Spiniform phase-encoded metagratings entangling arbitrary rational-order orbital angular momentum

A device that can generate light beams with continuously variable values of orbital angular momentum (OAM) has been demonstrated. Quantum entanglement between photons with integer or fractional OAM has been realized, but until now it has been difficult to generate light beams that have an arbitrary rational amount of OAM. Now, Cheng-Wei Qiu of the National University of Singapore and co-workers have designed and tested an analog vortex transmitter of rational-order OAM, that consists of two tilted gratings under a circular aperture. Plane waves of green light incident on the aperture are converted into vortex beams with a continuously controllable amount of OAM that is determined by the effective variation in size of the aperture across the device. Potential applications for the device include selection and sorting of microparticles, spiral imaging and high-dimensional quantum entanglement.