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Laser Beam’s Behavior Runs Counter to Normal Laws of Refraction

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A type of laser beam, developed by a team at the University of Central Florida (UCF), does not follow the classical laws of refraction and could offer new opportunities for shaping the flow of light beams used for optical communications and laser applications.

Normally, light slows down when it passes through a dense material. The new class of laser beams, called spacetime wave packets, can be arranged to not change speed at all, or even to speed up in denser materials. “As such, these pulses of light can arrive at different points in space at the same time,” professor Ayman Abouraddy, the study’s principal investigator, said.

The researchers created the spacetime wave packets by using a spatial light modulator to reorganize the energy of a light pulse so that its properties in space and time were no longer separate. This allowed the researchers to control the group velocity of the pulse.

When a pulsed beam was given precise spatiotemporal spectral correlations, it showed refractory phenomena such as group-velocity invariance with respect to the refractive index, group-delay cancellation, anomalous group-velocity increase in higher-index materials, and tunable group velocity by varying the angle of incidence. In experiments, the researchers verified a law of refraction for spacetime wave packets that encompassed these effects.

These abilities are counter to Fermat’s principle, Abouraddy said. “What we find here ... is no matter how different the materials are that light passes through, there always exists one of our spacetime wave packets that could cross the interface of the two materials without changing its velocity,” he said. “So, no matter what the properties of the medium are, it will go across the interface and continue as if it’s not there.”

For optical communication, this means that the speed of a message traveling in spacetime wave packets would not be affected even if the packet traveled through different materials of different densities. Pulses could be arranged to propagate so that they arrive at different destinations at the same time.

“This new field that we’re developing is a new concept for light beams,” Abouraddy said. “As a result, everything we look into using these beams reveals new behavior.

“All the behavior we know about light really takes tacitly an underlying presumption that its properties in space and time are separable. So, all we know in optics is based on that. It’s a built-in assumption. It’s taken to be the natural state of affairs. But now, breaking that underlying assumption, we’re starting to see new behavior all over the place.”

Next steps for the researchers will include studying the interaction of these new laser beams with devices such as laser cavities and optical fibers, in addition to applying the insights they have gained to matter rather than to lightwaves.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-020-0645-6).  

Photonics Spectra
Oct 2020
Research & TechnologyeducationAmericasCentral Florida Universitylaserslight sourcesopticspulsed lasersspacetime wave packetsCommunicationslight refractionlight-matter interactionFermat's lawTech Pulse

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