Broadband near-unidirectional absorption enabled by phonon-polariton resonances in SiC micropyramid arrays

Abstract: Inspired by moth eyes, nature's most powerful antireflex, we present a sub-wavelength SiC micropyramid design, which operates in the Reststrahlen band of SiC, namely the spectral band of strong phonon-photon coupling in the SiC material. While within this band SiC repels EM waves, we observe here a broad low-reflectivity window with unique attributes, with distinct characteristics different from typical dielectric moth-eye-like structures. To be specific, while the latter systems are entirely symmetric, the reflection response of our SiC micropyramid system can be highly asymmetric. In particular, the SiC micropyramid system can be near-reflectionless for light impinging from the tip side of the micropyramids and exhibits more than 90% reflection for light impinging from the base side of the micropyramids, over a broad wavelength range in the SiC Reststrahlen band. This strongly asymmetric reflection response emanates from the cascaded coupling of vortex-like cavity modes at each of the SiC blocks comprising the micropyramids and translates into a strongly uni-directional absorber response. We discuss how, by virtue of Kirchhoff's law, this strongly uni-directional superabsorber behavior implies a strongly uni-directional emission profile that is important for one-way infrared sources and passive radiative-cooling systems.