KELOWNA, B.C., August 21, 2018 – The exact nature of how light interacts with matter is central to an international research study led by Kenneth Chau of the University of British Columbia (UBC) and including researchers from Brazil and Slovenia.
The team measured surface displacements on picometer scale on a dielectric mirror mirror that is 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 almost entirely driven by the momentum of light.
Kenneth Chau is an associate professor of technology at the Okanagan campus of UBC. Thanks to UBC Okanagan.
"Until now, we had not determined how this momentum would be turned into power or motion," Professor Chau said. "Because the amount of light carried by light is very small, we do not have equipment that is sensitive enough to solve this."
The mirror-built mirror was equipped with acoustic sensors and heat shielding to minimize interference and background noise. The sensors were used to detect elastic waves as they moved across the surface of the mirror.
In 1619, Johannes Kepler suggested that pressure by sunlight could be responsible for the tail of a comet that always points away from the sun. In 1873, James Clerk Maxwell predicted that this radiation pressure was due to the momentum in the electromagnetic fields of the light itself. Now scientists have modeled and measured the momentum coupling between electromagnetic fields and matter.
"We can not directly measure photon momentum, so our approach was to detect its effect on a mirror by listening to the elastic waves that had passed through it.
"We were able to reduce the characteristics of those waves to the momentum in the light pulse itself, which opens the door to eventually determine and model how light pulses exist in materials," Chau said.
The simulation platform created by the team allowed spatio-temporal tracking of energy and momentum distribution in random configurations and enabled the team to identify different elastic wave types generated by light-matter interaction.
The discovery, in addition to promoting the fundamental understanding of light, can potentially be used for material characterization, because optically induced elastic waves offer unique characteristics, depending on local optical and viscoelastic properties.
The research was published in Nature Communications (doi: 10.1038 / s41467-018-05706-3).