  Acquiring full control over a large number of diffraction orders can be
strongly attractive in the case of realizing multifunctional devices such as
multichannel reflectors. Recently, the concept of metagrating has been
introduced which enables obtaining the desired diffraction pattern through a
sparse periodic array of engineered scatterers. In this letter, for the first
time, a tunable all-graphene multichannel meta-reflector is proposed for
operating at terahertz (THz) frequencies. In the supercell level, the designed
metagrating is composed of three graphene ribbons of different controllable
chemical potentials which can be regarded as a five-channel THz meta-reflector.
Several illustrative examples have been presented in which by choosing proper
distribution of DC voltages feeding the ribbons, our design can realize
different intriguing functionalities such as anomalous reflection,
retro-reflection, and three-channel power splitting within a single shared
aperture and with high efficiency. This work paves the way toward designing
highly-efficient and tunable THz multichannel meta-reflectors with many
potential applications in photonics and optoelectronics.
