Comment on "Optical Orbital Angular Momentum from the Curl of Polarization"
A Comment on the Letter by Xi-Lin Wang et al., Phys. Rev. Lett. 105, 253602 (2010). The authors of the Letter offer a Reply.
[Show abstract] [Hide abstract] ABSTRACT: We theoretically investigate the tight focusing properties of hybridly polarized vector beams. Some numerical results are obtained to illustrate the intensity, phase, and polarization of tightly focused hybridly polarized vector beams. It is shown that the shape of the focal pattern may change from an elliptical beam to a ring focus with increasing radial index. The phase distribution around the tightly focused ring is shown to be the helical phase profile, indicating that the radial-variant spin angular momentum of hybridly polarized vector beams can be converted into the radial-variant orbital angular momentum.
- "Such phenomenon is attributed to the conversion of the curl of polarization of the beam into the orbital angular momentum (OAM) . While a different view point for the origin of the OAM is given in Ref  . They argued that the observed orbital motion of trapped particles may also be driven partially by the converted OAM from the spin-to-orbital angular momentum interaction in strong focusing. "
[Show abstract] [Hide abstract] ABSTRACT: It is known that internal energy flow in a light beam can be divided into the orbital flow, associated with the macroscopic energy redistribution within the beam, and the spin flow originating from instantaneous rotation of the field vectors inherent in circular or elliptic polarization. In contrast to the orbital one, experimental observation of the spin flow constituent seemed problematic because (i) it does not manifest itself in the visible transformation of the beam profile and (ii) it converts into the orbital flow upon tight focusing of the beam, usually employed for the energy flow detection by the mechanical action on probe particles. We propose a two-beam interference technique that permits to obtain appreciable level of the spin flow in moderately focused beams and to detect the orbital motion of probe particles within a field where the transverse energy circulation is associated exclusively with the spin flow. This result can be treated as the first demonstration of mechanical action of the spin flow of a light field.
- "(6), (7), quite similar SMD should appear in polarization-inhomogeneous beams. Such situations were recently discussed  but wrongly interpreted  as manifestations of a new category of the orbital angular momentum. Besides, high-NA focusing reported in Ref.  gives no certainty that the observed orbital motion of trapped particles is not caused by the OMD generated due to the spin-to-orbital conversion. "
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