Science : Ultracool acoustic laser hits the right note

By Andrew Watson INSIDE a bar of cold glass Jean-Yves Prieur and his team at the University of Paris-South have created a saser—the acoustic equivalent of a laser. They say it could open up a new way of detecting subatomic particles. Lasers use a medium, usually a gas or crystal, with atoms that can occupy two or more different energy levels. Left alone, the atoms go for the lowest possible energy level. The trick with lasers is to overpopulate the upper energy level, by pumping light into the system to excite the atoms. Passing photons can then provoke the excited atoms into giving up their excess energy as a cascade of photons with a single, pure wavelength. Prieur realised that it should be possible to do the same with sound waves. Glass is a strong absorber of ultrasound. At low temperatures, incoming packets of vibrational energy, or phonons, are trapped at absorbing centres by the bonds between atoms. Just as the medium in a laser will release light if it is overloaded with excited atoms, these absorbing centres may later release this energy as amplified sound of a pure wavelength. The basic idea is not new. In the early 1960s, Ed Tucker of General Electric’s research laboratory in Schenectady, New York, demonstrated a similar phenomenon in crystals of ruby. But his device operated only at a single ultrahigh frequency, and the work was quickly forgotten. Prieur’s saser has the advantage that it can work at a range of ultrasonic frequencies. Prieur’s team used a bar of pure silica glass, 2 centimetres long and chilled to 0.05 degrees above absolute zero, with a transducer at each end for converting electrical signals to ultrasonic vibrations and back again. One transducer primes the system by injecting a pulse of sound, called the pump pulse, which varies in intensity and frequency throughout its duration. The absorbing centres in glass prefer to vibrate out of step with an applied vibration, as this is a lower-energy option than vibrating in step. The secret of achieving laser-like behaviour is to get the absorbing centres moving in step. Prieur achieved this by carefully controlling the strength, frequency and timing of the pump pulse. With the system primed, the second transducer sends a pulse of vibration through the bar in the opposite direction. This stimulates the excited absorbing centres to emit phonons and drop back down to the lower energy level. The resulting amplified pulse is picked up by the first transducer, operating in its second role as a detector. The team found that they could amplify the second pulse by a factor of 1.6 (Physica B, vol 219/220, p 235). “It’s not very much,” Prieur admits. But he notes that the dramatic gains in intensity achieved with lasers come from passing the light through the medium many times. Prieur’s team is now trying to do the same with the sound pulses in the saser. The glass bar’s flat polished ends reflect vibrational waves, so the amplified pulse already travels to and fro along the bar. But so, too, does the original pump pulse, and when reflected back along the bar, it destroys all the excited absorbing centres it created on its first pass. The researchers are now looking for ways to limit the pump pulse to a single pass, while allowing the amplified pulse to move back and forth unhindered. Prieur does not envisage hi-fis powered by acoustic lasers, however, because the system only works at low temperatures and because the sound frequencies are very high,
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