Excitation enhancement of hot electrons by ultrafast optical pumping in heavily p-doped graphene stacks

In this work, we intentionally explored the heavily p-doped graphene stacks by degenerate femtosecond pump-probe spectroscopy, and observed an excitation enhancement of hot electrons at weak pump fluence. 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. Furthermore, large modulation depth is achieved in graphene stacks with small variation of pump fluence. We suggest that this effect can have potential applications on harvesting energy from excited hot electrons, and may provide a unique way to achieve high-speed modulators, photodetectors, solar cells, and photocatalysts.