▎ 摘　　要

When light is absorbed by a semiconductor, photoexcited charge carriers enhance the absorption of far-infrared radiation due to intraband transitions. We observe the opposite behavior in monolayer graphene, a zero-gap semiconductor with linear dispersion. By using time domain terahertz (THz) spectroscopy in conjunction with optical pump excitation, we observe a reduced absorption of THz radiation in photoexcited graphene. The measured spectral shape of the differential optical conductivity exhibits non-Drude behavior. We discuss several possible mechanisms that contribute to the observed low-frequency non-equilibrium optical response of graphene. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4795858]