Putting light in a spin generates a ring of fire on gold film

Ars Technica » Scientific Method 2017-03-24

Enlarge / The evolution of the vortex over femtoseconds. (credit: Spektor et. al.)

The late 20th and early 21st century have seen a revolution in the study of light. Far from the old days of seeing things dimly through microscopes, we are now in the position to freeze light, use it to make materials transparent, and watch it spiral around on a gold surface.

Watching light do its thing is very difficult. This sounds a bit silly, as we observe the world through the effects of light. But what we actually see is an average effect. Light, shade, colors, and texture all come to us via the intensity of light, provided by lots of individual photons. We are in no position to see the femtosecond flickering of the field that averages to our spectacular view of the world.

All the interesting stuff we see is related to the amplitude and phase of the light field, though. And the amplitude of a light wave changes very fast, going through a complete cycle in two to three femtoseconds. The wavefront (phase) also travels very fast, moving around 300 nanometers every femtosecond. Tracking this sort of motion is tricky, but it reveals all sorts of intriguing stuff.

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