  Intelligent reflecting surface (IRS)-assisted wireless communication is
widely deemed a key technology for 6G systems. The main challenge in deploying
an IRS-aided terahertz (THz) link, though, is the severe propagation losses at
high frequency bands. Hence, a THz IRS is expected to consist of a massive
number of reflecting elements to compensate for those losses. However, as the
IRS size grows, the conventional far-field assumption starts becoming invalid
and the spherical wavefront of the radiated waves must be taken into account.
In this work, we focus on the near-field and analytically determine the IRS
response in the Fresnel zone by leveraging electromagnetic theory.
Specifically, we derive a novel expression for the path loss and beampattern of
a holographic IRS, which is then used to model its discrete counterpart. Our
analysis sheds light on the modeling aspects and beamfocusing capabilities of
THz IRSs.
