A team of physicists has managed to demonstrate a new method for the time-reversal of a wave of optical light. It doesn’t mean that the researchers have actually handled the reversal of the flow of time – instead, they have discovered a technique to simulate an optical wave to reconstruct a forward path in reverse, returning to its point of birth.
This is the first time optical waves’ reversal has been attained with complete control over all of the levels of freedom of light at the same time. The outcome would be a significant achievement in and of itself, but the high degree of spatiotemporal control needed has benefits for applications like imaging, micromanipulation, and nonlinear optics.
Optical Light Waves’ Time Reversal
The time-reversal of waves takes place when a wave, having multiplied through a medium, is re-emitted in such a manner from the other side that it accurately retraces its path back to the origin. The two routes are mathematically identical, except for the direction of time.
The achievement has been attained with low-frequency waves, including acoustic waves, water waves, and microwaves. Researchers have also managed to achieve partial spatiotemporal control of optical waves; however, much higher frequencies of optical waves are difficult to measure and thus to control. This is what makes the research of physicists from the University of Queensland (UQ) in Australia and Nokia Bell Labs so outstanding.
UQ physicist Mickael Mounaix explained: “Imagine launching a short pulse of light from a tiny spot through some scattering material, like fog. 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 and arrives on the other side at many different locations at many different times. 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.”
The light that’s re-emitted retraces the first scattering process to arrive back at the source – at a single point in time. The physicists’ device that made the discovery possible has a pulse shaper for controlling the shape of laser pulses and multi-plane conversion, which enables the researchers to spatially transform light. This way, the team could manipulate the light on two spatial levels, namely amplitude and phase, and one temporal degree as it is moved through an optical fiber.
According to the researchers, the consequent time-reversed wave can be compared to a random-looking cloud composed of light.
“To create that light cloud, you need to take an initial ball of light flying into the system, and then sculpt it into the 3D structure you want,” explained UQ physicist Joel Carpenter. “That sculpting needs to take place on time scales of trillionths of a second, so that’s too fast to sculpt using any moving parts or electrical signals – think of it like shooting a ball of clay at high speed through a static apparatus with no moving parts, which slices up the ball, diverts the pieces, and then recombines the pieces to produce an output sculpture, all as the clay flies through without ever slowing down.”
The unprecedented control achieved by the team of physicists can be observed in a series of images. They designed the device so that, at the distal end, the light formed various shapes, such as a smiley face or the letters of the alphabet.
These images are of significant interest as this degree of control can enable a wave to be focused on a region that might not be possible to reach utilizing traditional methods. The medium itself can be employed to focus re-scattering light.
“This new type of control in optics,” the researchers wrote in their paper, “could open up many possibilities that are not just generalizations of previous demonstrations for lower frequency phenomena, with applications such as nonlinear microscopy, micromachining, quantum optics, optical trapping, nanophotonics and plasmonics, optical amplification, and other new nonlinear spatiotemporal phenomena, interactions, and sources.”
A paper detailing the findings was published in the prestigious journal Nature Communications.