Using light to cool sound

Laser light has been used to cool atoms down to near absolute zero. The technique is simple, if versatile. (And includes some history involving a little-known Indian physicist.)

Laser light is shined on an atom that’s made to move towards the source of light. When the atom absorbs a photon, it slows down because of the law of conservation of momentum. The atom then emits the photon from a different direction.

By Newton’s third law, it should then receive a ‘kick’ in the direction opposite to this emission. But because the photons will be emitted in various random directions, their total ‘kick’ will be far smaller than the brakes applied by swallowing photons from just one direction.

By carefully tuning the laser’s frequency and intensity, scientists can ensure that the atom absorbs and emits enough photons to slow down. And when an atom slows down, it simply means – in the language of thermodynamics – that it has cooled down.

This entire process involves a coupling between light and matter, nothing else. The atom absorbs the photons and then spits them out – i.e. the atom interacts with electromagnetic radiation. The resulting drop in temperature is simply the result of the atom losing its kinetic energy. There are no other forms of energy involved.

However, because laser-cooling is such a cool technique, scientists have been curious about whether it could be used to slam the brakes on the kinetic energy of objects other than atoms. In a new study, published November 27, that’s what scientists say they have done (preprint here).

And this time, what they have done might just be cooler: they have used laser to slow down sound waves.

The technique is the same – and equally simple – except for one small change. In the case of atoms, photons mediated the interaction between the laser light and the atom. In the case of sound waves, there is a second mediator: Brillouin scattering.

We know sound in the air is simply a series of blocks of compressed and rarefied air. Another way to describe this is as a wave. The air is less dense in the rarefied parts and more dense in the compressed parts, so the sound is effectively a density wave. When sound passes through a solid, it does so through a similar density wave.

All waves carry some energy (according to the Planck-Einstein relation: E = hv, where h is Planck’s constant and v is the wave’s frequency). For example, the electromagnetic wave carries energy that, at certain frequencies, we call light or heat. The energy carried by a density wave moving through a solid is, at some frequencies, perceived by the human ear as sound.

So when photons from a laser can be used to remove energy from the density wave, it will effectively reduce the energy of the sound waves. We just need to figure out how to create a coupling between the laser photons and the density waves. This isn’t hard because part of the answer is in the language itself.

How do you couple a particle to a wave? You can’t – unless you can describe both of them as waves or both of them as particles. This is possible in physics through the wave-particle duality. You’ll remember from high school that light is both waves and particles. It’s just two different ways to describe the transport of electromagnetic energy.

You can do this with sound as well. It can be described as a density wave or a particle moving through a medium – two ways to describe the transport of acoustic energy. These ‘sound particles’ are called phonons (cf. quasiparticles).

So to cool a sound wave using lasers, you need to couple the laser photons with the phonons. Put another way, one packet of one kind of energy has to transform into a packet of a different kind of energy. The scientists accomplished this by colliding photons and phonons in a waveguide (a fancy term for any medium that’s carrying a wave).

When a photon is scattered off of a phonon, it can either lose some of its energy to the sound particle or gain energy. When the scattering is such that the photon gains energy, the phonon slows down according to the same mechanism at play between photons and an atom – based on the law of conservation of momentum. This interaction is called a Brillouin scattering.

In their experiment, the scientists, from North Arizona University and Yale University, used a silicon waveguide 2.3 cm long and carrying sound waves at 6 GHz. When they shined laser light of frequency close to the infrared part of the EM spectrum, they observed that the waveguide cooled by 30 K due to interactions between the photons and its phonons.

They used other techniques to make sure that this was the case, and that the material didn’t cool in other ways. For example, they measured the duration for which phonons of certain frequencies persisted in the system. For another, the phonons were found to slow down (a.k.a. “cool down” in thermodynamic-speak) only in one direction – the direction in which the laser was incident – and not others.

There are two more ways in which this experiment is interesting.

First, the scientists found that they didn’t have to setup a closed space, typically called an optomechanical cavity, to perform this experiment. Previous experiments involving light-matter coupling have required the use of such cavities to produce amplified effects. In this experiment, the effect was pronounced in an (relatively) open space itself.

Second, the scientists were able to show that they could influence different groups of phonons in the continuum of the solid simply by changing the frequency of laser light being shot at them.

The applications are obvious. Many devices in our lives, from ultra-sensitive instruments studying gravitational waves to machines that are used regularly, carry unnecessary vibrations that interfere with their purposes. The new study suggests that they can all be damped out simply by using lasers tuned to the right frequencies.