Spin-orbit coupling in free-space Laguerre-Gaussian light beams.

Abstract

The expression for the azimuthal force on an atom in a circularly polarized Laguerre-Gaussian beam is shown to contain a term that depends upon the coupling of the intrinsic spin and orbital angular momenta of the light. Consequently, the state of polarization of the light affects the gross motion of the atom and not simply its internal dynamics. The effect arises from the spin-orbit coupling exhibited in the linear momentum density of the free-space mode. @S1050-2947~96!50505-6# The concepts of electron spin and orbital angular momentum are a commonplace in the theory of atomic structure. The detailed origin and labeling of the fine structure of the energy-level scheme arise from the coupling between them. These joint concepts are not so well known for light. The spin of the photon is well understood and explains the polarization of light beams, but although the orbital angular momentum of the photon is known as a concept, it is rarely cited in discussions of dipole radiation and is more commonly associated with multipole radiation @1#. It is not customary, moreover, in this regime to think of a beam of light with a discrete, quantized, amount of orbital angular momentum. More important, there is no evidence of spin-orbit coupling in a beam of free-space light. A good deal of activity has followed the prediction @2# that free-space Laguerre-Gaussian laser modes possess quantized orbital angular momentum. Theoretical activity includes an eigenfunction description of such beams, the way in which the orbital angular momentum of a beam of light may be analogous to the angular momentum of the harmonic oscillator, as well as studies of the properties of their Poynting vector @3#. In addition, the property has been shown to occur outside the paraxial approximation @4#. Experimental work has included the production of Laguerre-Gaussian beams in the visible, microwave and millimeter-wave regimes @5# culminating, for the moment at least, with the direct qualitative observation of the transfer of orbital angular momentum to absorptive particles @6#.