Physicists have found a highly efficient way to transfer angular momentum from a light beam to a tiny cylindrical object, causing it to spin in a way that could be used as a motor, such as pump in a microfluidic circuit.

By using resonant enhancement, the new device has drastically improved on previous approaches, achieving efficiency levels that surprised even its creators.

“We compared our result to a theoretically perfect transfer of angular momentum for a waveplate, and found our efficiency was six times higher than the limit. It did not make sense at first,” said Ivan Toftul, PhD student in the Nonlinear Physics Centre.

When the team looked into the result, they realised the unexpected gains came because the Mie resonances in the target cylinder were allowing higher-order angular momentum values in the scattering – the theoretical maximum applies to converting a 100 percent right-hand polarised light beam (angular momentum of +1ħ) to 100 percent left-hand polarisation.

But resonances, which arise from the dimensions of the target object, have their own angular momentum – with judicious choice you can achieve multiple resonances at once. This resonance matching can boost the torque further, achieving change in the angular momentum larger then 2ħ per photon on average,

“We got lucky,” Mr Toftul said. “The resonances overlap, and at the same time the particle is stable in the beam like a spinning top.”

One part of the ‘luck’ was Mr Toftul’s choice of a cylinder as the target shape.

“I was aiming for the simplest shape with some resonances – it turned out to be better than a sphere, in which the resonances don’t overlap,” he said.

“Perhaps more modelling, even with machine learning, could find even more efficient shapes.”

But as a starting place, the cylinder, made of silicon, was good enough. Mr Toftul performed the modelling in dimensionless parameters – at the scale of the standard telecommunications wavelength of 1550 nm this meant a cylinder with radius of 270 nm and height of 435 nm. The team used vector spherical harmonics expansion as a tool of their choice.

“The equations are very beautiful and symmetrical, and have well-known properties with respect to angular momentum,” Mr Toftul said.

“When you do the expansion, you can clearly see which mode caused the enhancement and what angular momentum it had.”

The research is published in ACS Photonics.

The investigation was prompted by Mr Toftul’s background in optical manipulation of matter (e.g. optical tweezers). When he joined the Nonlinear Physics Centre, who have a history of remarkable achievements leveraging Mie resonances, it was an obvious choice to see if resonant effects could enhance light-powered force effects.

As with optical tweezers, the cylinder is placed in two counterpropagating circularly polarized beams, and where it is trapped in a node of the standing wave. The counterpropagating nature prevents any photon “wind” from causing the particle to drift.

With the higher efficiency of momentum transfer, light powered rotators will now be able to achieve higher spin rates: while the rate could also be increased with higher-powered lasers, there is a practical limit to the power that can be injected, placed by the melting point of the cylinder material.

One puzzle this may help solve surrounds experiments with particles spinning at a rate of gigahertz, in which an unexplained nonlinear dependence on the air pressure has been observed.

“There may be more opportunities that will arise from this technique, due to the combination of high torque and the radiation-induced optical forces,” Mr Toftul said.