Ultrafast Modulation Spectroscopy in Four-Level System

Based on the interference between two two-photon processes, we reported a four-level ultrafast modulation spectroscopy in Na vapor, where 3S, 3P, 5S, and 4D formed a four-level system. We studied the temporal behavior of the four-wave mixing signal intensity and obtained signal modulation with period 25fs, corresponding to the splitting between 5s and 4D. This technique can achieve Doppler-free precision in the measurement of the energy level splitting between two excited states which are dipolar forbidden from the ground state.