Spectroscopic ellipsometry of graphene and an exciton-shifted van Hove peak in absorption

TL;DR

This paper uses spectroscopic ellipsometry to analyze graphene's optical properties, deriving its electronic dispersion and identifying a shifted van Hove peak influenced by excitonic effects.

Contribution

It introduces a simple formula linking electronic spectra to optical absorption in 2D systems and applies it to extract graphene's electronic dispersion from optical data.

Findings

Optical transparency governed by the fine structure constant.

Reconstruction of graphene's electronic dispersion near the K point.

Observation of a shifted van Hove singularity peak at 4.6 eV.

Abstract

We demonstrate that optical transparency of any two-dimensional system with a symmetric electronic spectrum is governed by the fine structure constant and suggest a simple formula that relates a quasi-particle spectrum to an optical absorption of such a system. These results are applied to graphene deposited on a surface of oxidized silicon for which we measure ellipsometric spectra, extract optical constants of a graphene layer and reconstruct the electronic dispersion relation near the K point using optical transmission spectra. We also present spectroscopic ellipsometry analysis of graphene placed on amorphous quartz substrates and report a pronounced peak in ultraviolet absorption at 4.6 eV because of a van Hove singularity in graphene's density of states. The peak is downshifted by 0.5 eV probably due to excitonic effects.