A new type of laser uses sound waves to help to detect weak forces

Most people are familiar with optical lasers through their experience with laser pointers. But what about a laser made from sound waves?

What makes optical laser light different from a light bulb or the sun is that all the light waves emerging from it are moving in the same direction and are pretty much in perfect step with each other. This is why the beam coming out of the laser pointer does not spread out in all directions.

In contrast, rays from the sun and light from a light bulb go in every direction. This is a good thing because otherwise it would be difficult to illuminate a room; or worse still, the Earth might not receive any sunlight. But keeping the light waves in step – physicists call it coherence – is what makes a laser special. Sound is also made of waves.

Recently there has been considerable scientific interest in creating phonon lasers in which the oscillations of light waves are replaced by the vibrations of a tiny solid particle. By generating sound waves that are perfectly synchronized, we figured out how to make a phonon laser – or a “laser for sound.”

In work we recently published in the journal Nature Photonics, we have constructed our phonon laser using the oscillations of a particle – about a hundred nanometers in diameter – levitated using an optical tweezer.

A red laser beam from a high-power lab laser. Doug McLean/Shutterstock.com

Waves in sync

An optical tweezer is simply a laser beam which goes through a lens and traps a nanoparticle in midair, like the tractor beam in “Star Wars.” The nanoparticle does not stay still. It swings back and forth like a pendulum, along the direction of the trapping beam.

Since the nanoparticle is not clamped to a mechanical support or tethered to a substrate, it is very well isolated from its surrounding environment. This enables physicists like us to use it for sensing weak electric, magnetic and gravitational forces whose effects would be otherwise obscured.

To improve the sensing capability, we slow or “cool” the nanoparticle motion. This is done by measuring the position of the particle as it changes with time. We then feed that information back into a computer that controls the power in the trapping beam. Varying the trapping power allows us to constrain the particle so that it slows down. This setup has been used by several groups around the world in applications that have nothing to do with sound lasers. We then took a crucial step that makes our device unique and is essential for building a phonon laser.