Limit to the orbital angular momentum per unit energy in a light beam that can be focussed onto a small particle

Within a ray-optical approximation we show that there is a limit to the orbitaI angular momentum per unit energy that can be focussed onto a small particle. This limit depends solely on the geometry of the focussing optics and the size of the particle and is independent of the wavelength of the light. (C) 2000 Elsevier Science B.V. All rights reserved.