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Many promising optoelectronic devices, such as broadband photodetectors,nonlinear frequency converters, and building blocks for data communicationsystems, exploit photoexcited charge carriers in graphene. For these systems,it is essential to understand, and eventually control, the cooling dynamics ofthe photoinduced hot-carrier distribution. There is, however, still an activedebate on the different mechanisms that contribute to hot-carrier cooling. Inparticular, the intrinsic cooling mechanism that ultimately limits the coolingdynamics remains an open question. Here, we address this question by studyingtwo technologically relevant systems, consisting of high-quality graphene witha mobility >10,000 cm$^2$V$^{-1}$s$^{-1}$ and environments that do notefficiently take up electronic heat from graphene: WSe$_2$-encapsulatedgraphene and suspended graphene. We study the cooling dynamics of these twohigh-quality graphene systems using ultrafast pump-probe spectroscopy at roomtemperature. Cooling via disorder-assisted acoustic phonon scattering andout-of-plane heat transfer to the environment is relatively inefficient inthese systems, predicting a cooling time of tens of picoseconds. However, weobserve much faster cooling, on a timescale of a few picoseconds. We attributethis to an intrinsic cooling mechanism, where carriers in the hot-carrierdistribution with enough kinetic energy emit optical phonons. During phononemission, the electronic system continuously re-thermalizes, re-creatingcarriers with enough energy to emit optical phonons. We develop an analyticalmodel that explains the observed dynamics, where cooling is eventually limitedby optical-to-acoustic phonon coupling. These fundamental insights into theintrinsic cooling mechanism of hot carriers in graphene will play a key role inguiding the development of graphene-based optoelectronic devices.

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