  Orbital angular momentum associated with the helical phase-front of optical
beams provides an unbounded \qo{space} for both classical and quantum
communications. Among the different approaches to generate and manipulate
orbital angular momentum states of light, coupling between spin and orbital
angular momentum allows a faster manipulation of orbital angular momentum
states because it depends on manipulating the polarisation state of light,
which is simpler and generally faster than manipulating conventional orbital
angular momentum generators. In this work, we design and fabricate an
ultra-thin spin-to-orbital angular momentum converter, based on plasmonic
nano-antennas and operating in the visible wavelength range that is capable of
converting spin to an arbitrary value of OAM $\ell$. The nano-antennas are
arranged in an array with a well-defined geometry in the transverse plane of
the beam, possessing a specific integer or half-integer topological charge $q$.
When a circularly polarised light beam traverses this metasurface, the output
beam polarisation switches handedness and the OAM changes in value by $\ell =
\pm2q\hbar$ per photon. We experimentally demonstrate $\ell$ values ranging
from $\pm 1$ to $\pm 25$ with conversion efficiencies of $8.6\pm0.4~\%$. Our
ultra-thin devices are integratable and thus suitable for applications in
quantum communications, quantum computations and nano-scale sensing.
