# Limits on Broadband Absorption Enhancement in the Presence of Multiple Lossy Materials

## TL;DR

This paper rigorously analyzes how parasitic losses in multiple materials affect the fundamental limits of broadband electromagnetic absorption enhancement, showing that surpassing traditional ray-optics limits is possible even with significant loss.

## Contribution

It provides a theoretical framework for understanding the impact of parasitic loss on absorption limits and demonstrates that these limits can be exceeded with nanophotonic structures.

## Key findings

- Absorption enhancement limits can surpass ray-optics bounds despite parasitic losses.
- Numerical verification with a metal-insulator-metal waveguide confirms theoretical predictions.
- Parasitic loss does not necessarily constrain broadband absorption enhancement.

## Abstract

Enhancing the absorption and emission of electromagnetic waves over a broad range of wavelengths is a topic of fundamental and applied interest in photonics and energy research. In the context of light trapping in solar cells, for example, significant interest in the past decade has focused on overcoming limits in the ray-optics regime with nanophotonic structures. However, many such structures, in particular plasmonic structures, or PT-symmetric systems can posses multiple materials with varying values of intrinsic loss. Here, we rigorously determine the effect of parasitic loss on the achievable absorption enhancement in arbitrary electromagnetic structures. We show that the fundamental limit of broadband absorption enhancement, even in the presence of large parasitic loss in an alternate material, can exceed conventional ray-optics limits on light trapping and absorption enhancement. We numerically verify this behavior by determining the absorption enhancement factor of a canonical system, a metal-insulator-metal waveguide, whose core is a low-index organic semiconductor, in the presence of varying intrinsic loss values in the metal.