Quadrupole absorption rate and orbital angular momentum transfer for atoms in optical vortices

Abstract: Recent experiments involving the interaction of optical vortices with atoms in quadrupole transitions have been shown to be accompanied by the exchange of orbital angular momentum (OAM) between the electronic states of the atom and the optical vortex field. Earlier work by both theory and experiment had ruled out the transfer of a vortex OAM to the electronic degrees of freedom in an electric dipole atomic transition and it has been confirmed that the lowest multipolar order involving an OAM transfer to the electronic motion is indeed the electric quadrupole. Hitherto, the quadrupole transition involving optical vortices has not been quantified and we have thus set out to evaluate the absorption rate accompanied by an OAM transfer with reference to the $6^2S_{1/2}\rightarrow 5^2D_{5/2}$ in Cs when caesium atoms are subject to the field of a linearly polarized optical vortex. Our results assuming typical experimentally accessible parameters indicate that the absorption rate for moderate light intensities is smaller than the quadrupole spontaneous emission rate, but should still be within the measurement capabilities of modern spectroscopic techniques.