The experiment makes “shots” of light, stops light, uses light to change the properties of matter


Credit: Petr Kratochvil / public domain

Light travels at a speed of about 300,000,000 meters per second as particles of light, photons, or the equivalent of electromagnetic field waves. The experiments, led by Hrvoje Petek, a professor at RK Mellon at the Department of Physics and Astronomy, examined ideas about the origin of light, taking images of light, stopping light and using it to change the properties of matter.

On Friday, he worked on experiments with students and associates of prof. Chen-Bin (Robin) Huang from Tsing Hua National University in Taiwan and Atsushi Kubo from Tsukuba University in Japan. Their findings were published in the paper “Plasmon topological quasiparticle on a nanometer and femtosecond scale”, which was published in the December 24 issue. Nature magazine.

Friday thanked graduate student Yanan Dai for his anticipation and work in the process.

“The mutual outcome of the research is that Yanan, who performed the experiments and provided theoretical modeling, showed that he was educated far above the level of his professor and could insightfully interpret nanofemto topological properties and interactions of optical fields,” he said.

The team performed an ultrafast microscopy experiment, where they captured 20 fs (2×10) green light pulses-14 s) duration as complex waves of fluctuation of light electron density, known as surface plasmon polaritons, and pictured their propagation on the surface of silver at the speed of light. But they did so with a twist, so that the light waves merged on two sides to create a light vortex where light waves seem to orbit around a stationary common core like a whirlwind of waves. They could generate a film about how light waves rotate on a nanometer (10-9 m) a wavelength scale by recording electrons emitted from the surface by two light photons that are created together.

Gathering all such electrons with an electron microscope creates images where the light has passed, thus allowing researchers to take their own image. Of course, if nothing is faster than light, its image cannot be taken, but by sending two light pulses with their time separation advanced by 10-16 In steps, we could imagine how light waves merge causing their joint amplitude to rise and fall at fixed points in space forming a light vortex at nano (10-9 m) -femto (10-15 c) scale.

Such light vortices occur when you illuminate your red or green laser pointer on a rough surface and see a spotted reflection, but they also have cosmological significance. Light vortex fields can potentially cause transitions in the quantum-mechanical phase sequence in solid materials, so that the transformed structure of the material and its mirror image cannot be upgraded. In other words, the sense of vortex rotation creates two materials that are topologically different.

Friday said such topological phase transitions are at the forefront of physics research because they are thought to be responsible for some aspects of the structure of the Universe.

“Even forces of nature, including light, are thought to have emerged as symmetry transitions in the primordial field. Thus, the ability to record optical fields and plasmon vortices in an experiment paves the way for ultrafast microscopic studies of light-related triggered phase transitions in condensed materials in laboratory proportions, ”he said.

Illuminating nanoscale dynamics

More information:
Plasmon topological quasiparticle on nanometer and femtosecond scale, Nature (2020). DOI: 10.1038 / s41586-020-3030-1,

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