Researchers observe hidden distortions in complex light fields

Daily experience teaches us that light reflected from a perfectly flat mirror will give us a correct image without any distortion. Interestingly, this is not the case when the light field itself is structured in a complex way. Tiny deformations appear.

These were first observed in the laboratory by researchers at the University of Tampere. The results confirm the prediction of this fundamental optical effect made more than ten years ago. They also show how it can be used, for example, as a method for determining material properties.

The first observation of this fundamental optical effect is now presented in the article “Observation of topological aberrations of twisted light” published in Natural communications on September 17, 2024.

Light is a wave. Although this simple statement has been known to scientists for over a century, new properties and applications of light waves are regularly discovered and explored by researchers in optics and photonics. At the University of Tampere, the Experimental Quantum Optics (EQO) group studies the nuances of the shape – or structure, as it is often called – of light. The structure of light has become an important topic in modern optics, with advances ranging from the fundamentals of quantum physics to information science and optical communications.

In their latest work, the researchers showed that the shape of a light beam is slightly distorted when it is reflected by a perfectly flat object such as a mirror. Although the deformation is very small, it conveys important information about the object itself, such as the material it is made of. This topological aberration effect, predicted more than ten years ago by British researchers, was observed for the first time.

“Although the general idea of ​​observing a deformation seems rather simple, it took us over a year to perfect our experiment and adjust the original theory to distinguish the effect from all other natural beam deformations at the experimental research”, explains the associate professor. Robert Fickler, group leader of the EQO team.

Swirls of light and darkness

With recent technological advances in shaping light waves, the field of structured light has grown in recent decades. Much of the interest in this area is due to so-called twisted light waves, which not only travel at the speed of light, but also rotate as they travel.

“The curious thing about these twisted light fields is that they have completely dark spots, optical vortices as we call them, like swirls in water themselves without water. What we did, “it’s to observe how these vortices thread and move around when the beam interacts with a flat object, and what we can learn from these movements,” adds Rafael Barros, a postdoctoral researcher at the Academy. lead author of the study.

The dynamics of vortices in optical fields has been the subject of long-standing research and is generally considered a complex mathematical problem. In their work, the authors studied how vortices in a twisted light field move when reflected by an object.

They showed that although each optical vortex moves in complex ways, their collective motion is determined by the object’s properties in simple and predictable ways. The researchers emphasize that their work will inspire new ways to measure material properties with structured waves. This will bring a new twist to optical technologies.