▎ 摘　　要

Carrier dynamics in graphene films on CaF2 have been measured in the mid infrared region by femtosecond pump-probe spectroscopy. The relaxation kinetics shows two decay times. The fast time component is similar to 0.2 ps, which is attributed to the mixture of initial few ultrafast intraband and interband decay channels. The slow component is similar to 1.5 ps, which is primarily assigned to optical phonon-acoustic phonon scattering. The contribution of fast component exhibits an increase trend in the probe photon frequencies from 2600 to 3100 cm(-1). At the probe frequency of 2700 cm(-1), the accelerated carrier relaxation was detected, which resulted from the interband triple-resonance electron-phonon scattering in graphene. At the probe frequency of 3175 cm(-1), a clear instant negative differential transmission signal was observed, which is due to stimulated two-phonon emission involved with G phonons in graphene. This result indicates that graphene can be used as a source of coherent ultrashort sound-wave emission. (C)2012 Optical Society of America