Scientists Uncover Hidden Property of Light That Twists Matter Sideways

Light can push and twist matter, a concept first proposed by James Clerk Maxwell in the 1870s. This idea led to the development of optical tweezers in the 1970s by Arthur Ashkin. Optical tweezers use focused laser beams to trap and move tiny particles.

However, measuring these small forces at the nanoscale has been difficult. Thermal motion constantly affects tiny objects, making weak forces hard to detect.

A New Way to Measure Forces

Scientists at Hokkaido University have created a new method to measure these forces precisely. They discovered that light can make tiny objects rotate sideways, which is perpendicular to the light's direction.

Professor Yoshito Y. Tanaka from Hokkaido University explained that their new "micro-drone" platform allows for the first full three-dimensional study of optical forces and torques on nanostructures.

The micro-drone is a small, cross-shaped device. A nanostructure sits in its center. Four laser beams hold the platform steady, much like optical tweezers. By tracking the platform's movements, researchers can figure out the forces acting on the object inside.

This method overcomes the limits of traditional optical tweezers, which could only measure rotation along one axis. The micro-drone captures motion and rotation in all directions, giving a complete 3D view. It makes tiny nanoscale forces easier to measure by turning them into larger movements of the platform.

Unexpected Sideways Twist

To test their method, the researchers used small gold structures shaped like the letter "V." When light hit these structures inside the micro-drone, they showed a "transverse optical torque." This means they twisted sideways instead of along the light's path.

Tanaka noted that this sideways rotation had not been seen experimentally at the nanoscale before.

This effect was surprising because earlier theories suggested such motion depended on the light's angular momentum. However, the team found that a different property, called optical helicity, was responsible. Optical helicity describes the "handedness" or twist of the light's electromagnetic field.

The researchers confirmed this by removing angular momentum while keeping helicity. The sideways rotation still happened, proving that helicity is the key factor.

This discovery helps us better understand how light interacts with matter at very small scales. It could lead to new ways of controlling nanoscale systems, potentially for light-driven nanomachines and advanced sensors.

Tanaka believes this work offers a new way to measure nanoscale optomechanics. He hopes it will open up new possibilities for studying mechanical phenomena at the nanoscale that were previously out of reach.

Deep Dive & References

Transverse optical torque observed at the nanoscale - Nature Physics, 2026