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Phys Rev Lett. 2002 Feb 4;88(5):053601. Epub 2002 Jan 16.

Intrinsic and extrinsic nature of the orbital angular momentum of a light beam.

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Department of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, Scotland.


We explain that, unlike the spin angular momentum of a light beam which is always intrinsic, the orbital angular momentum may be either extrinsic or intrinsic. Numerical calculations of both spin and orbital angular momentum are confirmed by means of experiments with particles trapped off axis in optical tweezers, where the size of the particle means it interacts with only a fraction of the beam profile. Orbital angular momentum is intrinsic only when the interaction with matter is about an axis where there is no net transverse momentum.

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