Ultrafast modulation spectroscopy in cascading three-level and four-level systems

We study theoretically ultrafast modulation spectroscopy in Doppler-broaden ed cascading three-level and four-level systems. The pump beams consist of two frequency components that drive two-pathway excitation. The four-wave m ixing (FWM) signal exhibits damping oscillation as the relative time delay tau between the two pump beams is varied, from which the resonant frequenci es between energy levels can be deduced. Furthermore, Doppler-free precisio n in the measurement of the resonant frequencies can be achieved when the p ump beams have either narrow-band or broadband linewidths. We study two-pho ton resonant FWM induced by time-delayed correlated fluctuating fields in a Doppler-broadened cascading three-level system. It is related intrinsicall y to the sum-frequency trilevel echo when the pump beams have broadband lin ewidths. However, unlike the corresponding incoherent-light stimulated phot on echo, the maximum of the FWM signal is shifted from tau similar or equal to 0. If we consider the fluctuations of the pump beams, the FWM signal ex hibits damping oscillation when the laser frequency is off resonant from th e two-photon transition. (C) 2001 Optical Society of America.