Angular momentum of a rotating light beam

The structure and mechanical properties of a light beam, which is forcedly rotated around the propagation axis, are considered on the ground of the beam representation by superposition of axially symmetric optical-vortex beams (for example, Laguerre-Gaussian modes) with concertedly shifted frequencies. It is shown that a rotating light beam carries the angular momentum proportional but directed oppositely to the angular velocity of rotation. This paradoxical feature, connected with the complicated 3D spatial configuration inherent to any rotating beam, is explained and commented with the help of simple classical and quantum models. (c) 2005 Elsevier B.V. All rights reserved.