Excitation-Energy-Dependent Enhancement of Hot-Carrier Lifetimes in Graphene

Abstract

Understanding hot-carrier relaxation in graphene is important for ultrafast optoelectronic and terahertz technologies. Here, we investigate carrier cooling dynamics in mono- and bilayer graphene using mid-infrared pump pulses (0.22 to 0.73 eV) and terahertz probe pulses. We uncover a pronounced, reproducible, and nonmonotonic dependence of the carrier relaxation time on excitation photon energy. Within a narrow spectral window from 0.42 to 0.48 eV, the carrier lifetime increases by nearly an order of magnitude compared to the few-picosecond cooling observed at lower and higher excitation energies. The observed excitation-energy dependence is consistent with a hot-phonon bottleneck scenario in which the accumulation and reabsorption of nonequilibrium optical phonons suppress the energy transfer to the lattice. A phenomenological coupled carrier–phonon model supports this interpretation. These findings reveal a previously unreported excitation-energy dependence of hot-carrier cooling in graphene and provide new insight into nonequilibrium carrier–phonon interactions near the optical-phonon bottleneck.