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Physical Review A

Time-dependent analysis of few-photon coherent control schemes

Shaohao Chen, Agnieszka Jaroń-Becker, Andreas Becker

Published Jul 20, 2010• 6 citations
Publisher

Abstract

We investigate the coherent control of nonresonant two-photon excitation and ($2+1$) photon ionization processes from the time-dependent perspective. To this end, we have solved the time-dependent Schr\"odinger equation for a hydrogen atom interacting with an ultraviolet laser field. Results are obtained for symmetric and antisymmetric phase distributions leading to dark and bright pulses, for which the temporal field distributions consist of many subpulses. For the two-photon excitation process, it is found that at intermediate time between the subpulses, the population in the excited state can exceed the final state population and, in particular, is nonzero for dark pulses. The results for the ($2+1$) photon ionization closely follow those for the excitation process, which shows that the ionization of the electron proceeds as a resonant process via the excited state in all cases considered.

DOI
10.1103/physreva.82.013414

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We present and experimentally demonstrate for the first time the observation and femtosecond coherent control over the temporal evolution of a transient population that is excited via nonresonant two-photon absorption. Based on symmetry properties of the two-photon absorption process, the exciting femtosecond pulses are phase-shaped to photoinduce different evolutions of the transient excited population for a given final excited population. As a study case, we focus here on the attractive case of two-photon dark pulses that, although inducing zero final population (hence, the terminology of "dark pulses"), they induce a transient excited population during the pulse irradiation that can significantly deviate from zero. This nonzero transient population can be viewed as the bright side of such dark pulses. The symmetry-based coherent control is demonstrated first with dark pulses that we shape to induce transient excited population that at all times is kept below different target levels. Then, it is further demonstrated with pairs of dark pulses where one is rationally shaped to induce temporal evolution of the transient excited population that is the inverse of the evolution induced by the other. The work is conducted in the weak-field regime with the sodium atom as the model system. The approach developed here is general, conceptually simple, and very effective.
We investigate the coherent control of the photoelectron angular distribution in bichromatic atomic ionization. Neon is selected as target since it is one of the most popular systems in current gas-phase experiments with free-electron lasers (FELSs). In particular, we tackle practical questions, such as the role of the fine-structure splitting, the pulse length, and the intensity. Time-dependent and stationary perturbation theory are employed, and we also solve the time-dependent Schr\"odinger equation in a single-active electron model. We consider neon ionized by a FEL pulse whose fundamental frequency is in resonance with either $2p\text{\ensuremath{-}}3s$ or $2p\text{\ensuremath{-}}4s$ excitation. The contribution of the nonresonant two-photon process and its potential constructive or destructive role for quantum coherent control is investigated.
Abstract The effects of pulse chirping on the excitation and ionization process in hydrogen atoms are theoretically investigated by numerically solving the full time-dependent Schrödinger equation with a pseudospectral method. We specifically compare the dynamical results of single-photon resonant ionization (SRI) and two-photon resonant ionization (TRI) across laser intensities ranging from 10 12 to 10 14 W cm 2 . The ionization probabilities of SRI and TRI tend to exhibit opposite chirp dependent behaviors as the laser field changes from weak to strong, which are elucidated with the strong field approximation theory. Furthermore, the sensitivity of ionization probability to chirp can be suppressed at specific laser intensities when the laser field is synthesized by two-color resonant chirped pulses. Our study provides a basis for theoretical investigations into chirp effect on atomic photoionization, offering new insights into the coherent control of electron dynamics.
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We present a theoretical study of the main characteristics of two-color photoionization spectra obtained when atoms are subjected to an intense radiation pulse containing the fundamental of an infrared laser and one of its higher harmonics. The frequency of the latter is chosen to be high enough that the atom can be ionized by a single photon. The calculation is performed by numerical integration of the time-dependent Schrödinger equation for a (three-dimensional) hydrogen atom. We present the general properties of the spectra and angular distributions as well as the possible occurrence of interferences between the laser-assisted process and above-threshold ionization. Our results indicate that such interference effects, which can lead to partial coherent control of the photoionization process, can be observed at moderate laser intensities within the reach of currently operated laser and harmonic sources.