Single-bubble sonoluminescence

As a result of some spectacular experiments, it has recently become possible to drive a single gas bubble of a few microns to such an extreme nonlinear motion that light is emitted which is visible to the naked eye in a darkened room. This phenomenon, associated with stable acoustic cavitation, is termed here as single-bubble sonoluminescence (SBBL). Even though the emitted light appears continuous to the naked eye, it actually consists of a sequence of individual flashes which are highly synchronous with the driving acoustic field. Each flash, containing about one million photons, is established to last for about 50 ps. The spectrum of the emitted light is known to be broadband, with higher energy content in the ultraviolet. A black-body fit to the spectrum suggests temperatures nearing 105 K under certain conditions. However, whether SBSL is associated with simple black-body radiation obeying the Planck distribution has not yet been clearly established. Irrespective of that, the phenomenon of SBSL does involve spontaneous energy amplification by roughly 11 orders of magnitude since very low level acoustic energy in the liquid is converted to highly energetic photons. It is now proven beyond doubt that light emission occurs during the violent collapse phase of an acoustically driven gas bubble, this being the mechanism for energy amplification. The intensity of SBSL, in terms of the number of photons emitted per flash, is known to depend on several parameters, viz. drive pressure amplitude and frequency, temperature of the liquid sample and the amount and type of dissolved gas in the liquid. The upper bound on the intensity is yet to be established. The primary aim of this review article is to provide experimental evidence to substantiate many of the above statements without much discussion, since some crucial aspects of the phenomenon are still not understood.