We present an easy, efficient and fast method to generate arbitrary linear
combinations of light orbital angular momentum eigenstates ℓ=±2
starting from a linearly polarized TEM00 laser beam. The method exploits
the spin-to-orbital angular momentum conversion capability of a
liquid-crystal-based q-plate and a Dove prism inserted in a Sagnac polarizing
interferometer. The nominal generation efficiency is 100\%, being limited only
by reflection and scattering losses in the optical components. When closed
paths are followed on the polarization Poincar\'{e} sphere of the input beam,
the associated Pancharatnam geometric phase is transferred unchanged to the
orbital angular momentum state of the output beam.Comment: 5 pages and 5 figure