Direct view on the ultrafast carrier dynamics in graphene (1304.2615v1)

Abstract: The ultrafast dynamics of excited carriers in graphene is closely linked to the Dirac spectrum and plays a central role for many electronic and optoelectronic applications. Harvesting energy from excited electron-hole pairs, for instance, is only possible if these pairs can be separated before they lose energy to vibrations, merely heating the lattice. While the hot carrier dynamics in graphene could so far only be accessed indirectly, we here present a direct time-resolved view on the Dirac cone by angle-resolved photoemission (ARPES). This allows us to show the quasi-instant thermalisation of the electron gas to a temperature of more than 2000 K; to determine the time-resolved carrier density; to disentangle the subsequent decay into excitations of optical phonons and acoustic phonons (directly and via supercollisions); and to show how the presence of the hot carrier distribution affects the lifetime of the states far below the Fermi energy.

• The paper demonstrates near-instantaneous thermalization (~30 fs) of hot carriers immediately after photoexcitation using TR-ARPES.
• It reveals two decay regimes with optical phonon interactions at ~160 fs and acoustic phonon/supercollisions at ~2700 fs.
• The study highlights deviations from the Fermi-Dirac distribution, supporting the potential for carrier multiplication in graphene-based devices.

Direct View on the Ultrafast Carrier Dynamics in Graphene: A Technical Overview

The paper "Direct view on the ultrafast carrier dynamics in graphene" elucidates the intricate dynamics of excited carriers in graphene using time and angle-resolved photoemission spectroscopy (TR-ARPES). This research focuses on the ultrafast dynamics of hot carriers, which are pivotal for enhancing electronic and optoelectronic applications by exploiting the Dirac spectrum of graphene. The ability to directly observe these carriers' dynamics provides a more comprehensive understanding of graphene's intrinsic properties and the potential for energy harvesting from electron-hole pairs.

The experimental set-up at the Artemis facility allowed for the measurement of TR-ARPES data from the Dirac cone in graphene, overcoming the challenges posed by the position of the Dirac cone in the Brillouin zone. The paper relies on quasi-free-standing monolayer graphene (QFMLG) samples to approximate the electronic properties of pristine graphene. This approach ensures minimal electron-phonon coupling and effective decoupling from the substrate, facilitating the observation of many-body effects.

One significant finding of this paper is the near-instantaneous thermalization of the electron gas to a temperature exceeding 2000 K, as confirmed by TR-ARPES. This rapid thermalization occurs on a timescale of approximately 30 fs, faster than the experimental temporal resolution. Notably, the experiments disclosed deviations from the expected Fermi-Dirac distribution immediately following photoexcitation, underlining an out-of-equilibrium carrier distribution.

The decay of the hot electron population is identified as involving sequential interactions with optical and acoustic phonons. The decay constants derived are 160 fs for optical phonon interactions and 2700 fs for acoustic phonons/supercollisions. These findings corroborate the theoretical predictions of carrier dynamics in graphene, highlighting the essential role of supercollisions once optical phonon interactions are saturated.

Intriguingly, the paper assesses carrier multiplication (CM), a concept underlining the potential for generating more electron-hole pairs per absorbed photon in low-fluence conditions, relevant for practical device applications. Despite the experimental conditions limiting CM to below unity (~0.5), the results align with theoretical expectations, suggesting device applications could achieve CM > 1 under different conditions.

Furthermore, the research details how the photohole lifetime is affected by the presence of hot carriers, with increased decay rates post-excitation attributed to scattering processes with high-energy holes. The paper posits that these processes dissipate quickly, reflecting the depletion of high-energy holes.

The direct measurement techniques employed provide robust insights into the carrier dynamics, offering a framework for future advancements in graphene-based technologies. TR-ARPES, by disentangling electronic and photohole lifetimes, provides an incisive tool for probing the interplay between ultrafast carrier dynamics and many-body interactions in graphene.

Overall, the implications of this research are substantial for the development of more efficient graphene-based electronic and optoelectronic devices. The insights gained pave the way for exploiting carrier multiplication and optimizing energy transport in graphene, reinforcing its potential as a cornerstone material in future technologies. As the field progresses, further experimentation under varied conditions may unlock enhanced applications and deeper understanding of graphene's ultrafast carrier dynamics.