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 the relaxation dynamics after photoexcitation. These dynamics contain a sub-100 fs thermalization phase, which occurs through carrier–carrier scattering and leads to a carrier distribution with an elevated temperature. This is followed by a picosecond cooling phase, where different phonon systems play a role: graphene acoustic and optical phonons, and substrate phonons. Here, we address the cooling pathway of two technologically relevant systems, both 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: WSe 2 -encapsulated graphene and suspended graphene. We study the cooling dynamics using ultrafast pump–probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to substrate phonons is relatively inefficient in these systems, suggesting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a time scale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the high-energy tail of the hot-carrier distribution emit optical phonons. This creates a permanent heat sink, as carriers efficiently rethermalize. We develop a macroscopic model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights will guide the development of graphene-based optoelectronic devices.

Chiral transport of hot carriers in graphene in the quantum Hall regime

“…We observe that the PC for low LLs does not saturate, whereas the high LL PC saturates much easier. This is consistent with our model since the PC for low LLs is mainly a sum of the electron and hole currents; whereas in the high LL regime the PC saturates much easier since it is the difference of electron and hole currents and thus bottlenecked by the difference of relaxation rates of electrons and holes [25,45]. This is evidenced by the observed markedly different saturation powers, P 0 for low and high LL ( We also see a peculiar double-bent saturation behavior which occurs predominately at high LLs, as shown in the inset of Fig.…”

Electrically Tunable Nonequilibrium Optical Response of Graphene

“…The HFD can be detected in a pump–probe experiment as a photobleaching (PB) signal, 56 − 58 i.e., decreased probe absorption compared with equilibrium, because of Pauli blocking of interband transitions caused by the photogenerated e/h. The excess energy of the hot charge-carriers is released to the lattice via electron–phonon scattering with optical phonons, 62 − 64 anharmonically coupled to acoustic phonons. 62 − 65 Hot carriers’ cooling occurs on a few-ps time-scale 56 − 58 , 60 , 65 and is influenced, through the activation of additional relaxation channels, by the dielectric environment (e.g., via near-field coupling to hyperbolic optical phonons of substrate or encapsulant material 66 ).…”

Properties and challenges of hot-phonon physics in metals: MgB$_2$ and other compounds

“…The presence and an active role of hot phonons is also commonly revealed in the analysis of the time dependence of electronic properties upon nonequilibrium conditions. Given the plethora of ultrafast pump-probe experiments nowadays accessible, the physical properties under investigation can vary in a wide range, from optical probes (transmission [77,84,85,87,150,151], reflectivity [13,89,[110][111][112][152][153][154], absorption [91,[155][156][157][158]) to non-linear optics [86], time-resolved photoelectron spectroscopy [108,132,133,[159][160][161][162], photoluminescence [83,163], time-dependent Raman probes [164], ultrafast diffraction [6,44,109,126,[165][166][167][168][169][170][171].…”