ULTRAFAST MODULATION SPECTROSCOPY IN 4-LEVEL SYSTEM

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