  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.
