We have studied theoretically a time-delayed laser-induced double gratings (TDLIDG) in a Doppler-broadened three-level system. We have shown that the overall accuracy of using TDLIDG for the energy-level splitting measurement is determined by the homogeneous linewidths of the optical transitions.
We have employed chaotic and phase-diffusion models to study the effect of laser coherence on time-delayed laser-induced double gratings (TDLIDG). It is found that, although the temporal behavior of the four-wave mixing signal depends on the stochastic properties of the lasers and the relaxation time of the grating, the modulation of the signal is always damped out when the relative time delay between two pump beams is much longer than the laser coherence time. We performed a TDLIDG experiment mediated by thermal effects to study the temporal behavior of the modulation, and we obtained the frequency difference between two light beams with an accuracy limited by the laser linewidth. We also studied the influence of the phase change in the optical path on TDLIDG.
We have studied theoretically a beat effect in a four-level system due to fifth-order optical polarization, and considered the cases that the pump beams have either narrowband or broadband linewidth. We have found that the accuracy for the energy-level splitting meaurement is determined by the homogeneous linewidths of the optical transitions. That is to say, this technique can achieve Doppler-free precision in the measurement of the eaergy level splitting between two excited states which are dipolar forbidden from the ground state.
This paper describes a new class of techniques, called time-delayed laser saturation spectroscopy, which combine frequency- and time-domain methods of laser spectroscopy to provide a way of studying a molecular system as it evolves from an initially prepared stationary state to a second, final state. The specific example analyzed here is three-level free induction decay, in which the time-dependent gain of a Doppler-broadened molecular transition is probed after the sudden termination of an intense field resonating with a coupled transition. The theoretical calculation is based on the coupled density-matrix equations of motion in the slowly-varying envelope approximation. The time-delayed line shapes, which may be studied in either transmission or side fluorescence, exhibit linewidth asymmetries, line-shape deformations, Ramsey-type fringes, power broadening and dephasing, and dynamic Stark splittings and oscillatory decays. The technique provides a unique way of distinguishing the influence of Raman-type processes from that of population saturation and a means to separately measure the associated decay rates. The relationship of the present work to other studies is also discussed.
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.