Controlled generation of array beaMS of higher order orbital angular momentum and study of their frequency-doubling characteristics

Abstract

Optical vortices, beams carrying orbital angular momentum (OAM) per photon are of paramount interest in recent times for their wide variety of applications in particle trapping and micromanipulation, material lithography and quantum information. Due to the presence of a helical phase variation in propagation and an undefined phase at the center, these beams have a phase singularity in their wavefront, resulting in the doughnut-shaped intensity distribution. Though the vortex beams have been widely explored in the past, the recent advancements on multi-channel quantum-communication systems, multiple-particle trapping, single-shot material ablation, demand an array of optical vortices in a simple experimental scheme.

The present work elaborates on a novel experimental scheme to generate high power, ultrafast, higher order optical vortex arrays. Simply by using a dielectric microlens array (MLA) and a plano-convex lens we have generated array beams carrying the spatial property of the input beam. Considering the MLA as a 2D sinusoidal phase grating, we have numerically calculated the intensity pattern of the array beams in close agreement with the experimental results. We have also theoretically derived the parameters controlling the intensity pattern, size and the pitch of array and verified experimentally. The single-pass frequency doubling of the vortex array at 1064 nm in a 1.2 mm BiBO crystal produced green vortex arrays of orders as high as l_sh = 12, twice the order of the pump array beam, with a conversion efficiency as high as ~3.65%.

Additionally, the near field diffraction effects of MLA, have been studied in this work, indicating the self-imaging effect at Talbot planes. Using the MLA as an array illuminator at 1064 nm on to a 1.2 mm BiBO crystal, we have experimentally verified the second harmonic Talbot effect at 532 nm. In order to overcome the stringent dependence of the MLA parameters on the Talbot planes, we have devised a generic experimental scheme based on Fourier transformation technique, to independently tune the Talbot lengths.