Modification of a vacuum Rabi splitting via a frequency-modulated cavity mode

The spectrum radiated by an initially excited two-level atom in a very-high-Q cavity is studied. We consider the situation where the frequency of the vacuum field cavity mode is varied time-dependently during Rabi cycles. Under the resonance condition where the modulation frequency is the same as the vacuum Rabi frequency, we show that the vacuum Rabi splitting can be further split into two doublets. The relative heights of spectral peaks are shown to depend on the modulation phase. Furthermore, we show that the interference between different spectral peaks can break the symmetry of the spectrum.