Hot-Carrier Cooling in High-Quality Graphene is Intrinsically Limited by Optical Phonons

Abstract

Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand, and eventually control, the cooling dynamics of the photoinduced hot-carrier distribution. There is, however, still an active debate on the different mechanisms that contribute to hot-carrier cooling. In particular, the intrinsic cooling mechanism that ultimately limits the cooling dynamics remains an open question. Here, we address this question by studying two technologically relevant systems, consisting of high-quality graphene with a mobility >10,000 cm 2 V −1 s −1 and environments that do not efficiently take up electronic heat from graphene: WSe2-encapsulated graphene and suspended graphene. We study the cooling dynamics of these two high-quality graphene systems using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to the environment is relatively inefficient in these systems, predicting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a timescale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the hot-carrier distribution with enough kinetic energy emit optical phonons. During phonon emission, the electronic system continuously re-thermalizes, re-creating carriers with enough energy to emit optical phonons. We develop an analytical model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights into the intrinsic cooling mechanism of hot carriers in graphene will play a key role in guiding the development of graphene-based optoelectronic devices.

Hot carriers in graphene – fundamentals and applications

“…83 More recently, it was shown that this cooling mechanisms leads to bi-exponential decay, with a sub-picosecond initial decay related to direct coupling to optical phonons, and a few-picosecond decay due to the hot-phonon bottleneck. 84 This study showed that a larger peak temperature and smaller E F typically give rise to slower cooling dynamics for this cooling channel. For graphene where alternative cooling channels (see below) are suppressed, this cooling channel ultimately determines the intrinsic limit of the hot-carrier lifetime of high-quality graphene (with a mobility >10 000 cm 2 V −1 s −1 ) at room temperature.…”

“…Indeed, several experimental studies showed a temporary thermal decoupling between the electronic system and the phonon system, allowing the electronic system to heat up efficiently, before it cools down via phonons. 82 Recently however, 83,84 it was shown that optical phonons can still play an important role in hot-carrier cooling even for carrier distributions with a temperature close to room temperature. This is based on the idea that (at least at room temperature and above) there is a significant fraction of carriers in the tail of the Fermi-Dirac distribution with a kinetic energy that is large enough to couple to optical phonons.…”

“…For graphene where alternative cooling channels (see below) are suppressed, this cooling channel ultimately determines the intrinsic limit of the hot-carrier lifetime of high-quality graphene (with a mobility >10 000 cm 2 V −1 s −1 ) at room temperature. 84 3.2.2 Acoustic phonons. Charge carriers with a kinetic energy that is not high enough to couple to optical phonons can couple to acoustic phonons.…”

Time-Resolved Raman Scattering in Exfoliated and CVD Graphene Crystals

“…32 The temperature of the carrier subsystem is much higher than the lattice temperature and within ∼150 fs 15,17,21 loses a significant part of energy through carrier−phonon intraband scattering, creating, in turn, a nonequilibrium population of hot optical phonons. Very recently, Pogna et al 30 using a variety of time-resolved experimental techniques showed that electrons with proper energy coupled to optical phonons decaying, through certain relaxation channels, to lower energy phonons via anharmonic interactions.…”