Sonoluminescence: Bogolubov coefficients for the QED vacuum of a   time-dependent dielectric bubble
  We extend Schwinger's ideas regarding sonoluminescence by explicitly calculating the Bogolubov coefficients relating the QED vacuum states associated with changes in a dielectric bubble. Sudden (non-adiabatic) changes in the refractive index lead to an efficient production of real photons with a broadband spectrum, and a high-frequency cutoff that arises from the asymptotic behaviour of the dielectric constant. 
Matt Visser, S. Liberati, F. Belgiorno, D.W. Sciama