Probing carrier dynamics in photo-excited graphene with time-resolved ARPES

TL;DR

This study uses time-resolved ARPES to investigate the ultrafast carrier dynamics in photo-excited graphene, revealing impact ionization as the dominant early scattering process and Auger recombination at later stages, informing optoelectronic applications.

Contribution

It provides the first direct measurement of impact ionization and Auger recombination timescales in monolayer graphene using ultrafast spectroscopy.

Findings

Impact ionization dominates the first 25 fs after excitation.

Auger recombination occurs between 100 and 250 fs.

Carrier dynamics depend on the number of non-thermal carriers.

Abstract

The dynamics of photo-generated electron-hole pairs in solids are dictated by many-body interactions such as electron-electron and electron-phonon scattering. Hence, understanding and controlling these scattering channels is crucial for many optoelectronic applications, ranging from light harvesting to optical amplification. Here we measure the formation and relaxation of the photo-generated non-thermal carrier distribution in monolayer graphene with time- and angle-resolved photoemission spectroscopy. Using sub 10fs pulses we identify impact ionization as the primary scattering channel, which dominates the dynamics for the first 25fs after photo-excitation. Auger recombination is found to set in once the carriers have accumulated at the Dirac point with time scales between 100 and 250fs, depending on the number of non-thermal carriers. Our observations help in gauging graphene's potential as a solar cell and TeraHertz lasing material.