  We report experimental observation of electrically-tunable coherent perfect
absorption (CPA) of terahertz (THz) radiation in graphene. We develop a
reflection-type tunable THz cavity formed by a large-area graphene layer, a
metallic reflective electrode and an electrolytic medium in between. Ionic
gating in the THz cavity allows us to tune the Fermi energy of graphene up to
1eV and to achieve critical coupling condition at 2.8 THz with absorption of
99%. With the enhanced THz absorption, we were able to measure the Fermi energy
dependence of the transport scattering time of highly doped graphene.
Furthermore, we demonstrate flexible active THz surfaces that yield large
modulation in the THz reflectivity with low insertion losses. We anticipate
that the gate-tunable CPA will lead efficient active THz optoelectronics
applications.
