Abstract. Sonoluminescence is the intriguing phenomenon of strong light flashes from tiny bubbles in a liquid. The bubbles are driven by an ultrasonic wave and need to be filled with noble gas atoms. Approximating the emitted light by blackbody radiation indicates very high temperatures. Although sonoluminescence has been studied extensively, the origin of the sudden energy concentration within the bubble collapse phase is still controversial. It is hence difficult to further increase the temperature inside the bubble for applications like sonochemistry and table top fusion. Here, we show that the strongly confined noble gas atoms inside the bubble can be heated very rapidly by a weak but highly inhomogeneous electric field as might occur naturally during rapid bubble deformations. An indirect proof of the proposed quantum optical heating mechanism would be the detection of the non-thermal emission of photons in the optical regime prior to the light flash. Our model implies that it is possible to increase the temperature inside the bubble with the help of appropriately detuned laser fields.

GENERAL SCIENTIFIC SUMMARY
Introduction and background. Over recent years, single-bubble sonoluminescence experiments have been perfected in the laboratory. Stable bubbles are created which periodically change their radius when driven by an ultrasonic wave. Each cycle results in a sudden collapse of the bubble, which is accompanied by a strong picosecond light flash (see figure). The spectrum of the emitted light and the possible formation of dense plasma indicate temperatures as high as 10⁶ K.

Main results. Our work does not contradict current models for the description of sonoluminescence experiments. Instead it addresses certain, still controversial aspects by revealing a previously unconsidered quantum optical heating mechanism. It is shown that a weak but highly inhomogeneous electric field, as might occur naturally during rapid bubble deformations, can increase the temperature of strongly confined particles by several orders of magnitude, even on the relevant nanosecond time scale. Finally, our model implies that it is possible to further enhance the energy concentration in sonoluminescence experiments with the help of appropriately detuned laser fields.

Wider implications. Our model is based on the build up of quantum coherences on a nanosecond time scale. This contradicts the widely held belief that quantum mechanics applies only at relatively low temperatures. However, quantum physics is currently very successful in predicting, for example, the dynamics of atoms and molecules in intense laser fields on comparable time scales. Further studies of the sonoluminescence phenomenon might hence lead to new applications in sonochemistry and table top fusion.

Figure. Time dependence of the driving sound pressure and of the bubble radius in a typical single-bubble sonoluminescence cycle. Point A marks the beginning of the collapse phase. At point B, the temperature within the bubble is significantly increased and a strong light flash occurs. Point C denotes the beginning of the expansion phase in which the bubble regains stability.