Upper bound for light absorption assisted by a nanoantenna

TL;DR

This paper derives an environment-dependent upper bound for light absorption by a dipolar absorber, revealing that hot spots are not always optimal for absorption enhancement and providing a new figure of merit for system comparison.

Contribution

It introduces a first-principle derived upper bound for absorption that isolates environmental effects and challenges the emphasis on field enhancement alone.

Findings

The upper bound depends on the ratio of field enhancement to local density of states.

Hot spots are not necessarily optimal for absorption enhancement.

The theoretical framework extends beyond scalar approximation and plane-wave illumination.

Abstract

We study light absorption by a dipolar absorber in a given environment, which can be a nanoantenna or any complex inhomogeneous medium. From first-principle calculations, we derive an upper bound for the absorption, which decouples the impact of the environment from the one of the absorber. Since it is an intrinsic characteristic of the environment regardless of the absorber, it provides a good figure of merit to compare the ability of different systems to enhance absorption. We show that, in the scalar approximation, the relevant parameter is not the field enhancement but the ratio between the field enhancement and the local density of states. Consequently, a plasmonic structure supporting hot spots is not necessarily the best choice to enhance absorption. We also show that our theoretical results can be applied beyond the scalar approximation and the plane-wave illumination.