Angular momentum of focused beams: beyond the paraxial approximation

Details Angular momentum of focused beams: beyond the paraxial approximation Angular momentum of focused beams: beyond the paraxial approximation

2009-02-18

arxiv.org/abs/0902.3214v1

Physical Review A 79, 033830 (2009)

Physics

Optics

10 pages, 9 figures

Scientific paper

10.1103/PhysRevA.79.033830

We investigate in detail the focusing of a circularly polarized Laguerre-Gaussian laser beam ($\hbar \ell$ orbital angular momentum per photon; $\sigma=1/-1$ for left/right-handed polarization) by a high numerical aperture objective. The diffraction-limited focused beam has unexpected properties, resulting from a strong interplay between the angular spatial structure and the local polarization in the non-paraxial regime. In the region near the beam axis, and provided that $|\ell|\ge 2$ and $\ell$ and $\sigma$ have opposite signs, the energy locally counter-propagates and the projection of the electric field onto the focal plane counter-rotates with respect to the circular polarization of the incident beam. We explicitly show that the total angular momentum flux per unit power is conserved after focusing, as expected by rotational symmetry, but the spin and orbital separate contributions change.

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