Publish Date: 27.08.2018

Category: News from the University

Scientific Article on Listening to the Reflections of Light Published in the Renowned Nature Communications Journal

The researchers from the Faculty of Mechanical Engineering of the University of Ljubljana in cooperation with three research institutions from Canada and Brazil were able to listen to the reflections of the light. The scientific article about the discovery was published in the renowned scientific journal Nature Communications. In order to describe their measurements, they developed a new research approach, which can be modelled entirely in a computer environment.

Although the idea that light carries linear momentum is not new, the exact nature of the interaction between light and matter has been a mystery for almost 150 years, ever since the existence of the linear momentum of light was predicted. The new research at the Faculty of Engineering of the University of Ljubljana, recently published in the renowned Nature Communications journal, presents an important step toward final explanation of this mystery of light.

The authors of the article Isolated detection of elastic waves driven by the momentum of light accurately analysed the motion of the sample mirror, from which they acquired the data about the basic interaction between light and matter. They listened and analysed a very weak ultrasound, which is the consequence of light pressure or a “hit” of a laser pulse lasting a few billionths of a second against a high-reflectivity mirror. The sound is so quiet that the atoms, put into motion by the sound, get displaced by less than a hundredth of their diameter; but it is also strong enough to provide an experimental access to the characteristic fingerprint, which the reflection of light leaves behind.

With this experiment they want to discern how the linear momentum carried by light is transferred to the mirror during the process of reflection. This question is important, since its answer can decide, which of the competitive formalisms of the classical theory of electromagnetism is correct. Namely, different formalism predict different distributions of optical forces. These distributions of forces cause different sound waves, which propagate through the mirror. By detecting the displacements, induced by the sound waves during their reflection off the mirror surface, they can decipher information carried by these elastic waves. Namely, they contain the fingerprint unique for each formalism. This was also the motivation for the research, carried out by the authors, the researchers from the Faculty of Mechanical Engineering, Tomaž Požar, Jernej Laloš, Aleš Babnik, and Rok Petkovšek, who worked in collaboration with foreign colleagues Max Bethune-Waddell, Kenneth J. Chau, Gustavo V. B. Lukasievicz and Nelson G. C. Astrath.

With this achievement they set the ground for the new experimental approach, which can be computer modelled in its entirety. The comparison between the initial experiments and the results of simulations offer great hopes for the final experimental definition of the part of the electromagnetic theory, which describes how light exchanges linear momentum and energy with matter.

A statement by Tomaž Požar about the discovery and the publication of the article:

“The basic motivation for our international collaboration was the experimental resolution of an about 150-year-old problem about how light interacts with matter exclusively due to light pressure. Our research efforts in this field are closely monitored by developers and users of optical tweezers, since they still have no suitable theoretical description for manipulation of deformable matter.”

The research was financially supported by: Slovenian Research Agency (ARRS), the Brazilian agencies CAPES, CNPq and Fundaçao Araucária, and the Canadian NSERC.

Video presentation of the discovery