Physicists managed to send light waves back in time

(ORDO NEWS) — Like watching a movie in rewind, physicists have demonstrated a new technique for reversing a wave of optical light in time.

This does not mean that they have actually turned the tide of time; rather, they found a way to induce the optical wave to travel in the opposite direction, returning to its starting point.

For the first time, optical wave reversal was achieved with complete control over all degrees of freedom of light simultaneously.

This would be a tremendous achievement in itself, but the high degree of spatio-temporal control required is important for applications such as imaging, nonlinear optics, and micromanipulation.

Wave time inversion occurs when a wave, having propagated through a medium, is re-radiated from the other side in such a way that it accurately repeats its path back to the source. These two paths are mathematically exactly the same, except for the direction of time.

Physicists have previously achieved partial space-time control of optical waves; but at higher frequencies of optical waves they are more difficult to measure and, therefore, to control.

This is what makes the work of physicists at the University of Queensland (UQ) in Australia and Nokia Bell Labs so great.

“Imagine passing a short pulse of light from a tiny point through some scattering material such as fog,” explains UQ physicist Mikael Munaix.

“Light arises in one place in space and at the same time, but is scattered when it passes through the fog and reaches the other side in different places at different times. We found a way to accurately measure where all the scattered light comes and at what time, then create a “reverse” version of the light and send it back through the fog.”

The re-emitted light repeats the original scattering process to return to the single point from which the first ray was emitted at the initial time.

The command device consists of a pulse shaper for controlling the shape of laser pulses and multi-plane light conversion, which allows the command to transform light in space.

Thus, the researchers could control light in two spatial degrees – amplitude and phase, as well as in one time degree when it passed through the optical fiber.


The exceptional control achieved by the team can be seen in a series of shots. They tuned the device so that the light forms shapes like the letters of the alphabet or a smiley face.

While the images are cute, they also generate a lot of interest: this level of control can allow the wave to focus on an area that traditional means cannot reach. The medium itself can be used to focus the re-scattered light.

“This new type of control in optics,” the researchers write in their paper, “could open up many possibilities that are not simply generalizations of previous demonstrations for low frequency phenomena with applications such as nonlinear microscopy, micromachining, quantum optics, optical capture, nanophotonics, and plasmonics, optical amplification and other new nonlinear space-time phenomena, interactions and sources”.


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