Extremely thin perfect absorber by generalized multipole bianisotropic effect

TL;DR

This paper introduces a novel ultra-thin terahertz perfect absorber leveraging symmetry-breaking induced bianisotropy in resonators, achieving near-perfect unidirectional absorption with a thickness far below fundamental limits.

Contribution

The work demonstrates a new approach to creating extremely thin perfect absorbers by exploiting bianisotropic effects from symmetry breaking and extends Onsager-Casimir relations to include high-order multipoles.

Findings

Achieved a wavelength-to-thickness ratio of 25,000 in a terahertz perfect absorber.

Demonstrated unidirectional perfect absorption with nearly perfect reflection from the opposite side.

Validated the theoretical model through experimental measurements.

Abstract

Symmetry breaking plays a crucial role in understanding the fundamental physics underlying numerous physical phenomena, including the electromagnetic response in resonators, giving rise to intriguing effects such as directional light scattering, supercavity lasing, and topologically protected states. In this work, we demonstrate that adding a small fraction of lossy metal (as low as $1\times10^{-6}$ in volume), to a lossless dielectric resonator breaks inversion symmetry thereby lifting its degeneracy, leading to a strong bianisotropic response. In the case of the metasurface composed of such resonators, this effect leads to unidirectional perfect absorption while maintaining nearly perfect reflection from the opposite direction. We have developed more general Onsager-Casimir relations for the polarizabilities of particle arrays, taking into account the contributions of quadrupoles, which shows that bianisotropy is not solely due to dipoles, but also involves high-order multipoles. Our experimental validation demonstrates an extremely thin terahertz-perfect absorber with a wavelength-to-thickness ratio of up to 25,000, where the material thickness is only 2% of the theoretical minimum thickness dictated by the fundamental limit. Our findings have significant implications for a variety of applications, including energy harvesting, thermal management, single-photon detection, and low-power directional emission.