  The reconfigurable intelligent surface (RIS) is a promising technology to
provide smart radio environment. In contrast to the well-studied
patch-array-based RISs, this work focuses on the metasurface-based RISs and
simultaneously transmitting and reflecting (STAR)-RISs where the elements have
millimeter or even molecular sizes. For these meticulous metasurface
structures, near-field effects are dominant and a continuous electric current
distribution should be adopted for capturing their electromagnetic response
instead of discrete phase-shift matrices. Exploiting the electric current
distribution, a Green's function method based channel model is proposed. Based
on the proposed model, performance analysis is carried out for RISs and
STAR-RISs. 1) For the transmitting/reflecting-only RIS-aided single-user
scenario, closed-formed expressions for the near-field/far-field boundary and
the end-to-end channel gain are derived. Then, degrees-of-freedom (DoFs) and
the power scaling laws are obtained. It is proved that the near-field channel
exhibits higher DoFs than the far-field channel. It is also confirmed that when
communication distance increases beyond the field boundary, the near-field
power scaling law degrades to the well-known far-field result. 2) For the
STAR-RIS-aided multi-user scenario, three practical STAR-RIS configuration
strategies are proposed, namely power splitting (PS), selective element
grouping (SEG), and random element grouping (REG) strategies. The channel gains
for users are derived within both the pure near-field regime and the hybrid
near-field and far-field regime. Finally, numerical results confirm that: 1)
for metasurface-based RISs, the field boundary depends on the sizes of both the
RIS and the receiver, 2) the received power scales quadratically with the
number of elements within the far-field regime and scales linearly within the
near-field regime.
