Role of very-high-frequency excitation in single-bubble sonoluminescence

The fundamental and tenth harmonics were used to produce stable single-bubble sonoluminescence in water; By varying the phase difference between the harmonics, it was possible to enhance the sonoluminescence light emission by as much as a factor of 2.7 compared with single-frequency excitation. Absolute measurements of the bubble radius evolution were carried out using the two-detector technique. Unlike previous observations, these measurements and complementary fits of the Rayleigh-Plesset equation reveal that the maximum bubble radius does not change significantly with phase angle between the harmonics. Therefore, increased sonoluminescence intensity does not have to correlate with increases in maximum bubble radius prior to collapse. We believe that a more violent bubble collapse rate (driven by the very-high-frequency component) is responsible for the enhanced light emission under this type of mixed excitation. It was further found that the presence of the tenth-harmonic frequency component led to significant enhancements in the stability of the bubble undergoing sonoluminescence. This allowed the bubble to be driven at the fundamental frequency at 2.0 bars pressure amplitudes, which are significantly above often-reported thresholds of 1.4 bar itself, thereby leading to increased levels of light emission (by more than 250%).