  We report a highly efficient tunable THz reflector in graphene. By applying a
small gate voltage (up to 3 V), the reflectance of graphene is modulated from a
minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures
at room temperature, and the reflection tuning is uniform within a wide
spectral range (0.1 - 1.5 THz). Our observation is explained by the Drude
model, which describes the THz wave-induced intraband transition in graphene.
This tunable reflectance of graphene may contribute to broadband THz mirrors,
deformable THz mirrors, variable THz beam splitters and other optical
components.
