High-frequency coherent phonons in graphene on silicon

High-frequency coherent vibrations have been investigated in graphene on silicon substrates using pump-probe anisotropic reflectivity spectroscopy with 7.5fs laser pulses. The coherent phonons of both the intralayer C=C stretching mode (G-mode) and the disorder-induced mode (D-mode) are clearly observed even in monolayer graphenes. The G-mode shifts to higher frequencies as the number of graphene layers decreases, whereas the D-mode does not exhibit such hardening. Moreover, the D-mode softens upon photoexcitation and then shifts to higher frequencies during the relaxation process. The different frequency shift behaviors in the G-and D-modes are discussed.