A new way to cool materials with light

New findings by University of Michigan researchers overturn scientists’ understanding of how light and sound interact in the process called Brillouin scattering.

First explained by French physicist Léon Brillouin in 1922, the phenomenon describes how light waves and acoustic waves are coupled in almost any transparent material, such as glass, water, crystals such as  silicon, and air.

In 1964, laser pioneer Charles Townes used this phenomenon to demonstrate ‘Brillouin lasers’ in which acoustical waves replace the electron vibrations in conventional lasers. Previous researchers focused on the emitted light in a Brillouin laser, however, these lasers also contain an amplified acoustical wave in the host material that was rarely investigated.

“For the better part of the century since its discovery, it has been textbook knowledge that the component in the Brillouin scattering that amplifies sound, which involves heating, is always dominant” said Gaurav Bahl, a postdoctoral researcher in the Department of Electrical Engineering and Computer Science and first author of a paper on the new findings published in the print edition of Nature Physics.

“Our work attempts to change this notion, by providing the first experimental evidence of the cooling of an acoustical density wave in a solid using Brillouin scattering.”

“We’re tipping the balance,” added Tal Carmon, assistant professor in the Department of Electrical Engineering and Computer Science and principal investigator. “Where previously the heat element was dominant, we found a way to use Brillouin scattering for cooling.”

They did this by using a glass device that only allowed certain colors of light to circulate in a resonator. The colors of light that would have a cooling effect—those toward the blue end of the spectrum, were allowed to circulate 1 million times in the resonator, enhancing their cooling ability. The redder colors that would heat were filtered out of the system.

In this manner, the researchers used light to cool a single mode of vibration of a glass sphere to an effective temperature of -425.2 degrees Fahrenheit (19 Kelvin).

The work advances the scientific understanding of laser cooling technologies currently being pursued to explore the boundary between classical and quantum physics.

Research on the cooling of solids can lead to, as Bahl describes, “exquisitely sensitive physical sensors,” such as gyroscopes, accelerometers, and picogram mass sensors that are capable of weighing tiny objects such as cells or large proteins.

The paper, titled “Observation of spontaneous Brillouin cooling,” by Gaurav Bahl, Matthew Tomes, Florian Marquardt, and Tal Carmon, is published in Nature Physics. It is also is featured in a News & Views piece in Nature Physics.

Story by: Nicole Casal Moore (734-647-7087)