Checkpoint for helicity conservation in fluorescence at the nanoscale: Energy and helicity transfer (hFRET) from a rotating donor dipole.

Orientation factor (κ2 ) for FRET from a rotating donor dipole, transferred helicity of the donor field and torque exerted on the acceptor by the donor have been investigated in the framework of classical electrodynamics. It is shown that for rotating dipole, κ2 is significantly higher as compared to linear dipole independently of the orientation distribution of the donor and acceptor and whether the static or the dynamic rotational regimes are used for averaging κ2 . By this property of κ2 , FRET serves as an example for a phenomenon where local field interference may take place in a "natural" way for emitters possessing rotating dipoles in their excited states by nature. The overlapping spatial distributions for the helicity of donor local field, torque exerted on the acceptor by the donor and for the FRET orientational factor suggest that transfer of both energy and helicity take place predominantly in the plane of rotation by keeping the original direction of helicity, i.e. in accordance with the conservation law for helicity. Orienting FRET has been proposed by engineering local field structure by using elliptically polarized light for donor excitation or by using linearly polarized light coupled with electromagnetic modification of the donor environment. The phenomenon of increased κ2 can be exploited for checking helicity conservation for different FRET donors without the need for polarized detection optics. Modulation of FRET with changing ellipticity of the excited donor state might supply structural and dynamical information on the orientational distribution of dye-holding matrices even on the surface of living cells, e.g. on the level of cell surface receptor clusters. Furthermore, it might also be exploited in sensing local electromagnetic fields. Rotating excited donor states might also facilitate turning on photoswitchable acceptors.