Engineering professor works together with research team from Slovenia and Brazil

The idea that light has momentum is not new, but the exact nature of how light interacts with matter has remained a mystery for almost 150 years. New research from UBC's campus Okanagan, recently published in Nature Communications, has perhaps uncovered the key to one of the darkest secrets of light.

Johannes Kepler, the famous German astronomer and mathematician, suggested for the first time in 1619 that the pressure of the sunlight could be responsible for the tail of a comet that always points away from the sun, says co-author and UBC Okanagan engineer, Kenneth Chau. It took until 1873 before James Clerk Maxwell predicted that this radiation pressure was due to the momentum in the electromagnetic fields of the light itself.

"Until now, we had not determined how this momentum would be converted into force or movement," 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."

Now that technology is sensitive enough, Chau, with its international research team from Slovenia and Brazil, sheds light on this mystery.

To measure these extremely weak interactions between light photons, the team has built a special mirror with acoustic sensors and heat shielding to keep interference and background noise to a minimum. Then they made laser pulses in the mirror and used the sound sensors to detect elastic waves as they moved across the surface of the mirror, such as looking at ripples on a pond.

"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," Chau said. "We were able to reduce the characteristics of these waves to the momentum in the light pulse itself, which opens the door to eventually define and model how light pulses exist in materials."

The discovery is important in promoting our fundamental understanding of light, but Chau also points to practical applications of radiation pressure.

"Imagine traveling to distant stars on interstellar yachts powered by solar energy," Chau said. "Or maybe, here on earth, develop an optical tweezer that could assemble microscopic machines."

"We are not there yet, but the discovery in this work is an important step and I am delighted to see where it takes us."

The research was published in Nature Communications on 21 August with funding from the Natural Sciences and Engineering Research Council of Canada, the Slovenian research center, CAPES, CNPq and Fundação Araucária.

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