Non-thermal hot electrons ultrafastly generating hot optical phonons in graphite

TL;DR

This study uses time-resolved photoemission spectroscopy to reveal ultrafast, non-thermal hot electron-driven generation of optical phonons in graphite, challenging traditional two-temperature models.

Contribution

It demonstrates a mechanism of ultrafast optical phonon generation in graphite driven by hot electrons, beyond the conventional two-temperature framework.

Findings

Observation of non-thermal electron distributions

Detection of coupled optical phonons from early times

Evidence of phonon generation beyond two-temperature models

Abstract

Ultrafast dynamics of graphite is investigated by time-resolved photoemission spectroscopy. We observe spectral features of direct photoexcitations, non-thermal electron distributions, and recovery dynamics occurring with two time scales having distinct pump-power dependences. Additionally, we find an anomalous increase of the spectral intensity around the Fermi level, and we attribute this to spectral broadenings due to coupled optical phonons in the transient. The fingerprints of the coupled optical phonons occur from the temporal region where the electronic temperature is still not definable. This implies that there is a mechanism of ultrafast-and-efficient phonon generations beyond a two-temperature model.