Ultrafast pseudospin dynamics in graphene

TL;DR

This paper investigates ultrafast pseudospin dynamics in graphene, revealing that isotropization of non-equilibrium charges occurs mainly through optical phonon emission within 100 fs, differing from conventional semiconductors.

Contribution

It provides a detailed analysis of the ultrafast isotropization process in graphene, highlighting the dominant role of optical phonons over electron-electron scattering.

Findings

Isotropic distribution achieved within 100 fs.

Optical phonon emission governs isotropization.

Electron-electron scattering influences initial anisotropy.

Abstract

Interband optical transitions in graphene are subject to pseudospin selection rules. Impulsive excitation with linearly polarized light generates an anisotropic photocarrier occupation in momentum space that evolves at timescales shorter than 100fs. Here, we investigate the evolution of non-equilibrium charges towards an isotropic distribution by means of fluence-dependent ultrafast spectroscopy and develop an analytical model able to quantify the isotropization process. In contrast to conventional semiconductors, the isotropization is governed by optical phonon emission, rather than electron-electron scattering, which nevertheless contributes in shaping the anisotropic photocarrier occupation within the first few fs.