We study theoretically ultrafast modulation spectroscopy in Doppler-broadened cascading three-level and four-level systems. The pump beams consist of two frequency components that drive two-pathway excitation. The four-wave mixing (FWM) signal exhibits damping oscillation as the relative time delay τ between the two pump beams is varied, from which the resonant frequencies between energy levels can be deduced. Furthermore, Doppler-free precision in the measurement of the resonant frequencies can be achieved when the pump beams have either narrow-band or broadband linewidths. We study two-photon resonant FWM induced by time-delayed correlated fluctuating fields in a Doppler-broadened cascading three-level system. It is related intrinsically to the sum-frequency trilevel echo when the pump beams have broadband linewidths. However, unlike the corresponding incoherent-light stimulated photon echo, the maximum of the FWM signal is shifted from τ≃0. If we consider the fluctuations of the pump beams, the FWM signal exhibits damping oscillation when the laser frequency is off resonant from the two-photon transition.
© 2001 Optical Society of America
(300.2570) Spectroscopy : Four-wave mixing
(300.6240) Spectroscopy : Spectroscopy, coherent transient
(300.6320) Spectroscopy : Spectroscopy, high-resolution
(300.6500) Spectroscopy : Spectroscopy, time-resolved
Panming Fu, Yanbang Wang, Qian Jiang, Xin Mi, and Zuhe Yu, "Ultrafast modulation spectroscopy in cascading three-level and four-level systems," J. Opt. Soc. Am. B 18, 370-378 (2001)