Generating optical orbital angular momentum at visible wavelengths using a plasmonic metasurface

TLDR

Light beams with helical phase fronts carry orbital angular momentum, and recent work shows that spin–orbit coupling in complex media can convert changes in spin angular momentum into orbital angular momentum. This study proposes and demonstrates that the birefringence of plasmonic nanostructures can convert circularly polarized light into light carrying orbital angular momentum. A visible‑wavelength device is fabricated from a space‑variant array of subwavelength plasmonic nano‑antennas. Experimental results show that circularly polarized light transmitted through the device acquires ±2 ħ orbital angular momentum with at least 1 % efficiency, offering a route to ultrathin OAM generators for spectroscopy, sensing, and communications.

Abstract

Abstract Light beams with a helical phase-front possess orbital angular momentum along their direction of propagation in addition to the spin angular momentum that describes their polarisation. Until recently, it was thought that these two ‘rotational’ motions of light were largely independent and could not be coupled during light–matter interactions. However, it is now known that interactions with carefully designed complex media can result in spin-to-orbit coupling, where a change of the spin angular momentum will modify the orbital angular momentum and vice versa . In this work, we propose and demonstrate that the birefringence of plasmonic nanostructures can be wielded to transform circularly polarised light into light carrying orbital angular momentum. A device operating at visible wavelengths is designed from a space-variant array of subwavelength plasmonic nano-antennas. Experiment confirms that circularly polarised light transmitted through the device is imbued with orbital angular momentum of ±2 ħ (with conversion efficiency of at least 1%). This technology paves the way towards ultrathin orbital angular momentum generators that could be integrated into applications for spectroscopy, nanoscale sensing and classical or quantum communications using integrated photonic devices.