Fundamentals and model of resonance helicity (RHELT) and energy (RET) transfer between two magnetoelectric chiral particles.

We establish a classical electrodynamic theory for the non-radiative transfer of field helicity (RHELT) and energy (RET) between a donor and an acceptor, both being magnetoelectric and bi-isotropic, chiral in particular, with rotating excited dipoles. We introduce orientational factors that control this process. Also, a RHELT and RET interaction radius is put forward. The detection of RHELT adds a wealth of information contained in the helicity of the transferred fields, never used or established to date. The nature of these magnetoelectric bi-isotropic particles and/or molecules with induced dipoles possessing angular momentum, enriches the number of variables and associated effects. Hence the landscape involved in this transfer phenomenon, never explored before, is significantly broader than in conventional FRET. In this way, chiral interacting objects convey terms in the equations of transfer rate of helicity and energy, that are discriminatory, so that one can extract information on their structural chirality handedness and polarization rotation. As such, not only the rate of electromagnetic helicity transfer, but also that of energy transfer may be negative, which for the latter means an enhanced emission from the donor in pressence of acceptor, a phenomenon which does not exist in conventional FRET. Importantly, both the RHELT and RET rates, as well as the RHELT interaction radius, are very sensitive to changes in the helicity, or state of polarization, of the illumination, as well as to the polarization of the excited electric and magnetic dipole moments of donor and acceptor.