Ultrafast modulation of optical transitions in monolayer and multilayer graphene

We study ultrafast modulations of absorption spectra for both monolayer and multilayer graphene, by performing time-resolved transmission measurements with tuning probe photon energy. While reduced absorptions by photo-excited carriers are observed in monolayer graphene irrespective of the probe energy, multilayer graphene shows increased absorption at around 0.6 eV, which is explained by the optical transitions between subband states. Intraband carrier relaxation and electron–hole recombination times are found to be as fast as 0.5 and 10 ps, respectively. Modifications of ultrafast carrier dynamics are also studied with changing temperature and excitation density.