Ultrasound equivalent of laser built
15 June 2006 Champaign, Ill., USA. Researchers at the University of
Illinois at Urbana-Champaign and at the University of Missouri-Rolla have
built an ultrasound analogue of the laser. Called a uaser (pronounced
wayzer) — for ultrasound amplification by stimulated emission of radiation —
the instrument produces ultrasonic waves that are coherent and of one
frequency.
The potential applications are the study of laser dynamics and the detection
of subtle changes, such as phase changes, in modern materials. "We have
demonstrated that the essential nature of a laser can be mimicked by
classical mechanics — not quantum mechanics — in sound instead of light,”
said Richard Weaver, a professor of theoretical and applied mechanics at
Illinois. To make a uaser, Weaver, Illinois research associate Oleg Lobkis
and UMR physics professor Alexey Yamilov begin by mounting a number of
piezoelectric auto-oscillators to a block of aluminium, 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.
“The phases must be correct also,” Weaver said. “By carefully designing the
transducers, we can assure the correct phases and produce stimulated
emission. As a result, the power output scales with the square of the number
of oscillators.” The uaser more closely resembles a “random laser” than it
does a conventional, highly directional laser, Weaver said. “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. Nevertheless, uasers
may be useful. With their longer wavelengths and more convenient
frequencies, uasers could prove useful for modeling and studying laser
dynamics. They 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. “Uasers can produce an
ultrasonic version of acoustical feedback — an ultrasonic howl similar to
the squeal created when a microphone is placed too close to a speaker,”
Weaver said. “By slowly changing the temperature while monitoring the
ultrasonic feedback frequency, we could precisely measure the phase change
in various materials.” The research was presented at the annual meeting of
the Acoustical Society of America, held at the Rhode Island Convention
Center in Providence, June 5-9. The work was funded in part by the National
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