Single-Bubble Sonoluminescence as Dicke Superradiance at Finite   Temperature
  Sonoluminescence is a process in which a strong sound field is used to produce light in liquids. We explain sonoluminescence as a phase transition from ordinary fluorescence to a superradiant phase. We consider a spin-boson model composed of a single bosonic mode and an ensemble of $N$ identical two-level atoms. We assume that the whole system is in thermal equilibrium with a reservoir at temperature $\beta^{-1}$. We show that, in a ultrastrong-coupling regime, between the two-level atoms and the electromagnetic field it is possible to have a cooperative interaction of the molecules of the gas in the interior of the bubble with the field, generating sonoluminescence. 
M. Aparicio Alcalde, Hernando Quevedo and Nami Fux Svaiter
cond-mat.mtrl-sci hep-th quant-ph