Interaction of twisted light with a trapped atom: Interplay between electronic and motional degrees of freedom

We present a theoretical study of nondipole excitation of a single trapped atom by twisted light. Special emphasis is placed on effects that arise from the interplay between internal (electronic) and vibrational (centerof-mass) degrees of freedom of an atom. In order to provide a fully quantum mechanical understanding of the excitation, we used the density-matrix approach based on the Liouville-von Neumann equation. The developed theory has been applied to the particular case of the 4s 2S1/2 -> 3d 2D5/2 electric quadrupole (E2) transition in a 40Ca+ ion induced by Laguerre-Gaussian modes. It was found that the Rabi oscillations can show unconventional anharmonic behavior that is attributed to the strong coupling between vibrational levels of the trap. This effect is accompanied by the transfer of angular momentum to the center-of-mass motion and becomes most pronounced when the Rabi frequency is comparable to the trapping frequency.