A two-level system is driven by two strong fields, one on resonance and one
close to resonance. There is a resonance-like response of the coupled atom
-held system when the detuning of the second field is an integer fraction o
f the Rabi frequency of the resonant field. These subharmonic resonances ar
e monitored by a transition to a third level and give a characteristic spec
trum dominated by a series of doublets.