Physicists create time-reversed optical waves

Optical researchers at the University of Queensland and Nokia Bell Labs in the US have developed a new technique to demonstrate the inversion of optical waves in time that could transform the fields of advanced biomedical imaging and telecommunications.

Time reversal of waves in physics does not mean going back to the future. It describes a special type of wave that can retrace a path backwards through an object as if playing a movie of the moving wave in reverse order.

Dr. Mickael Mounaix and Dr. Joel Carpenter from UQ demonstrate together with the team of Dr. Nick Fontaine of Nokia Bell Labs first reversed the time of optical waves with a new device they developed that enables full 3D control of light through an optical fiber.

“Imagine launching a short pulse of light from a tiny point through a scattering material like fog,” said Dr. Mounaix.

“The light starts in a single place in space and at a specific time, but is scattered as it moves through the fog and arrives at many different places on the other side at many different times.

“We found a way to measure exactly where and at what time all the scattered light is arriving, then make a ‘backwards’ version of that light and send it back through the fog.

“This new time-reversed light wave will reproduce the original scattering process like looking back at a film and will ultimately arrive at the source exactly as it began: a single position at a single point in time.”

Dr. Carpenter said the reverse version of the beam of light, known as a time-reversed wave, is a random-looking 3D object, like a tiny cloud of light.

“To create this cloud of light, you have to let a first ball of light fly into the system and then shape it into the desired 3D structure,” said Dr. Carpenter.

“This sculpting has to be done on a trillionth of a second time scale. So that’s too fast to sculpt with moving parts or electrical signals slices, rerouting the pieces, and then recombining them to create a base sculpture as the clay flies through without ever slowing down.

Dr. Fontaine said there was no device that could completely control and shape a beam of light in 3D before the team developed this technique.

“It is very important to control the light output as precisely as possible for many applications, from imaging to capturing objects with light to generating very intense laser beams,” said Dr. Fontaine.

With the new device, researchers can carry out experiments that were previously impossible and put theoretical concepts to the test in many areas.

This research was published in Nature communication.