Measurements of optical forces in water refute most current theories of light-matter interaction

date: 25.04.2022

category: Sporočila za javnost


An international team of researchers, coordinated by Nelson G. C. Astrath, a professor at the Universidade Estadual de Maringá, Brazil, and with which a scientist from the Faculty of Mechanical Engineering, University of Ljubljana has been participating for a long time, measured elastic waves caused by an optical effect called electrostriction. In this phenomenon, the presence of light compresses matter.

Employing a laser pulse, they illuminated a thin layer of ultra-pure water, bounded on both sides by glass plates, and observed how water’s optical properties are altered by the transient presence of light solely due to electrostriction. As laser light travels through the layer of water, the optical densification of matter occurs in the direction perpendicular to the direction of light propagation. Using a nanosecond laser pulse, the compression occurs in the same short time interval, thus emitting elastic waves—propagating acoustic pressure disturbances.

The measurement was special in the sense that the phenomena that usually dominate the light-matter interactions were avoided. By bounding the water surface with a glass plate, they got rid of the bulging of the water surface at the location of the transmission of the light pulse into and out of the water, which was first detected by the Nobel laureate Arthur Ashkin, who attributed this phenomenon to the increase of the linear momentum of light as it enters from the matter with a lower into the one with a higher refractive index. In addition, they made sure that water was heated as little as possible by the light pulse, thus avoiding thermal effects. The remaining effects of the interaction were separated in space and time, thus observing only that part of the signal corresponding to the elastic waves launched from within the illuminated water column due to 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, but it is known to lack an important term describing magnetostriction, an electrostriction-like phenomenon. 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.

Even though this is basic research in photonics, digging deep to the very foundations of the coupling between electromagnetism and condensed matter, its immediate applicability is foreseen in the field of optical manipulation of deformable matter. 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.

The results of the research will be presented at the 21st International Conference on Photoacoustic and Photothermal Phenomena, this year hosted by Slovenia in co-organization with the Faculty of Mechanical Engineering, University of Ljubljana.

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.

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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]

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]

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