Angular spectral dyadic Green's function
A dyadic Green's function representation is derived between two disjoint volumes with arbitrary external scattering. The result is a double-integral of transmit and receive plane waves multiplied with a dyadic kernel indicating their amplitude and polarization interactions. The kernel, which is named the angular spectrum of the system, is fully described in terms of vector spherical harmonics. It provides a solid mathematical and physical foundation to the scattering response matrix that hereto has only been conjectured and modeled in the literature.
J. Xu, "Angular spectral dyadic Green's function," 2014 IEEE Antennas and Propagation Society International Symposium (APSURSI), 2014, pp. 115-116, doi: 10.1109/APS.2014.6904389.