Carrier Heating and Negative Photoconductivity in Graphene

TL;DR

This study explores negative photoconductivity in graphene using ultrafast terahertz techniques, revealing that it occurs independently of interband excitation and is driven by thermalized free carriers cooling via electron-phonon interactions.

Contribution

It demonstrates negative photoconductivity in graphene at various photon energies without requiring interband transitions, challenging existing population inversion models.

Findings

Negative photoconductivity observed at all photon energies tested.

Interband excitation not necessary for negative photoconductivity.

Results support a thermalized carrier cooling mechanism.

Abstract

We investigated negative photoconductivity in graphene using ultrafast terahertz techniques. Infrared transmission was used to determine the Fermi energy, carrier density and mobility of p-type CVD graphene samples. Time-resolved terahertz photoconductivity measurements using a tunable mid-infrared pump probed these samples at photon energies between 0.35eV to 1.55eV, approximately one half to three times the Fermi energy of the samples. Although interband optical transitions in graphene are blocked for pump photon energies less than twice the Fermi energy, we observe negative photoconductivity at all pump photon energies investigated, indicating that interband excitation is not required to observe this effect. Our results are consistent with a thermalized free carrier population that cools by electron-phonon scattering, but inconsistent with models of negative photoconductivity based on population inversion.