  Spin-to-orbital-angular-momentum conversion has attracted considerable
interest as a tool to create exotic light beams, leading to the emergence of
novel devices that implement this function. These converters exploit the
geometrical phase to create helical beams of handedness determined by the
chirality of the incident light. This property is finding important
applications in quantum optics thanks to the demonstration of liquid crystal
spin-to-orbital angular momentum converters (SOC) known as q-plates. Here we
demonstrate high-efficiency SOCs in the visible based on dielectric
metasurfaces that generate vortex beams with high and even fractional
topological charge and show for the first time the simultaneous generation of
collinear helical beams with different and arbitrary orbital angular momentum.
We foresee that this versatile method of creating vortex beams, which
circumvents the limitations of q-plates, will significant impact microscopy and
vector beam shaping.
