Spectrum of spontaneous emission into the mode of a cavity QED system
  We study the probe spectrum of light generated by spontaneous emission into the mode of a cavity QED system. The probe spectrum has a maximum on-resonance when the number of inverted atoms for an input drive is maximal. For a larger number of atoms N, the maximum splits and develops into a doublet, but its frequencies are different from those of the so-called vacuum Rabi splitting. 
M. L. Terraciano, R. Olson, D. L. Freimund, L. A. Orozco, and P. R.   Rice