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.

Near‐Unity and Narrowband Thermal Emissivity in Balanced Dielectric Metasurfaces

“…The large permittivity near the TO phonon of 3C‐SiC enables metasurfaces with deep subwavelength periods, providing more consistent performance with varying angle of incidence in a nanoscale unit cell that is not possible with Mie resonances in nonresonant materials at such long free‐space wavelengths. This compares favorably with work involving SiC emitters utilizing surface phonon polariton modes within the Restrahlen band . Our design utilizes a simple architecture that allows for operation at normal incidence and maintains high emissivity and narrow linewidths throughout a wide range of viewing angles.…”

“…This compares favorably with work involving SiC emitters utilizing surface phonon polariton modes within the Restrahlen band. [24,26,79] Our design utilizes a simple architecture that allows for operation at normal incidence and maintains high emissivity and narrow linewidths throughout a wide range of viewing angles. The large permittivity and dispersion near the edge of the TO phonon also significantly narrows the linewidths of the Mie type resonances, allowing for quality factors in excess of 100.…”

Terahertz Optics of Materials with Spatially Harmonically Distributed Refractive Index

“…The direct application of Equations (A1)-(A5) for calculating of band structure of material with n(z) distributrd by Equation (1) gives results presented in Figure 1. The intermediate calculations show that in this case the coefficients k ε m vanishing extremely fast and for satisfactory reproduction of band structure it is enough to consider only three terms n = −1; 0; 1.…”

“…The dielectric based periodic structures are widely investigated in the visible frequency range [ 1 , 2 , 3 ] and often may be considered as graded refractive index materials [ 4 , 5 ], being a practical realization of a gradient-index optics [ 6 , 7 , 8 ]. Graded refractive index materials can be found in nature (in particular, butterfly wings where colors appear due to a certain shape of chitin, gyroid) [ 4 , 9 , 10 ] and may be produced artificially [ 3 , 11 ].…”

Tunable deflection and asymmetric transmission of THz waves using a thin slab of graphene-dielectric metamaterial, with and without ENZ components

“…This variety can be further extended by using periodic structures and components made of PDs , e.g., see Refs. [48][49][50][51]. In this paper, the consideration is restricted to the case when the PD slab is made of LiF, whose complex permittivity is given as follows:…”