Sonoluminescence

Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound. The effect was first discovered at the University of Cologne in 1934 as a result of work on sonar. Interest in the topic rose again when an inner temperature of such a bubble well above one million degrees Celsius was postulated, making it a possible source for nuclear fusion energy.

A major breakthrough occurred when Gaitan et al. were able to produce single bubble sonoluminescence (SBSL), in which a single bubble, trapped in a standing acoustic wave, emits light with each pulsation. This development allowed the systematic study of the phenomenon, because it allowed the isolation of the effects of just one bubble, rather than the complex environment of many bubbles.

Here are some facts about sonoluminescence:

The light flashes from the bubbles are extremely short - between 35 and a few hundred picoseconds long.
The bubbles are very small when they emit the light - about 1 micrometer in diameter.
Single-bubble sonoluminescence pulses can have very stable periods and positions. In fact, the frequency of light flashes can be more stable than the rated frequency stability of the oscillator making the sound waves driving them.
For unknown reasons, the addition of a small amount of noble gas (such as helium, argon, or xenon) to the gas in the bubble increases the intensity of the emitted light dramatically.

The mystery of how a low-energy-density sound wave can concentrate enough energy in a small enough volume to cause the emission of light is still unsolved. It requires a concentration of energy by about a factor of 10¹² (one trillion). To make matters more complicated, the wavelength of the emitted light is very short - the spectrum extends well into the ultraviolet. Shorter wavelength light has higher energy, and the observed spectrum of emitted light seems to indicate a temperature in the bubble of at least 10,000 degrees Celsius, and possibly a temperature in excess of one million degrees Celsius.

Such a high temperature makes the study of sonoluminescence especially interesting for the possibility that it might be a means to achieve thermonuclear fusion. If the bubble is hot enough, and the pressures in it high enough, fusion reactions like those that occur in the Sun could be produced within these tiny bubbles: this is sometimes referred to as bubble fusion. Recent experiments of Taleyarkhan et.al. in deuterated acetone show measurements of tritium and neutron output consistent with fusion, but these measurements have not been confirmed and are highly debated.

The achievement of fusion through sonoluminescence was fictionalized in the movie Chain Reaction, starring Keanu Reeves and Morgan Freeman.

References

Putterman, S. J. "Sonoluminescence: Sound into Light," Scientific American, Feb. 1995, p.46.(Available Online)
H. Frenzel and H. Schultes, Z. Phys. Chem. B27, 421 (1934)
D. F. Gaitan, L. A. Crum, R. A. Roy, and C. C. Church, J. Acoust. Soc. Am. 91, 3166 (1992)
M. Brenner, S. Hilgenfeldt, and D. Lohse, "Single bubble sonoluminescence", Rev. Mod. Phys., April (2002).
R. P. Taleyarkhan, C. D. West, J. S. Cho, R. T. Lahey, Jr. R. Nigmatulin, and R. C. Block, "Evidence for Nuclear Emissions During Acoustic Cavitation," Science 295, 1868 (2002). (see bubble fusion article for direct link)
A how to guide to setting up a sonoluminescence experiment - " class="external">http://www.ucl.ac.uk/~ucapwas/sl/sono.html