  We demonstrate experimentally an optical process in which the spin angular
momentum carried by a circularly polarized light beam is converted into orbital
angular momentum, leading to the generation of helical modes with a wavefront
helicity controlled by the input polarization. This phenomenon requires the
interaction of light with matter that is both optically inhomogeneous and
anisotropic. The underlying physics is also associated with the so-called
Pancharatnam-Berry geometrical phases involved in any inhomogeneous
transformation of the optical polarization.
