Abstract
We show that when strongly focusing a linearly polarized optical vortex with the topological charge 2 (or −2) in the near-focus region, there occurs not only a reverse energy flow (where the projection of the Poynting vector is negative) but the right- (or left-) handed circular polarization of light as well. Notably, thanks to spin–orbital conversion, the on-axis polarization vector handedness is the same as that of the transverse energy flow, i.e., anticlockwise (clockwise). An absorbing spherical microparticle centered on the optical axis placed in the focus may be expected to rotate anticlockwise (clockwise) around its axis and its center of masses. We also show that in the case of sharp focusing of light with linear polarization (without an optical vortex) before and after focus, the light has an even number of local regions with left- and right-handed circular (elliptical) polarizations. Theoretical predictions are corroborated by the numerical simulation.
Highlights
When strongly focusing a circularly polarized Gaussian beam, a near-focus orbital energy flow has been generated thanks to spin–orbital conversion [1,2,3,4,5,6,7]
A non-zero longitudinal component of the electric vector that occurs in the strong focus leads to the generation of a transverse energy flow, which produces the non-zero longitudinal orbital angular momentum (OAM) component
Due to the presence of an optical vortex with a topological charge of 2, the reverse flow was formed on the optical axis, and its magnitude was approximately equal to 30% of the maximum value of the forward energy flux
Summary
When strongly focusing a circularly polarized Gaussian beam, a near-focus orbital energy flow has been generated thanks to spin–orbital conversion [1,2,3,4,5,6,7]. In the original plane, such a beam has no orbital angular momentum (OAM), only having a non-zero on-axis projection of the spin angular momentum (SAM) vector thanks to circular polarization. In this work, using Richard–Wolf formulae, we derived analytical relationships to describe projections of the Poynting vector (the energy flow) and the SAM vector when tightly focusing a linearly polarized optical vortex with a topological charge of 2. It is important to mention that a vortex beam with the topological charge (TC) m = 2 has a specific feature—that of generating an on-axis reverse energy flow in the tight focus (characterized by the negative longitudinal projection of the Poynting vector). Two transverse projections (x and y components) of the electric field vector (E-vector) have a relative phase shift of π, generating a circularly polarized beam, which, in turn, generates the longitudinal component of the SAM vector
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