NIR/MIR Induced Ultrafast Carrier-Dynamics in Graphene

We are reporting on ultrafast carrier dynamics in quasi-free-standing bilayer and monolayer graphene using MIR-pump, THz-probe pulses. A wide range of mid IR wavelengths were used for the excitation of the sample and the subsequent carrier-relaxation dynamics has been monitored by THz pulses. We observed a large variation in the carrier relaxation time t indicating a strong dependence on the mid IR excitation wavelength of the excitation pulses. It is well established that after the excitation of the graphene, the electron gas is internally thermalized in about 200 to 300 femtosecond via Coulomb interactions. Subsequently, carrier relaxation occurs via electron-lattice cooling involving the strongly coupled optical and acoustic phonons. Multiple theoretical and experimental studies suggest shorter decay time for photoexcitation energies E_n larger than the strongly-coupled optical phonon energy (E_phonon~200 meV). However, up to an order of magnitude longer decay times have been reported for E_n < 2E_F, where E_F is the Fermi energy. We report on much longer decay-times for a range of excitation energies, where E_n > 2E_F. Consequently, our measurements do not fully support these observations and further investigations are ongoing to understand the underlying phenomenon.