New Understanding of Light Momentum

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 may have uncovered the keys 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 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.

Kenneth Chau measured the extremely weak interactions between light photons with a special mirror fitted with acoustic sensors and heat shielding. (Source: UBC)

“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 that tech­nology is sensitive enough, 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.” (Source: UBC)

Reference: T. Požar et al.: Isolated detection of elastic waves driven by the momentum of light, Nat. Commun. 9, 3340 (2018); DOI: 10.1038/s41467-018-05706-3

Link: Nanophotonics (K. Chau), School of Engineering, University of British Columbia, Kelowna, Canada

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