  In this paper, we study the performance of wideband terahertz (THz)
communications assisted by an intelligent reflecting surface (IRS).
Specifically, we first introduce a generalized channel model that is suitable
for electrically large THz IRSs operating in the near-field. Unlike prior
works, our channel model takes into account the spherical wavefront of the
emitted electromagnetic waves and the spatial-wideband effect. We next show
that conventional frequency-flat beamfocusing significantly reduces the power
gain due to beam squint, and hence is highly suboptimal. More importantly, we
analytically characterize this reduction when the spacing between adjacent
reflecting elements is negligible, i.e., holographic reflecting surfaces.
Numerical results corroborate our analysis and provide important insights into
the design of future IRS-aided THz systems.
