A new type of experiment is proposed to study optical nutation and free induction decay in a Doppler-broadened transition, by observing the transmission of a weak probe field resonating at a coupled transition. These techniques open new possibilities for direct detailed observations of relaxation phenomena and effects of velocity dephasing in gases, and provide an experimental method for separately studying the effects of population saturation and Raman-type processes in three level systems.
A new type of experiment is proposed to study optical nutation and free induction decay in a Doppler-broadened transition, by observing the transmission of a weak probe field resonating at a coupled transition. These techniques open new possibilities for direct detailed observations of relaxation phenomena and effects of velocity dephasing in gases, and provide an experimental method for separately studying the effects of population saturation and Raman-type processes in three level systems.
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.
A review of coherent transients is presented in this article. After a brief discussion of single-wave transients (free induction decay, optical nutation, etc...), two-wave transients, i.e. time-resolved saturation spectroscopy, are treated in more detail. It is shown that this technique of combining both frequency-and-time-resolution in laser spectroscopy allows one to study the various quantum-mechanical processes contributing to the lineshape, in particular (i) the interaction between the laser field and the atomic (or molecular) system, and (ii) the dynamics of the relaxation of the system. Applications to the analysis of Raman-type processes in three-level systems, power-broadening and velocity diffuston are given.
We present a theoretical and experimental study of the time evolution of the saturation-spectroscopy line shape when the saturating beam is suddenly switched on. We consider two metastable levels coupled by two Doppler-broadened transitions with a common excited level. In this special case, the fast transients from the upper level are rapidly damped, and the slow transients from populations and from Raman coherence in the metastable levels are easy to observe and are used for studying the collisional relaxation. For the sake of clarity, the theoretical analysis is developed according to a perturbation calculation. Nevertheless, saturation of the metastable levels must be taken into account in a more realistic theory (velocity-selective optical pumping): We give only a few results that are in excellent agreement with the experiments performed on neon metastable levels 3P0 and 3P2.
When an intense and short laser pulse or a fast-switching-on/off laser field interacts with a resonant absorbing medium, several optical coherent transient effects can be observed, including the self-induced transparency effect, photon echo effect, optical nutation effect, and the optical free induction decay effect. The study of these effects enables us to gain a deeper understanding of the coherent transient interaction of an intense pulsed or rapidly switched optical field with the resonant medium, and to measure the magnitude of the dipole-moment matrix elements, relaxation parameters, and spectral broadening mechanisms of the investigated materials. On the other hand, these optical coherent transient effects can also be applied for developing special techniques of optical pulse delay, pulse reshaping, and optical pulse manipulation.
We present a theoretical analysis of high-frequency optical heterodyne saturation spectroscopy in Doppler-broadened coupled three-level systems. The saturating beam, which is assumed to be amplitude modulated at frequency δ (double-sideband suppressed carrier amplitude modulation) is resonant for one of the transitions. Through non-degenerate four-wave mixing processes, this modulation is transferred to a probe beam, resonant for the coupled transition. Saturating and probe fields may be either co-propagating or counter-propagating. The lineshape of the probe modulation is analysed via a third-order perturbation expansion of the atomic density matrix with respect to the incident field amplitudes. The resulting signal is integrated over velocities, in the Doppler limit approximation. Both population saturation effects and coherent (Raman-type, or two-photon) processes contribute to the signal. The various contributions appear as Lorentzian-type resonance doublets. We show that, in the absence of relaxation processes (collisional dephasing, or radiative cascades), destructive interferences between population saturation and coherent two-photon processes are responsible for the disappearance of one resonance doublet. Phase-interrupting collisions are thus predicted to lead to the existence of a « pressure-induced extra-resonance » (PIER) doublet, which could yield information on collisional processes in the impact regime. The properties of the predicted PIER doublets are analysed in relation with other types of PIER signals studied in four-wave mixing, and in time-resolved saturation spectroscopy.
We analyze the transient response induced by a resonant, weak probe laser in a three-level V-type system driven by a strong, resonant coupling laser. The Rabi oscillation of the probe transition contains two frequencies determined by the coupling laser, and the ratio of the two-frequency components can be controlled by a time delay between the probe and the coupling laser. In particular, when both lasers are turned on simultaneously, the optical nutation with the frequency approximately equal to the Rabi frequency of the coupling laser is dominant; when the probe laser is turned on well after the steady state is reached for the coupling transition, the Rabi oscillation of the probe transition still occurs, and the oscillatory frequency is approximately equal to half of the Rabi frequency of the coupling laser. When both the probe and the coupling lasers are strong, the mutual interaction of the two fields and the atomic system may revive the optical nutation with each other.