New class of laser beam doesn't follow normal laws of refraction

Date:
August 6, 2020
Source:
University of Central Florida
Summary:
Researchers have developed a new type of laser beam that
doesn't follow long-held principles about how light refracts and
travels. The findings could have huge implications for optical
communication and laser technologies.



FULL STORY ========================================================================== University of Central Florida researchers have developed a new type
of laser beam that doesn't follow long-held principles about how light
refracts and travels.


==========================================================================
The findings, which were published recently in Nature Photonics, could
have huge implications for optical communication and laser technologies.

"This new class of laser beams has unique properties that are not shared
by common laser beams," says Ayman Abouraddy, a professor in UCF's
College of Optics and Photonics and the study's principal investigator.

The beams, known as spacetime wave packets, follow different
rules when they refract, that is when they pass through different
materials. Normally, light slows down when it travels into a denser
material.

"In contrast, spacetime wave packets can be arranged to behave in
the usual manner, to not change speed at all, or even to anomalously
speed up in denser materials," Abouraddy says. "As such, these pulses
of light can arrive at different points in space at the same time."
"Think about how a spoon inside a water-filled glass looks broken at
the point where the water and air meet," Abouraddy says. "The speed
of light in air is different from the speed of light in water. And so,
the light rays wind up bending after they cross the surface between air
to water, and so apparently the spoon looks bent. This is a well-known phenomenon described by Snell's Law." Although Snell's Law still applies,
the underlying change in velocity of the pulses is no longer applicable
for the new laser beams, Abouraddy says. These abilities are counter
to Fermat's Principle that says light always travels such that it takes
the shortest path, he says.



========================================================================== "What we find here, though, 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," Abouraddy says. "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 communication, this means the speed of a message traveling in these packets is no longer affected by traveling through
different materials of different densities.

"If you think of a plane trying to communicate with two submarines at
the same depth but one is far away and the other one's close by, the
one that's farther away will incur a longer delay than the one that's
close by," Abouraddy says.

"We find that we can arrange for our pulses to propagate such that they
arrive at the two submarines at the same time. In fact, now the person
sending the pulse doesn't even need to know where the submarine is, as
long as they are at the same depth. All those submarines will receive
the pulse at the same time so you can blindly synchronize them without
knowing where they are." Abouraddy's research team created the spacetime
wave packets by using a device known as a spatial light modulator to
reorganize the energy of a pulse of light so that its properties in
space and time are no longer separate. This allows them to control the
"group velocity" of the pulse of light, which is roughly the speed at
which the peak of the pulse travels.

Previous work has shown the team's ability to control the group velocity
of the spacetime wave packets, including in optical materials. The
current study built upon that work by finding they could also control
the spacetime wave packets' speed through different media. This does
not contradict special relativity in any way, because it applies to the propagation of the pulse peak rather than to the underlying oscillations
of the light wave.



========================================================================== "This new field that we're developing is a new concept for light beams," Abouraddy says. "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." Co-authors of the study were Basanta
Bhaduri, lead author and a former research scientist with UCF's College
of Optics and Photonics, now with Bruker Nano Surfaces in California,
and Murat Yessenov, a doctoral candidate in the college.

Bhaduri became interested in Abouraddy's research after reading about it
in journals, such as Optics Express and Nature Photonics, and joined the professor's research team in 2018. For the study, he helped develop the
concept and designed the experiments, as well as carried out measurements
and analyzed data.

He says the study results are important in many ways, including the new research avenues it opens.

"Space-time refraction defies our expectations derived from Fermat's
principle and offers new opportunities for molding the flow of light
and other wave phenomena," Bhaduri says.

Yessenov's roles included data analysis, derivations and simulations. He
says he became interested in the work by wanting to explore more about entanglement, which in quantum systems is when two well-separated objects
still have a relation to each other.

"We believe that spacetime wave packets have more to offer and many more interesting effects can be unveiled using them," Yessenov says.

Abouraddy says next steps for the research include studying the
interaction of these new laser beams with devices such as laser cavities
and optical fibers, in addition to applying these new insights to matter
rather than to light waves.

The research was funded by the U.S. Office of Naval Research.


========================================================================== Story Source: Materials provided by
University_of_Central_Florida. Original written by Robert Wells. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Basanta Bhaduri, Murat Yessenov, Ayman F. Abouraddy. Anomalous
refraction
of optical spacetime wave packets. Nature Photonics, 2020; 14 (7):
416 DOI: 10.1038/s41566-020-0645-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200806133511.htm

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