Nano Lett. http://doi.org/sb3 (2014)

A light beam with orbital angular momentum has a helical wavefront, and these optical vortices could be of use in applications such as imaging and quantum cryptography. Inducing angular momentum in a typical laser beam is, however, a relatively cumbersome procedure, requiring bulky components that cannot be easily integrated into optical circuits. Natalia Litchinitser and colleagues at the University at Buffalo have now created a waveguide array with compact dimensions that can spin a linearly or circularly polarized light beam into a beam with optical angular momentum.

To generate a helical wavefront it is necessary to induce a phase delay of 2π around the plane of the beam. To achieve this, the researchers first studied the effect that a single waveguide, which consisted of a hole with a radius of tens of nanometres milled in a metal plate, would have on an incident light beam. They found that the waveguide induces a phase delay that increases as the radius of the hole increases. A waveguide array was then fabricated that had a circular distribution of waveguide radii, designed to create the 2π delay. Interference experiments between a beam modified by the waveguide array and a reference beam confirmed that the array can create an optical vortex.