Ultrafast modulation spectroscopy in a cascade three-level system

Based on the polarization interference between one-photon and two-photon processes, we demonstrated an ultrafast modulation spectroscopy in a cascade three-level system (UMSCTS) in Na vapor, which showed a modulation of the four-wave mixing signal intensity with a 55-fs period, corresponding to the beating between the resonant frequencies of the transitions from 3${\mathit{S}}_{1/2}$ to 3${\mathit{P}}_{3/2}$ and from 3${\mathit{P}}_{3/2}$ to 4${\mathit{D}}_{3/2,5/2}$. We have also discussed the possibility of using UMSCTS to measure the energy levels of the excited states with Doppler-free precision.