Measurement of the Optical Conductivity of Graphene

TL;DR

This study measures graphene's optical conductivity across a range of photon energies, confirming theoretical predictions at higher energies and revealing complex behaviors at lower energies due to doping and temperature effects.

Contribution

The paper provides experimental validation of the universal optical absorbance of graphene at higher energies and explains deviations at lower energies through doping and temperature considerations.

Findings

Graphene exhibits a flat optical absorbance of about 2.3% above 0.5 eV.

Deviations from universality occur at lower energies due to doping and temperature effects.

Experimental results align with theoretical models of non-interacting Dirac fermions at higher energies.

Abstract

Optical reflectivity and transmission measurements over photon energies between 0.2 and 1.2 eV were performed on single-crystal graphene samples on a transparent SiO2 substrate. For photon energies above 0.5 eV, graphene yielded a spectrally flat optical absorbance of (2.3 +/- 0.2)%. This result is in agreement with a constant absorbance of pi*alpha, or a sheet conductivity of pi*e^2/2h, predicted within a model of non-interacting massless Dirac Fermions. This simple result breaks down at lower photon energies, where both spectral and sample-to-sample variations were observed. This "non-universal" behavior is explained by including the effects of doping and finite temperature, as well as contributions from intraband transitions.