Far out physics: light has momentum (Introduction)

by David Turell @, Saturday, March 02, 2024, 21:35 (82 days ago)

Shown in a spinning mechanism:

https://phys.org/news/2024-03-laser-merry.html

"Light constantly nudges things and can even be used to push spacecraft. Light can also spin objects if it carries orbital angular momentum (OAM)—the property associated with a rotating object's tendency to keep spinning.

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"physicists at UMD, led by UMD Physics Professor Howard Milchberg, have discovered that light can carry its OAM in a vortex turned to the side—the phase spins like a wheel on a car, rolling along with the light. The researchers called these light structures spatio-temporal optical vortices (STOVs) and described the momentum they carry as transverse OAM.

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"In an article published in the journal Physical Review X, the team describes a novel technique they used to change the transverse OAM of a light pulse as it travels. Their method requires some laboratory tools, like specialized lasers, but in many ways, it resembles spinning a playground merry-go-round or twisting a wrench.

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"Their theory predicts that pushing on different parts of a merry-go-round light pulse can change its transverse OAM by different amounts and that if a bit of light is scattered off a speck of dust and leaves the pulse, then the pulse loses some transverse OAM with it.

"The team focused on testing what happened when they gave the transverse OAM vortices a shove. But changing the transverse OAM of a light pulse isn't as easy as giving a merry-go-round a solid push; there isn't any matter to grab onto and apply a force. To change the transverse OAM of a light pulse, you need to flick its phase.

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"In the new experiment, the team used a laser to make narrow columns of plasma, which they called transient wires, that are small enough and flash into existence quickly enough to target specific regions of the pulse mid-flight. The index of refraction of a transient wire plays the role of a child pushing the merry-go-round.

"The researchers generated the transient wire and meticulously aligned all their beams so that the wire precisely intercepted the desired section of the OAM-carrying pulse. After part of the pulse passed through the wire and received a flick, the pulse reached a special optical pulse analyzer the team invented. As predicted, when the researchers analyzed the collected data, they found that the refractive index flick changed the pulse's transverse OAM.

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"The team plans to continue exploring the physics associated with transverse OAM. The techniques they have developed could provide new insights into how OAM changes over time during the interaction of an intense laser beam with matter (which is where Milchberg's lab first discovered transverse OAM).

"The group plans to investigate applications of transverse OAM, such as encoding information into the swirling pulses of light. Their results from this experiment demonstrate that the naturally occurring fluctuations in the index of refraction of air are too slow to change a pulse's transverse OAM and distort any information it is carrying.

"'It's at an early stage in this research," Hancock says. "It's hard to say where it will go. However, it appears to have a lot of promise for basic physics and applications. Calling it exciting is an understatement.'"

Comment: this esoteric study may lead to different forms of communication at the speed of light.

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