Physicists Create Time-Reversed Optical Waves For The First Time

A pulse of light scattered through single and multi-mode optical fibers produces a complex shape in both time and space. Now these have been time reversed for the first time. University of Queensland

Physicists have achieved an unprecedented time reversal of optical waves. Sadly (or thankfully, depending on your perspective), this has nothing to do with time travel, but it could prove very useful none the less.

Interference can make a once simple wave complex as it spreads. Time reversal involves collecting all that complexity and inverting it to recreate the wave's original form. It's already common with water and sound waves, and even in relatively low-frequency electromagnetic waves, but Dr Mickael Mounaix and Dr Joel Carpenter of the University of Queensland have now achieved it at wavelengths we can almost see.

“Imagine launching a short pulse of light from a tiny spot through some scattering material, like fog,” Mounaix said in a statement. “The light starts at a single location in space and at a single point in time but becomes scattered as it travels through the fog...We have found a way to precisely measure where all that scattered light arrives and at what times, then create a ‘backwards’ version of that light, and send it back through the fog.”

Mounaix compares the process to watching a film in reverse.

If you've had a kidney stone broken up using ultrasound you may have already experienced the benefits of time reversing waves. Time reversing the pattern produced by waves scattering off stones helps doctors target their shock waves precisely on the object that needs to be broken up, not the surrounding organs.

However, ultrasound has frequencies of 20,000 to several billion hertz. Microwaves have maximum frequencies of 300 billion hertz or less. Visible light, on the other hand, starts at more than 1,000 times that, which meant Mounaix and Carpenter needed to do something quite different. Carpenter refers to creating the pattern to be directed back to sculpting a 3D structure out of the detected light waves. Instead of thousands or millionth of seconds, “That sculpting needs to take place on time scales of trillionths of a second, [for optical light]” he said. “So that’s too fast to sculpt using any moving parts or electrical signals.”

In Nature Communications Mounaiz and Carpenter announce they have succeeded, passing pulses of wavelengths around 1,551.4 nanometers through optical fibers that split the pulses along many optical paths to create a complex output that was collected and time reversed.

 

“Previous experiments in optics have demonstrated spatial control, temporal control or some limited combination of both,” the paper notes, whereas here they have combined both.

Although 1,551 nanometers is in the infrared, rather than visible light, Carpenter told IFLScience it's still considered an optical wavelength, and their work could be replicated with visible light lasers. The frequency is standard for telecommunications, being where glass is most transparent.

Carpenter admitted to IFLScience if the pulse “Had too many fine features we would not be able to represent it, or if it had too many features over too long a delay.” Nevertheless, he says the work should open up possibilities for amplifying lasers without distortion or for identifying the shape of irradiated organs within the body where intervening flesh produces a scattered pattern.

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