  The next generation of wireless networks is expected to tap into the
terahertz (0.1--10 THz) band to satisfy the extreme latency and bandwidth
density requirements of future applications. However, the development of
systems in this band is challenging as THz waves confront severe spreading and
penetration losses, as well as molecular absorption, which leads to strong
line-of-sight requirements through highly directive antennas. Recently,
reconfigurable intelligent surfaces (RISs) have been proposed to address issues
derived from non-line-of-sight propagation, among other impairments, by
redirecting the incident wave toward the receiver and implementing
virtual-line-of-sight communications. However, the benefits provided by a RIS
may be lost if the network operates at multiple bands. In this position paper,
the suitability of the RIS paradigm in indoor THz scenarios for 6G is assessed
grounded on the analysis of a tunable graphene-based RIS that can operate in
multiple wideband transparency windows. A possible implementation of such a RIS
is provided and numerically evaluated at 0.65/0.85/1.05 THz separately,
demonstrating that beam steering and other relevant functionalities are
realizable with excellent performance. Finally, the challenges associated with
the design and fabrication of multi-wideband graphene-based RISs are discussed,
paving the way to the concurrent control of multiple THz bands in the context
of 6G networks.
