Very Slow Cooling Dynamics of Photoexcited Carriers in Graphene Observed by Optical-Pump Terahertz-Probe Spectroscopy

TL;DR

This study investigates the slow cooling of photoexcited carriers in graphene at low temperatures using optical-pump terahertz-probe spectroscopy, revealing extended relaxation times due to inefficient phonon emission near the Dirac point.

Contribution

It provides a detailed model explaining the slow carrier cooling in graphene at low temperatures, aligning well with experimental data.

Findings

Relaxation transients slow down below 50K

Carrier cooling extends beyond hundreds of picoseconds at low temperatures

Model including scattering and recombination matches experimental results

Abstract

Using optical-pump terahertz-probe spectroscopy, we study the relaxation dynamics of photoexcited carriers in graphene at different temperatures. We find that at lower temperatures the tail of the relaxation transients as measured by the differential probe transmission becomes slower, extending beyond several hundred picoseconds at temperatures below 50K. We interpret the observed relaxation transients as resulting from the cooling of the photoexcited carriers via phonon emission. The slow cooling of the photoexcited carriers at low temperatures is attributed to the bulk of the electron and hole energy distributions moving close enough to the Dirac point such that both intraband and interband scattering of carriers via optical phonon emission becomes inefficient for removing heat from the carriers. Our model, which includes intraband carrier scattering and interband carrier recombination and generation, agrees very well with the experimental observations.