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Momentum of Light Drives Elastic Waves

Photonics Handbook
The exact nature of how light interacts with matter is the focus of an international research study including researchers from Brazil and Slovenia and led by professor Kenneth Chau at the University of British Columbia (UBC).

The team measured picometer-scale surface displacements on a solid-state dielectric mirror illuminated by a laser light pulse, under experimental conditions designed to minimize absorption. Simulations of momentum deposition and material deformation yielded waveforms that closely matched the experimental measurement, confirming that the measured surface displacements were driven almost entirely by the momentum of light.

Kenneth Chau is an associate professor of engineering at UBC’s Okanagan campus. Courtesy of UBC Okanagan.
Kenneth Chau is an associate professor of engineering at UBC’s Okanagan campus. Courtesy of UBC Okanagan.

“Until now, we hadn’t determined how this momentum is converted into force or movement,” Chau said. “Because the amount of momentum carried by light is very small, we haven’t had equipment sensitive enough to solve this.”

The mirror constructed by the team was fitted with acoustic sensors and heat shielding to keep interference and background noise to a minimum. The sensors were used to detect elastic waves as they moved across the surface of the mirror.

In 1619, Johannes Kepler suggested that pressure from sunlight could be responsible for a comet’s tail always pointing away from the sun. In 1873, James Clerk Maxwell predicted that this radiation pressure was due to the momentum residing within the electromagnetic fields of light itself. Now, scientists have modeled and measured momentum coupling between electromagnetic fields and matter.
“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,” Chau said.

“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 modeling how light momentum exists inside materials.”

The simulation platform created by the team enabled spatio-temporal tracking of energy and momentum distribution in arbitrary configurations and enabled the team to identify different elastic wave types generated by light-matter interaction.

The discovery, in addition to advancing the fundamental understanding of light, could potentially be used for materials characterization, as optically induced elastic waves provide unique signatures depending on local optical and viscoelastic properties.

The research was published in Nature Communications (doi:10.1038/s41467-018-05706-3).

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