  An intelligent reflecting surface (IRS) at terahertz (THz) bands is expected
to have a massive number of reflecting elements to compensate for the severe
propagation losses. However, as the IRS size grows, the conventional far-field
assumption starts becoming invalid and the spherical wavefront of the radiated
waves should be taken into account. In this work, we consider a spherical wave
channel model and pursue a comprehensive study of IRS-aided multiple-input
multiple-output (MIMO) in terms of power gain and energy efficiency (EE).
Specifically, we first analyze the power gain under beamfocusing and
beamforming, and show that the latter is suboptimal even for multiple meters
away from the IRS. To this end, we derive an approximate, yet accurate,
closed-form expression for the loss in the power gain under beamforming.
Building on the derived model, we next show that an IRS can significantly
improve the EE of MIMO when it operates in the radiating near-field and
performs beamfocusing. Numerical results corroborate our analysis and provide
novel insights into the design and performance of IRS-assisted THz
communication.
