Sonoluminescence (SL) was first observed in an ultrasonic water bath in 1934 by H. Frenzel and H. Schultes at the University of Cologne, an indirect result of wartime research in marine acoustic radar. By focusing ultrasonic waves of high intensity into a liquid, thousands of tiny bubbles appear. This process of breakup of the liquid is called acoustic cavitation. The bubbles begin to form a fractal structure that is dynamically changing in time. They also emit a loud chaotic sound because of their forced nonlinear oscillations in the sound field. The large mechanical forces on objects brought into contact with the bubbles enable the usage of cavitation in cleaning, particle destruction and chemistry. Marinesco and Trillat found that a photo plate in water could be fogged by ultrasound. This multi bubble sonoluminescence (MBSL) has been analyzed by many researchers, and a great amount of knowledge has been gained. Study of sonoluminescence then made little progress until 1988, when D. Felipe Gaitan succeeded in trapping a stable sonoluminescing bubble at the centre of a flask filled with degassed purified water, energised at its acoustic resonance - single-bubble sonoluminescence (SBSL). However their interest soon waned, and the research was subsequently taken up by Dr S. Putterman et. al., at UCLA, California. The discovery of Gaitan has been encouraging scientists to explore the phenomenon and the associated effects with a multitude of experiments, theories and simulations. The experimental results show picosecond synchronicity, quasiperiodic and chaotic variability of inter-pulse times, a black body spectrum and mass transport stability. The theories to explain the source of SBSL range from hotspot, bremsstrahlung, collision induced radiation and corona discharges to non-classical light. Numerical simulations have been focusing on the bubble dynamics, behavior of the gas content, properties in magnetic fields and the stability of the bubble. However, the final answer concerning the nature of SBSL still remains open.
Putterman pursued SBSL, published numerous papers, and established many of the characteristics which are now taken for granted. Once per acoustic cycle (1/30kHz), coincident with a sharp decrease in bubble size, bluey-white light is emitted in a brief flash in the order of 10 picoseconds in duration, with incredible regularity, and broadband spectrum, including at least the UV range. The spatial images show a bright spot in the source with a diameter of about 3 microns, or less, and a larger diffused region with a diameter of 50 to 100 microns. Scientists don't even know whether the bubbles emit X rays, a sign of very high temperatures. Water absorbs X rays, making it futile to try to detect them from outside the flask. Despite the results that have been obtained, the actual mechanism by which sound is converted to light remains elusive, not least because of the difficulty in measuring the conditions inside a pulsating bubble whose diameter is measured in micro-meters. It is known that the bubble contracts violently, and at the same time the brief flash is emitted, after which it expands again and oscillates about its original equilibrium radius, until it is again stable, ready for another pulse. 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. Conventional physics tries to explain SL as the adiabatic compression of the bubble which leads to very high interior temperatures. The issue is still hotly debated and possible explanations include shocks, plasmas, ionisation and photo-recombination, Bremsstrahlung radiation, and even fusion.
Considering one needs just about one watt of audio power to start observing such effects, sonoluminiscense is to say the least a very efficient energy converter. As you will see in the 'States of matter' table given below, the next higher energetic state of a gas is indeed plasma. But how on earth may one totally ionise a bubble into plasma with just one Watt of power. The trick is resonance, same as shattering a glass with singing, pulverising a kidney stone with ultrasonic or collapsing a bridge with resonant wind vibrations. Once you subject the octahedron structure of a gas to the correct resonance frequency, you need just enough power to 'get loose' the constituent tetrahedrons (plasma) from the gas structure. Current estimations for the bubble temperature and pressure indeed confirm plasma formation. Temperatures have been estimated to range from 10 to 100eV (1eV = 11,600K or 20,420 degrees F); that is as hot as the corona of our sun. The pressures are as high as 200Mbar (1Mbar = 1011 Pa) in the core of the imploding bubble. This pressure is equal to 1.974*108Pa or 19,743,336 atmospheres. The only state of matter which can exist under these conditions is plasma.