August 21, 2018
University of British
Columbia, Okanagan Campus
The idea that light has
momentum is not new, but the exact nature of how light interacts with matter has
remained a mystery for close to 150 years. New research may have uncovered the
key to one of the darkest secrets of light.
The idea that light has
momentum is not new, but the exact nature of how light interacts with matter
has remained a mystery for close to 150 years. New research from UBC's Okanagan
campus, recently published in Nature Communications, may have uncovered
the key to one of the darkest secrets of light.
Johannes Kepler, famed German
astronomer and mathematician, first suggested in 1619 that pressure from
sunlight could be responsible for a comet's tail always pointing away from the
Sun, says study co-author and UBC Okanagan engineering professor Kenneth Chau.
It wasn't until 1873 that James Clerk Maxwell predicted that this radiation
pressure was due to the momentum residing within the electromagnetic fields of
light itself.
"Until now, we hadn't
determined how this momentum is converted into force or movement," says
Chau. "Because the amount of momentum carried by light is very small, we
haven't had equipment sensitive enough to solve this."
Now, technology has caught up
and Chau, with his international research team from Slovenia and Brazil, are
shedding light on this mystery.
To measure these extremely
weak interactions between light photons, the team constructed a special mirror
fitted with acoustic sensors and heat shielding to keep interference and
background noise to a minimum. They then shot laser pulses at the mirror and
used the sound sensors to detect elastic waves as they moved across the surface
of the mirror, like watching ripples on a pond.
"We can't directly
measure photon momentum, so our approach was to detect its effect on a mirror
by 'listening' to the elastic waves that traveled through it," says Chau.
"We were able to trace the features of those waves back to the momentum
residing in the light pulse itself, which opens the door to finally defining
and modelling how light momentum exists inside materials."
The discovery is important in
advancing our fundamental understanding of light, but Chau also points to
practical applications of radiation pressure.
"Imagine travelling to
distant stars on interstellar yachts powered by solar sails," says Chau.
"Or perhaps, here on Earth, developing optical tweezers that could
assemble microscopic machines."
"We're not there yet, but
the discovery in this work is an important step and I'm excited to see where it
takes us next."
The study was published on
August 21 in Nature Communicationswith funding from the Natural Sciences
and Engineering Research Council of Canada, the Slovenian Research Agency,
CAPES, CNPq and Fundação Araucária.
Story Source:
Materials provided by University of British Columbia, Okanagan Campus. Original
written by Nathan Skolski. Note: Content may be edited for style and
length.
Journal Reference:
Tomaž Požar, Jernej Laloš,
Aleš Babnik, Rok Petkovšek, Max Bethune-Waddell, Kenneth J. Chau, Gustavo V. B.
Lukasievicz, Nelson G. C. Astrath. Isolated detection of elastic waves
driven by the momentum of light. Nature Communications, 2018; 9 (1)
DOI: 10.1038/s41467-018-05706-3
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