Phys. Rev. A 94, 013829 (2016)

Credit: APS

Caustic methods, which rely on phase manipulation and specific initial field distributions, make it possible to generate light trajectories other than straight lines, but so far most approaches have been limited to convex trajectories such as parabolae. Now, Yuanhui Wen and collaborators report the observation of light beams following arbitrary elliptical helical trajectories in free space. The key to their approach, based on a caustic technique that generates curved trajectories by interfering plane waves, was to consider a non-convex trajectory as a sequence of convex segments. To achieve the necessary phase and amplitude modulation of an incident light beam, the team fabricated a 0.5-mm-thick square quartz glass plate with a side length of 20 mm comprising a smaller effective area divided into 601 × 601 pixels. Each pixel was etched to determine a given phase shift, and a partial metal coverage realized specific transmittance patterns to modulate the beam amplitude.