Direction-dependent coupling between a nanofiber-guided light field and a two-level atom with an electric quadrupole transition

We study the directional dependence of the coupling between a nanofiber-guided light field and a two-level atom with an electric quadrupole transition. We examine the situation where the atom lies on the fiber transverse axis x , the quantization axis for the atomic internal states is the other orthogonal transverse axis y , the atomic upper and lower levels are the magnetic sublevels M ′ and M of hyperfine-structure levels of an alkali-metal atom, and the field is in a quasilinearly polarized fundamental guided mode HE 11 with the polarization ξ = x or y . We find that the absolute value of the quadrupole Rabi frequency depends on the propagation direction of the light field in the cases of ( M ′ − M = ± 1, ξ = y ) and ( M ′ − M = ± 2, ξ = x ). We show that the directional dependence of the coupling leads to the directional dependence of spontaneous emission into guided modes. We find that the directional dependence of the atom-field coupling in the case of quadrupole transitions is not entirely due to spin-orbit coupling of light: there are some other contributions resulting from the gradient of the spatial phase factor of the field.