Polarization-independent Spatial Kramers-Kronig medium for omnidirectional reflectionless absorption of unpolarized light

Abstract Spatial Kramers-Kronig (KK) media offer a possible route to obtain omnidirectional light absorption within a thin layer of material. However, the experimental realizations are typically limited to a specific polarization, i.e., either transverse electric (TE) or transverse magnetic (TM), hence lacking specific implementations for the absorption of unpolarized light. In this work, we propose theoretically and demonstrate experimentally a polarization-independent KK medium which performs omnidirectional reflectionless absorption for both TE and TM polarized waves. Our design makes use of a special matryoshka metamaterial, whose electric and magnetic responses can be independently controlled with minimized crosstalk. To extend the absorption spectrum, the inner truncation boundary of the KK medium is set at a position far away from the spatial Lorentz resonance, where the constitutive parameter of the metamaterial remains unitary over a broad frequency band. A mini anechoic chamber, 6.83-wavelength in diameter, is constructed using the designed annulus-shaped KK medium. The measured fields for both TE and TM polarizations confirm the polarization-independent omnidirectional and nearly reflectionless absorption in a broadband frequency range.