The Sound Laser

Lasers amplify light, masers amplify microwaves, and now, from France, comes the saser: a supercool glass rod that amplifies sound.

By Carl Zimmer|Friday, July 01, 1994

Lasers have become ubiquitous: they read the prices of our groceries, play our compact discs, and even clean our clogged arteries. Now, thanks to a group of French physicists, the world has its first saser- -a device that amplifies sound the way lasers amplify light. The saser isn’t of much use yet, but one day the researchers hope to turn it into the world’s most sensitive listening device.

A laser--the name stands for light amplification by stimulated emission of radiation--exploits the peculiar quantum interactions between atoms and light. An atom can absorb a photon, or light particle, by boosting one of its electrons to a higher energy, but it’s unstable in this state. Eventually it sheds another photon and returns to its calmer existence. When an already excited atom is hit by another photon, however, it can’t absorb it; instead it releases a photon of the same color, or frequency. A laser harnesses these free photons. Atoms in a gas, liquid, or solid are first pumped to high energy by a bright light or electric current. One of the pumped-up atoms then spontaneously relaxes and lets loose a photon that hits a neighboring atom, forcing it to surrender a photon of its own, and these two photons search out other atoms. Mirrors at the ends of the laser force the photons to ricochet back and forth many times, increasing their chances of hitting an atom. Released from the container, this light forms an intense beam made up of photons of uniform frequency.

Sound is another form of energy, one we are used to thinking of as a wave. But according to quantum theory it can be viewed as both wave and particle, just as light can--and at the scale of atoms it is the particle representation that works best. Sound quanta are called phonons, and atoms absorb and release them. In a solid, the absorption of a phonon makes an atom vibrate in its bonds with neighboring atoms; releasing the phonon allows the atom to relax. The arrangement of those bonds determines the frequency of phonons that an atom can absorb.

The saser built by Jean-Yves Prieur and his colleagues at the University of Paris-South in Orsay and the Pierre and Marie Curie University collects phonons the way a laser does photons. The device consists of an inch-long rod of ordinary glass, chilled almost to absolute zero to eliminate vibrations from heat. Attached to one end are piezoelectric crystals, which are crystals that emit a sound vibration when an electric current flows through them.

The physicists first pump up certain atoms in the glass with a two-and-half-microsecond sound pulse of carefully chosen intensity and frequency. Then they send through the pulse of sound they want to amplify. Those phonons travel through the rod, hit the already excited atoms, and-- without getting absorbed--force the atoms to release phonons of their own. The phonons multiply and the sound is intensified.

That proves the principle of the saser, but the device itself has tremendous practical limitations. Although with the proper stimulation the same glass rod can amplify a variety of sounds--glass is made up of a jumble of silica molecules with a wide range of bonds, which can absorb and release phonons of many different frequencies--it can amplify just one frequency at a time. (Prieur’s group did their test at 340 megahertz, which is inaudible ultrasound.) And so far it can amplify that frequency by only about 50 percent. The problem is that the sound passes through the rod only once, failing to make use of many excited atoms. Prieur and his colleagues are now trying to figure out how to make the sound bounce back and forth inside the rod, just as light bounces inside a laser. The sound could propagate back and forth forever, and you could get a hundred-decibel increase or more, says Prieur.

Finally, because the saser must be kept so cold, it won’t find many applications outside the laboratory. But its potential sensitivity could be of tremendous use there. Astrophysicists, for example, are searching for the exotic dark matter that many theorists claim makes up most of the universe. A dark-matter particle entering a piece of ordinary solid matter might, on rare occasion, hit an atom, make it vibrate, and create a faint sound. If researchers could only build a sensitive enough listening device, they could detect those few phonons. Perhaps, Prieur believes, all the astrophysicists need is an amplifier made of glass.

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