Twisted Light Shows Potential for Optical Anti-Counterfeiting

A collaborative research team from three universities has fabricated a 3D printed nano optical security label that provides 33 to the power of 100 possible combinations for security in optical anti-counterfeiting. (Author’s note: this figure, though unconfirmed, is significantly higher than the number of combinations in the May MIT article.) 

(© Singapore University of Technology and Design)

The research team achieved the feat by exploiting orbital angular momentum (OAM) beams, through the fabrication of 3D printed spiral phase plates. Importantly, these plates were miniaturised to a diameter smaller than that of a strand of human hair and further integrated with structural colour filters – spiky looking structures that allow specific colours of light through.

The physics of orbital angular momentum is most frequently used to describe the rotational motion of an object around a particular point or axis in space, which results in the object moving in a curved path or an orbit, such as the Earth’s motion around the Sun.

When the concept of OAM is applied to light, it refers to a property of light waves that describes their rotational motion around a central axis. Unlike the more familiar concept of spin angular momentum, which is associated with the polarisation of light, orbital angular momentum specifically relates to the spatial distribution of the light wave.

If you think of a light beam as a corkscrew being twisted, the OAM of light quantifies the amount of twisting in the light beam. The OAM of light has applications in various fields, such as optical communications, optical trapping, and quantum information processing. It enables the encoding of information in the twisted phase structure of light, allowing for increased data capacity and novel manipulation techniques in optical systems.

The researchers collaborating on the project include Associate Prof Joel Yang from the Singapore University of Technology and Design (SUTD), Prof Min Gu from University of Shanghai for Science and Technology (USST) and Associate Prof Cheng-Wei Qiu from National University of Singapore (NUS), along with their respective teams. Their work has been published in a paper in Nature Nanotechnology ¹.

‘OAM light beam is increasingly being used in exciting research spaces such as optical communications, super-resolution imaging and quantum computing and we wanted to explore its capabilities in the anti-counterfeiting field as well. But OAM requires coherent light sources like lasers. We wanted to see if we could use incoherent light from the sun or a light bulb to generate OAM beams instead,’ explained Hongtao Wang, first author of the paper and SUTD-NUS joint PhD student.

In their study, the researchers introduced colour, spatial position, and OAM of light onto a small, coloured vortex beam (CVB) generator (25 µm). With only a 10-by-10 CVB unit array to demonstrate, the optical security label they designed already showed promise for potential use for optical anti-counterfeiting.

‘We see things clearly when we hold them up to the light. What our team has done is to learn how to use the natural light that surrounds us and extract tiny beams from it that carry information encoded in not just colour, but also by how much we ‘twist’ its wavefront. This optical version of the combination lock that utilises high-dimensional structured light provides us with a powerful platform for advanced anti-counterfeiting and information security,’ explained principal investigator Associate Prof Yang.

A video showing the optical security label consisting of a coloured vortex beam array can be found at www.youtube.com/watch?v=byU83L_DKJs&t=12s.

1 - www.nature.com/articles/s41565-023-01319-0