A team of physicists has taken a step towards creating more powerful lasers out of sound instead of light, which could have a huge number of practical uses beyond just being pretty neat.

Typical lasers themselves are pretty cool, and these devices haven't actually been around on Earth that long, being first created by humans in the 1960s. "Lasers produce a narrow beam of light in which all of the light waves have very similar wavelengths. The laser’s light waves travel together with their peaks all lined up, or in phase. This is why laser beams are very narrow, very bright, and can be focused into a very tiny spot," NASA explains

While sound and light have their differences – for instance sound only propagating through a medium such as fluids and solids – but physicists have been working on creating sound lasers by manipulating phonons.

"Similar to the photons that make up beams of light, indivisible quantum particles called phonons make up a beam of sound. These particles emerge from the collective motion of quadrillions of atoms, much as a 'stadium wave' in a sports arena is due to the motion of thousands of individual fans. When you listen to a song, you’re hearing a stream of these very small quantum particles," Andrew N. Cleland, Professor of Molecular Engineering Innovation and Enterprise, University of Chicago Pritzker School of Molecular Engineering, explained in a 2023 piece for The Conversation. 

"Originally conceived to explain the heat capacities of solids, phonons are predicted to obey the same rules of quantum mechanics as photons. The technology to generate and detect individual phonons has, however, lagged behind that for photons."

In the new study, scientists took a microsphere (a tiny ball) of silicon oxide (SiO₂) and suspended it using beams of light. This vibrated the ball, giving it an internal sound like a very high-pitched beep as well as sound beyond human hearing.

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Next, they began manipulating the vibrating microsphere with an alternating electric field in order to cause resonance, amplifying the sound waves up to a thousand fold at those frequencies.

"By applying a single-colour electronic injection to this levitated system, giant enhancement can be achieved for all higher-order phonon harmonics, with more than 3 orders enhanced brightness and 5 orders narrowed linewidth," the team explains in their paper. 

While the experiment took place in a vacuum in order to better measure the sound waves (confined inside the microsphere), the study takes a step towards creating sonic lasers we could use for many things, from exploring and mapping the ocean using sound to advancing medical imaging techniques.

"This work, providing much stronger and better-quality signals of coherent phonon harmonics," the team concludes, "is a key step towards controlling and utilizing nonlinear phonon lasers for applications such as phonon frequency combs, broadband phonon sensors, and ultrasonic bio-medical diagnosis."

The study is published in the journal eLight.

[H/T: Sabine Hossenfelder]