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Scientists took snapshots of light

Light travels at the speed of 299,792,458 meters per second. In new experiments, Hrvoje Petek, an R.K. Mellon professor in the Department of Physics and Astronomy, examined ideas surrounding light’s origins.

Scientists took snapshots of light, stopped the light, and used it to change the matter’s properties.

In an ultrafast microscopy experiment, scientists trapped green light pulses of 20 fs (2×10-14 s) as composite light-electron density fluctuation waves, known as surface plasmon polaritons. They then imaged their propagation on a silver surface at the speed of light.

However, they did this with a twist, so the light waves met up from different sides to form a light vortex where light waves appear to circulate about a stationary common core as a whirlwind of waves. They could produce a movie of how light waves churn on their nanometer (10-9 m) wavelength scale by imaging electrons that two light photons coming together cause to emit from the surface.

Gathering all such electrons with an electron microscope forms images where the light had passed, empowering scientists to take a snapshot. By sending in two light pulses with their time separation advanced in 10-16 s steps, they could image how light waves come together, causing their joint amplitude to rise and fall at fixed points in space, forming a light vortex on the nano (10-9 m)-Femto (10-15 s) scale.

Such light vortices form when you shine your red or green laser pointer onto a rough surface and see a speckle reflection, but they also have a cosmological significance. The light vortex fields can potentially cause transitions in the quantum mechanical phase order in solid-state materials. The transformed material structure and its mirror image cannot be superimposed. In other words, the sense of the vortex rotation generates two topologically distinct materials.

Petek said, “such topological phase transitions are at the vanguard of physics research because they are thought to be responsible for some aspects of the structure of the Universe.

“Even the forces of Nature, including light, are thought to have emerged as symmetry-breaking transitions of a primordial field. Thus, the ability to record the optical fields and plasmonic vortices in the experiment opens the way to perform ultrafast microscopy studies of related light-initiated phase transitions in condensed matter materials at the laboratory scale.”

Journal Reference:
  1. Plasmonic topological quasiparticle on the nanometre and femtosecond scales, Nature (2020). DOI: 10.1038/s41586-020-3030-1

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