Wide-angle and polarization-selective plasmonic nano-metagrating absorbers

Abstract

Infrared (IR) polarimetric imaging is drawing significant interest because of its role in the enhancement of object recognition or detection ability. Conventional IR polarimetric imaging requires the use of polarizers or filters with IR cameras, which increases the complexity and cost of such systems, and degenerates performance. If uncooled IR sensors could selectively detect polarization without the need for polarizers or filters, then this would widen their range of applications. We have therefore investigated polarization-selective absorbers based on plasmonic metamaterials. Onedimensional (1D) plasmonic nano-metagrating absorbers (PNMAs) with high aspect ratios (<10) and narrow grooves (ca. 150 nm) are highly promising candidates for this purpose. Numerical calculations indicate that polarization selective absorption of over 90% absorbance is achieved. The incident electromagnetic wave is strongly confined in the narrow grooves and produces plasmonic resonance; the absorption wavelength is defined only by the groove depth and is independent of the incidence angle. Such high aspect ratio gratings with narrow grooves exhibit the optical properties of metamaterials rather than those of conventional metal gratings. We recently developed a top-down fabrication procedure for PNMAs using tapered-sidewall molds with Au deposition, which achieved 100 nm width grooves and an aspect ratio of 15. The absorption wavelengths obtained were larger than the period of the PNMA, and absorption over 90% was achieved. The absorption bandwidth can be controlled according to the groove shape, so narrow and broadband operation can be realized. PNMAs are therefore promising for uncooled IR polarimetric image sensors in terms of both sensor performance and mass production.