Much like planets orbiting the sun, light waves can spin around a central point as they travel, forming intricate corkscrew-like patterns. The strength of this motion—known as orbital angular momentum, or OAM—gives beams of light another identifying feature besides their colour and polarisation (the angles at which their waves travel).
OAM’s unique spiralling waves are intriguing as they offer a way to encode extra information into light beams, boosting optical technologies in many fields, according to Joel K. W. Yang, a Senior Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE).
“In communications, light’s OAM offers a theoretically infinite number of channels: each with a unique OAM, but all sharing the same frequency and space,” said Yang.
However, most light around us—the sun, electric bulbs, digital screens—produces ‘incoherent’ light waves. The clash of their differing frequencies and wavelengths causes them to spread and diffuse in uneven ways, not unlike a tangled clump of instant noodles.
“To date, we’ve needed lasers with very specific frequencies and coherence to generate beams with clear OAM,” said Yang. “Ordinary light sources create blurry OAM beams with mixed colours, lacking the precision for high-level applications like optical security.”
In a joint effort with colleagues from A*STAR’s Institute of High Performance Computing (IHPC), Singapore University of Technology and Design, National University of Singapore and University of Shanghai for Science and Technology, China, Yang co-developed a novel optical anti-counterfeiting device that generates OAM beams from ordinary light. Using miniaturised 3D spiral phase plates and structural colour filters, the device 'twists' incoherent light into multiple tiny beams with distinct OAM and colours.
“To make OAM information ‘visible’, we designed a set of phase plates that generates OAM beams by twisting light at a specific number of turns per oscillation,” Yang explained, comparing it to how a camera captures light’s intensity and colour.
Inspired by the historical ‘tiger tally’ of ancient China, the team then paired those phase plates with a second set that would ‘untwist’ the OAM beams produced, decoding the information they contained. “If the twisting and untwisting match each other, we can recover the original focused light; if not, the light takes on a distorted appearance,” said Yang.
Yang highlighted the team’s device as a breakthrough in manipulating light at the nanoscale to produce vortex beams with distinct colours and topological charges. Their device generated OAM beams with a doughnut-shaped intensity profile, which according to Yang is essential for advanced applications such as high-capacity optical communication, stimulated emission depletion microscopy and quantum computing.
The team’s method also offers an improved optical ‘lock-and-key’ mechanism for anti-counterfeiting and information security, as it can create numerous unique light combinations for security labels. “Our lock-key photonic device disrupts the one-to-one matching and validation scheme, extending it to a one-to-many pairing strategy,” Yang said.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE) and the Institute of High Performance Computing (IHPC).
