Effectively measuring the force that light exerts on certain molecules and cells has become one of the most relevant areas of Physics, Chemistry, and Biology in recent years. Up to now, researchers have used Maxwell’s stress tensor conservation law, which provides the actual force, or radiation pressure, that a beam of light exerts on an object. However, a recent discovery could change the way we understand this interaction.
A team of researchers composed of Manuel Nieto-Vesperinas, from the Institute of Materials Science in Madrid (ICMM-CSIC), and Xiaohao Xu, from the Chinese Academy of Sciences, has demonstrated the existence of a universal reactive force that opposes and, therefore, diminishes that radiation pressure. “We have discovered the existence of a universal phenomenon related to the electrodynamic (optical) forces exerted by light or other electromagnetic waves on charges, electric currents, and particles; a new paradigm of the mechanical efficiency of light on matter,” emphasized Nieto-Vesperinas.
The theory used up to now only made use of Maxwell’s stress tensor, describing only half of the physics that explains radiation pressure. The other half, which the researchers have unveiled and formulated in a study published in the journal Light: Science & Applications, is characterized by the imaginary part of a complex stress tensor, of which the Maxwell tensor is just its real part.
“We have discovered the existence of a universal phenomenon related to the electrodynamic and optical forces in particular, exerted by light or other electromagnetic waves on a distribution of charges and electric currents in general, and bodies or particles in particular,” Nieto-Vesperinas states.
This discovery opens up new possibilities in optical manipulation and propulsion with light. According to the researcher, “it constitutes a new paradigm of the mechanical efficiency of light on matter, and completes the panorama of electromagnetic forces in photonics and electrodynamics.” This new understanding will allow for better design of both lighting and matter, in optical manipulation and propulsion through light. By controlling the incident power, dissipation and heating caused by the interaction can be reduced.
Nieto-Vesperinas compares this law to Poynting’s theorem on the conservation of electromagnetic energy. According to this theorem, energy transport is measured by two variables: one real, which is known, and one imaginary, which depends on the electrons and is alternating. Knowing this reactive power is crucial to optimize the efficiency of the emitted energy. Similarly, this new study reveals that the force of light on matter also has an imaginary part, whose existence was not known until now.
The conservation law of the momentum of electromagnetic waves has always worked with a real variable: Maxwell’s stress tensor. This new work demonstrates the existence of an imaginary part of a complex stress tensor, touching the foundations of electrodynamics and affecting areas such as radiation pressure propulsion of matter, creation of optical bonds, and manipulation of objects through light.
In the current context of macro and nanoscience development, this discovery is particularly relevant. “It is such a basic conservation law that, over time, it is likely to be included in undergraduate and graduate textbooks in Physics and Engineering,” notes Nieto-Vesperinas. Both researchers acknowledge the practical difficulties in controlling photon propulsion and manipulation, but consider that the rapid advances and current maturity of the field now justify the formulation of this theory.
This innovative scenario completes an interpretative panorama of the dynamics in the science of light and electrodynamics, and could be fundamental for optimizing machines. Additionally, it suggests the existence of reactive forces in the mechanical action of sound waves, fluids, and matter waves, opening up a vast field of research, conclude the authors.
Referrer: MiMub in Spanish
