  Recent emergence of metasurfaces has enabled the development of ultra-thin
flat optical components through different wavefront shaping techniques at
various wavelengths. However, due to the non-adaptive nature of conventional
metasurfaces, the focal point of the resulting optics needs to be fixed at the
design stage, thus severely limiting its reconfigurability and applicability.
In this paper, we aim to overcome such constraint by presenting a flat
reflective component that can be reprogrammed to focus terahertz waves at a
desired point in the near-field region. To this end, we first propose a
graphene-based unit cell with phase reconfigurability, and then employ the
coding metasurface approach to draw the phase profile required to set the focus
on the target point. Our results show that the proposed component can operate
close to the diffraction limit with high focusing range and low focusing error.
We also demonstrate that, through appropriate automation, the reprogrammability
of the metamirror could be leveraged to develop compact terahertz scanning and
imaging systems, as well as novel reconfigurable components for terahertz
wireless communications.
