spectral resonance using amorphous-germanium two-dimensional fully-etched high index contrast gratings on a calcium fluoride substrate is presented. The resonance, centered at 7.42 µm wavelength, exhibits polarization-independent, notch-type characteristics with minimal change across a 0 to 30° incidence angle. The angle tolerance of such dielectric high contrast grating filters is found to be intermediate between the highly angle sensitive dielectric partially etched grating structures and least angle sensitive metallic nano-aperture structures.