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MEASUREMENTS OF OPTICAL FORCES IN WATER REFUTE MOST CURRENT THEORIES OF LIGHT-MATTER INTERACTION

Sr. Res. Assoc. PhD. Požar Tomaž, source: personal archive

Publish Date: 13.07.2022

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An international team of researchers with Sr. Res. Assoc. PhD. Požar Tomaž, a scientist from the Faculty of Mechanical Engineering, University of Ljubljana, measured elastic waves caused by an optical effect called electrostriction. In this phenomenon, the presence of light compresses matter. By measurement, they refuted most theories describing the basic interaction between light and matter. Even though this is basic research in photonics[1], its immediate applicability is foreseen in the field of optical manipulation of deformable matter. Example: More specifically, when laser tweezers are used in biology and medicine to manipulate single cells, it is important to know the correct spatial distribution of the force acting on the cells, because intense local optical forces have significant optomechanical effects on the cell. As water is usually the major component of organic soft matter, our description of the electrostriction effect should play an essential role in this scenario. Another application of the results of this work is in the very accurate optoacoustic determination of the absorption coefficient of light in low-loss liquids.

Grafika_Požar

Figure: Artistic illustration of the emission of electrostriction-generated elastic waves in water bounded by glass plates. The probe beam sensing the effects is illustrated in red [design: Mikko Partanen, Aalto University | based on the computer model of the experiment: Nelson G. C. Astrath, Universidade Estadual de Maringá]

It is a breakthrough of international researchers from Brazil, Slovenia, Finland, Norway, Switzerland, and the USA, coordinated by Nelson G. C. Astrath, a professor at the Universidade Estadual de Maringá, Brazil, and with which a scientist Sr. Res. Assoc. PhD. Požar Tomaž from the Faculty of Mechanical Engineering, University of Ljubljana has been participating for a long time. The research was financially supported by: Slovenian Research Agency (programme P2-0231), European Commission (MSCA), and foreign financers: CNPq, CAPES, Fundação Araucária, FINEP and COPEL. A scientific article on the discovery was published in the prestigious scientific journal Light: Science & Applications - Nature (IF: 17.782).

The team used the method of photo-induced lensing (by observing the deflection of additional, much weaker laser light) to measure the elastic waves caused by optical electrostriction. As they struggled to describe the phenomenon with hitherto known variants (formalisms) of the classical theory of coupling between an electromagnetic field and a dielectric, they found that most of these formalisms incorrectly predict the amplitude of the electrostriction-generated elastic waves. Thus, they disproved the validity of the theoretical variants proposed separately by Abraham, Minkowski, Chu, and Ampère. Only the theory proposed by Einstein and Laub agreed with the new measurements. "However, we already know that Einstein-Laub equation is wrong, because it does not include the term that would describe magnetostriction, and it is thus impossible to describe it correctly," explains Požar. Therefore, the authors built a completely new, microscopic theory of the coupling of an electromagnetic field with matter that correctly predicts the effects of optical electrostriction in water and at the same time does not have the shortcomings of the Einstein and Laub formalism.

Using a laser pulse, they illuminated a thin layer of very pure water, bounded on both sides by glass plates, and observed how optical properties of water are changed during the transient presence of light simply due to electrostriction. As laser light travels through the water layer, the phenomenon of optical densification of matter transverse to the direction of light propagation can be detected. Employing a nanosecond laser pulse compresses matter so quickly that elastic waves — traveling acoustic pressure disturbances — are emitted. The measurement was special in the sense that the phenomena that usually dominate the light-matter interactions were avoided. 

Animation: Temporal evolution of the pressure distribution in water and cuvette walls under pulsed excitation shown by the initial green beam of light. [design: Mikko Partanen, Aalto University | based on the computer model of the experiment: Nelson G. C. Astrath, Universidade Estadual de Maringá]

 

[1] Photonics is the science of light. It covers extremely wide areas of light generation in light sources, especially lasers, light propagation and modulation, its detection and use for various purposes. Source: Martin Čopič and Mojca Vilfan, Fotonika, Publishing house UL FMF (2020).

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