Abstract
Photons can carry angular momentum, not only due to their spin, but also due to their spatial structure. This extra twist has been used, for example, to drive circular motion of microscopic particles in optical tweezers as well as to create vortices in quantum gases. Here we excite an atomic transition with a vortex laser beam and demonstrate the transfer of optical orbital angular momentum to the valence electron of a single trapped ion. We observe strongly modified selection rules showing that an atom can absorb two quanta of angular momentum from a single photon: one from the spin and another from the spatial structure of the beam. Furthermore, we show that parasitic ac-Stark shifts from off-resonant transitions are suppressed in the dark centre of vortex beams. These results show how light's spatial structure can determine the characteristics of light–matter interaction and pave the way for its application and observation in other systems.
Highlights
Photons can carry angular momentum, due to their spin, and due to their spatial structure
This is expressed in the form of selection rules, which indicate which changes of angular momentum of the atom are possible when it interacts with a photon
Light beams with orbital angular momentum (OAM) have been used to drive motion of microscopic particles in optical tweezers as well as to generate vortices in degenerate quantum gases[13,14]. It was not clear if this extra angular momentum could affect the state of motion of bound electrons, that is, change the standard selection rules of optical excitation
Summary
Photons can carry angular momentum, due to their spin, and due to their spatial structure. It was not clear if this extra angular momentum could affect the state of motion of bound electrons, that is, change the standard selection rules of optical excitation. This issue was debated during the last two decades[15,16,17,18,19,20,21,22,23,24,25], and first experiments[26,27,28] did not observe these effects. Quadrupole transitions, driven by field gradients occurring in optical beams, are particulary interesting for they can even occur where there is no light intensity but only field gradient[30]
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