# Limits to the Optical Response of Graphene and 2D Materials

## TL;DR

This paper establishes fundamental upper limits on the optical responses of 2D materials like graphene, guiding the design of advanced nanophotonic devices by identifying optimal materials and configurations.

## Contribution

It derives general bounds on light--matter interactions for 2D materials based solely on optical conductivity, independent of shape or structure.

## Key findings

- Highly doped graphene is optimal in the infrared.
- Single-atomic-layer silver is optimal in the visible.
- Bounds can be approached in some geometries, indicating potential for design improvements.

## Abstract

2D materials provide a platform for strong light--matter interactions, creating wide-ranging design opportunities via new-material discoveries and new methods for geometrical structuring. We derive general upper bounds to the strength of such light--matter interactions, given only the optical conductivity of the material, including spatial nonlocality, and otherwise independent of shape and configuration. Our material figure of merit shows that highly doped graphene is an optimal material at infrared frequencies, whereas single-atomic-layer silver is optimal in the visible. For quantities ranging from absorption and scattering to near-field spontaneous-emission enhancements and radiative heat transfer, we consider canonical geometrical structures and show that in certain cases the bounds can be approached, while in others there may be significant opportunity for design improvement. The bounds can encourage systematic improvements in the design of ultrathin broadband absorbers, 2D antennas, and near-field energy harvesters.