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Physical Review A

Accumulative effects in temporal coherent control

D. Felinto, C. A. C. Bosco, L. H. Acioli, S. S. Vianna

Published Nov 19, 2001• 40 citations
Publisher

Abstract

This paper analyzes the result of a temporal coherent control experiment using trains of ultrashort laser pulses in the situation where the laser repetition period is smaller than the relaxation times of the system. We focus on the accumulative effects present in sequential two-photon absorption. It is shown that stimulated emission due to the accumulation of population in the excited state give an important contribution to the coherently controlled process. Excellent agreement is obtained in the comparison between experiments in rubidium vapor and theory, where resonant pulse propagation is also taken into account.

DOI
10.1103/physreva.64.063413

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This paper analyzes the result of a temporal coherent control experiment using trains of ultrashort laser pulses in the situation where the laser repetition period is smaller than the relaxation times of the system. We focus on the accumulative effects present in sequential two-photon absorption. It is shown that stimulated emission due to the accumulation of population in the excited state give an important contribution to the coherently controlled process. Excellent agreement is obtained in the comparison between experiments in rubidium vapor and theory, where resonant pulse propagation is also taken into account.
We investigate coherent accumulation processes in three-level atoms excited by a train of ultrashort pulses in the case where the atomic relaxation times are greater than the laser repetition period. In this situation the resonances of the laser field with the atomic system are determined by the laser frequency comb rather than by the spectrum of a single pulse. Using the density matrix formalism, we develop a perturbative theory that is valid for arbitrary pulse shapes. The excitation of a Doppler-broadened atomic vapor by hyperbolic-secant pulses and $0\ensuremath{\pi}$ pulses is analyzed. It is shown that pulse shape has a great influence on the accumulation process and can change the spectral periodicity of the pattern impressed on the Doppler profile of the medium due to the two-photon absorption process. The effect of interpulse phases is also investigated, and we show that the atomic populations can vary by more than one order of magnitude with small variations of the laser repetition rate, while being insensitive to variations of the laser offset. Finally, the theory is adapted for the temporal-coherent-control technique, and its results are compared with previously reported experimental data.
We study the effects of propagation through an extended sample of two- and three-level atoms on the coherent accumulation of excitation by an ultrashort pulse train. In the two-level case, previous pulses in the train may prepare the medium such that a later pulse may experience amplification and absorption in different positions inside the sample. For a large number of pulses, the atomic medium may be saturated by the train, and some pulses propagate without experiencing either absorption or amplification. In general, absorption of the resonant pulse frequency during propagation compromises the accumulation efficiency. In the three-level case, the coherent accumulation of excitation leads to electromagnetically induced transparency of the pulse train, and the pulses exit the medium without any distortion in their temporal profile.
We investigate the excitation of an atomic degenerate-$\ensuremath{\Lambda}$ system by a train of ultrashort pulses. An iterative analytic solution to the optical Bloch equations is found describing the temporal evolution of the density matrix elements. We show that electromagnetically induced transparency of the excitation pulses develops through the coherent accumulation of excitation (population and coherence) between the degenerate lower states. The influence of the pulse train parameters (area, repetition period, and phase between successive pulses) on establishing the transparency is investigated.
Investigations were made of the accumulation effect exhibited by transient electronic stimulated Raman scattering in atomic barium vapor pumped by a train of ultrashort pulses with a controlled time delay between the pulses. The essential feature of the accumulation effect is that the Stokes energy of a pulse is contributed to not only by its own exciting pump pulse, but also by the previous pump pulse, the influence of the latter depending strongly on the ratio of the transverse relaxation time T2 to the interpulse time interval. If one is to observe the accumulation effect in electronic stimulated Raman scattering, it is important to minimize the influence of depletion of the number of atoms. A method based on the accumulation effect for determination of T2 in one laser shot was developed.
We have considered the induced processes that occur in a driven two-level atom, where a laser photon is absorbed and emitted by the ground and by the excited states of the atom, respectively. In the low-intensity limit of the laser field, the induced spectra arising when a laser photon is absorbed by the ground state of the atom consist of two peaks describing induced-absorption and stimulated-emission processes, respectively, where the former prevails over the latter. Asymmetry of the spectral lines occurs at off-resonance and its extent depends on the detuning of the laser field. The physical, process where a laser photon is emitted by the excited state is the reverse of that arising from the absorption of a laser photon by the ground state of the atom. The former differs from the latter in that the emission of a laser photon by the excited state occurs in the low-frequency regime and that the stimulated-emission process prevails over that of the induced absorption. In this case, amplification of ultrashort pulses is likely to occur without the need of population inversion between the optical transitions. The computed spectra are graphically presented and discussed.
We consider the coherent population trapping effect with a train of ultrashort pulses in a $\ensuremath{\Lambda}$-type system with two bound states and a continuum replacing the upper state. Population transfer to the continuum can be totally suppressed provided that the splitting between the bound states is a multiple of the pulse repetition rate. Compared to the traditional case with only two laser fields coupling the bound states to the continuum, CPT with a pulse train allows one to avoid incoherent population losses due to interaction of the fields with ``wrong'' transitions, which results in only partial suppression of ionization. A method of suppression of excited-state absorption of pumping radiation in laser crystals, with pumping in the form of the pulse train, is proposed. As a particular example pumping dynamics of a Ti3+:YAlO3{\mathrm{Ti}}^{3+}:{\mathrm{YAlO}}_{3}Ti3+:YAlO3​ crystal using this technique is numerically analyzed.