Electronic-structural Dynamics in Graphene

TL;DR

This paper reviews experiments using time- and angle-resolved photoemission spectroscopy to study the transient electronic structure of graphene under optical excitation, revealing phenomena like population inversion, free carrier absorption, and enhanced electron-phonon coupling.

Contribution

It provides new insights into the ultrafast electronic dynamics of graphene, including population inversion and light-induced electron-phonon interactions, relevant for optoelectronic applications.

Findings

Population-inverted state near Dirac point at near-infrared excitation

Evidence of free carrier absorption at sub-400 meV pump energies

Transient enhancement of electron-phonon coupling with mid-infrared pulses

Abstract

We review our recent time- and angle-resolved photoemission spectroscopy experiments, which measure the transient electronic structure of optically driven graphene. For pump photon energies in the near infrared ($\hbar\omega_{\text{pump}}=950$meV) we have discovered the formation of a population-inverted state near the Dirac point, which may be of interest for the design of THz lasing devices and optical amplifiers. At lower pump photon energies ($\hbar\omega_{\text{pump}}<400$meV), for which interband absorption is not possible in doped samples, we find evidence for free carrier absorption. In addition, when mid-infrared pulses are made resonant with an infrared-active in-plane phonon of bilayer graphene ($\hbar\omega_{\text{pump}}=200$meV), a transient enhancement of the electron-phonon coupling constant is observed, providing interesting perspective for experiments that report light-enhanced superconductivity in doped fullerites in which a similar lattice mode was excited. All the studies reviewed here have important implications for applications of graphene in optoelectronic devices and for the dynamical engineering of electronic properties with light.