Excitonic Fano Resonance in Freestanding Graphene

TL;DR

This study reveals that excitonic Fano resonances significantly influence the optical absorption of freestanding graphene, linking the absorption features to the material's band structure topology.

Contribution

It provides the first quantitative analysis of excitonic Fano resonances in graphene using optical spectroscopy and Fano modeling.

Findings

Ultraviolet absorption dominated by asymmetric Fano resonance.

Excitonic resonance near van-Hove singularity couples to Dirac continuum.

Fano model accurately describes absorption across a broad energy range.

Abstract

We investigate the role of electron-hole correlations in the absorption of freestanding monolayer and bilayer graphene using optical transmission spectroscopy from 1.5 to 5.5 eV. Line shape analysis demonstrates that the ultraviolet region is dominated by an asymmetric Fano resonance. We attribute this to an excitonic resonance that forms near the van-Hove singularity at the saddle point of the band structure and couples to the Dirac continuum. The Fano model quantitatively describes the experimental data all the way down to the infrared. In contrast, the common non-interacting particle picture cannot describe our data. These results suggest a profound connection between the absorption properties and the topology of the graphene band structure.