Excitation Enhancement of Hot Electrons by Ultrafast Optical Pumping in Heavily p-Doped Graphene Stacks

Energetic photoinduced hot electrons have been attracting increased scientific attention owing to their potential applicability in numerous photoelectrical and photochemical processes. Normally, the energy of electrons quickly converts into heat by ultrafast cooling, which is considered as the bottleneck for highefficiency utilization of hot electrons. In this work, we explore intentionally heavily p-doped graphene stacks by degenerate femtosecond pump-probe spectroscopy, and observe an excitation enhancement of hot electrons at weak pump fluence. The time scale of hot-electron excitation is of the same order as that of fast decay via electron-electron and electron-optical-phonon scattering in our experiments. Physically, both Auger processes and population inversion are suppressed in this system, yet it becomes possible for the conduction bands to be effectively evacuated within the pulse duration through the ultrafast cooling of hot electrons, which may lead to an enhanced excitation of hot electrons. This excitation enhancement can be further strengthened by multiple layer-stacking processes or a thermal annealing pretreatment. The optical absorption of graphene stacks increases correspondingly, exhibiting a value larger than the linear limit at low pump fluence. Furthermore, the absorption modulation depth can reach approximately 0.79% in monolayer graphene for a small pump-fluence change (< 20 mu J/cm(2)), further increasing to approximately 1.2% and approximately 2.5% in double- and triple-stacking graphene layers, respectively, indicating that the absorption can be sufficiently altered by an extremely small variation of pump fluence. These outcomes can be applied for low-cost pulse operations. We suggest that this effect can have potential applications to harvesting energy from excited hot electrons, and may provide a unique way to achieve high-speed modulators, photodetectors, solar cells, and photocatalysts.