Researchers at the University of Missouri-Rolla and the University of Illinois at Urbana-Champaign have built an ultrasound analogue of the laser.
Called a uaser (pronounced WAY-zer) – for ultrasound amplification by stimulated emission of radiation, the instrument produces ultrasonic waves that are coherent and of one frequency, and could be used to study laser dynamics and detect subtle changes, such as phase changes, in modern materials.
“We exploit the fact that coherence and stimulated emission are classical concepts and, as such, can be applied to build a mechanical device – uaser – a classical analogue to the laser” says Dr. Alexey Yamilov, research assistant professor of physics at UMR who collaborates with Dr. Richard Weaver, professor of theoretical and applied mechanics, and research associate Oleg Lobkis at the University of Illinois at Urbana Champaign.
To make a uaser, the researchers begin by mounting a number of piezoelectric auto-oscillators to a block of aluminum, which serves as an elastic, acoustic body. When an external acoustic source is applied to the body, the oscillators synchronize to its tone. Like fireflies trapped in a bottle, the oscillators synchronize to the frequency of the source. In the absence of an external source, the tiny ultrasonic transducers become locked to one another by virtue of their mutual access to the same acoustic system.
Electronic circuit auto-oscillators interact with the reverberating acoustic cavity via piezoelectric transducers which can emit and absorb acoustic waves. This is similar to how an atom interacts with an optical resonator through emission and absorption of the electromagnetic waves. “Unlike atoms, where the quantum mechanics ensures that the wave emitted by stimulation is always in phase with the incident field, in the classical uaser, we achieve the correct phase with a careful design of the auto-oscillators – electro-mechanical ‘atoms,’” says Yamilov.
The uaser more closely resembles a “random laser” than it does a conventional, highly directional laser, Weaver says. “In principle, however, there is no reason why we shouldn’t be able to design a uaser to generate a narrow, highly directional beam.”
Optical lasers are useful because of their coherent emission, high intensity and rapid switching. These features are of little value in acoustics, where coherence is the rule and not the exception, intensity is limited by available power, and maximum switching speeds are limited by moderate frequencies.
The ultrasonic systems with their longer wavelengths and longer time scales can permit probes and controls to a degree not possible in optics. “Similar to how passive acoustic wave experiments allowed study the wave-functions of quantum dots and chaotic optical resonators, the ability to experiment with stimulated emission of ultrasound and monitor uasing states of the acoustic cavity will inform the research on lasing in complex media – random and chaotic lasers,” says Yamilov.
Uasers could also serve as highly sensitive scientific tools for measuring the elastic properties and phase changes of modern materials, such as thin films or high-temperature superconductors.
Weaver and Yamilov will describe the uaser and present the team’s latest experiments at the annual meeting of the Acoustical Society of America, to be held at the Rhode Island Convention Center in Providence, June 5-9 and at the annual meeting of the Optical Society of America in Rochester, N.Y., in October.