Light Trapping by Non-Hermitian Thin Films

TL;DR

This paper demonstrates how non-Hermitian thin films can unidirectionally trap and absorb incident light by combining surface corrugation and loss modulation, with numerical and experimental validation.

Contribution

It introduces a novel non-Hermitian thin film design that achieves unidirectional light trapping and absorption, combining theoretical analysis with experimental proof.

Findings

Unidirectional coupling of incident light into the film.

Efficient absorption of trapped light within the film.

Experimental validation of the trapping and absorption mechanism.

Abstract

One of the exceptional features of non-Hermitian systems is the unidirectional wave interactions. Simultaneous modulation of the real and the imaginary part of the interaction potentials (of the refractive index and the gain/loss in the case of optical systems) can result in unequal coupling coefficients between the fields of different parts of the system. The unidirectional coupling can also be arranged not only between the internal fields of the system but also between internal fields and external radiation. At a particular (exceptional) point the situation can be achieved, that the external radiation is efficiently coupled into the system, but the internal radiation cannot escape backwards. In this way, the incident radiation can be trapped inside the non-Hermitian system and, eventually, can be efficiently absorbed there. We realize this idea in non-Hermitically modulated thin films. The modulation consists of a Hermitian part - the periodic corrugation of the surfaces of a thin film, and a non-Hermitian part - the modulation of losses along the film. We prove numerically and demonstrate experimentally that the incident radiation, coupled with such a non-Hermitian thin film, is unidirectionally trapped into a planar mode of the film, does not escape from the film (or escape weakly due to experimental imperfections), and is efficiently absorbed there.