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Journal of Physics B Atomic Molecular and Optical Physics

Coherent control of autoionization in optically dense media

David Petrosyan, P. Lambropoulos

Published Apr 24, 2001• 3 citations
Publisher

Abstract

We present a self-consistent theory and analytical results for the three-photon-one-photon coherent control of ionization in an optically dense autoionizing medium. We show that for an optically thick medium, over a short scaled propagation distance, the phase and amplitude of the harmonic field settle to values that are fully determined by the atomic and fundamental field parameters and are completely independent of the initial relative phase between the two fields, thus preventing efficient phase control of the autoionization and its products. Control of the autoionization and the branching ratio of its products is still feasible, however, through the variation of the detuning of the fundamental field from the atomic transition resonance.

DOI
10.1088/0953-4075/34/9/309

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We present a self-consistent theory and analytical results for the three-photon-one-photon coherent control of ionization in an optically dense autoionizing medium. We show that for an optically thick medium, over a short scaled propagation distance, the phase and amplitude of the harmonic field settle to values that are fully determined by the atomic and fundamental field parameters and are completely independent of the initial relative phase between the two fields, thus preventing efficient phase control of the autoionization and its products. Control of the autoionization and the branching ratio of its products is still feasible, however, through the variation of the detuning of the fundamental field from the atomic transition resonance.
We present a self-consistent theory, as well as analytical and numerical results, for the three-photon--one-photon phase control of resonantly enhanced photoionization in an optically dense medium of xenon gas. We show that for an optically thick medium, the standard phase-control technique has a rather limited application since, independently of the initial relative phase between the two fields, over a very short scaled distance of propagation the medium tends to settle such a relative phase that exactly cancels the atomic excitation. Rather unusual results for an optically thin layer of atoms are also found.
We present a self-consistent theory, as well as an illustrative application to a realistic system, of phase control of photoabsorption in an optically dense medium. We demonstrate that, when propagation effects are taken into consideration, the impact on phase control is significant. Independent of the value of the initial phase difference between the two fields, over a short scaled distance of propagation, the medium tends to settle the relative phase so that it cancels the atomic excitation. In addition, we find some rather unusual behavior for an optically thin layer.
We present a formal theory and detailed calculations for phase-dependent laser-atom interactions involving autoionizing states. First, through simple models, we demonstrate that the simultaneous one- and three-photon excitation of one or two neighboring autoionizing states can exhibit profound changes of the line shape, as the relative phase of the two fields is varied from 0 to \ensuremath{\pi}. Through a proper choice of the field intensities and the phase, we obtain analytical results showing that one can cancel the transition to the discrete or the continuum part of the wave function, thereby leading to a flat or a completely symmetric line shape, respectively. At higher intensities, additional effects come into play, providing additional coupling between the discrete and continuum parts, which also exhibits a phase dependence. Finally, our theory is applied to a much more complex situation in Xe, involving many channels, not amenable to simple analytical expressions, but exhibiting nevertheless equally profound effects, including a modification of the branching ratio of two different products. The theory, which is here developed for atomic autoionization, is in fact fairly general and should pertain to related problems in any system involving discrete states embedded in continua.
We present results from perturbative calculations on a scheme of coherent control in Mg. The scheme involves the excitation of Mg in the vicinity of an autoionizing resonance lying above the first two ionization thresholds by a two-color field composed of a fundamental frequency and its third harmonic, whose relative phase can be continuously controlled. Four as well as two-photon coherent excitation pathways, involving appropriate combinations of the two frequencies, proceed through the atomic continuum simultaneously contributing to the excitation, while three-photon ionization by the fundamental is also energetically possible. As a result, some of the excitation processes involve above-threshold ionization (ATI). The calculated ionization yields for the various groups of photoelectrons reveal significant modulation as the relative phase of the field component frequencies is varied. The modulation patterns for the different photoelectron groups are mutually shifted due to the multiphoton matrix element phases associated both with the ATI process and the multichannel nature of the final state. The effect is particularly pronounced when the autoionizing state is in near four-photon resonance with the fundamental frequency. The role of an autoionizing state as a final state as well as that of the continuum as an intermediate state in coherent control processes is discussed.
We study the role of atomic coherence in the propagation dynamics of two-color lasers in an autoionizing medium. By solving the coupled equations for atoms and fields simultaneously, it is found that the asymptotic value of the phase difference after a long propagation distance critically depends on the detuning from an autoionizing resonance and the asymmetry parameter, suggesting that inherent atomic coherence of an autoionizing system, which is absent in a bound state system, plays an important role during the propagation processes. Our numerical findings are consistent with our theoretical analysis using a series expansion method of the coupled equations.
We examine the excitation dynamics of a model atom excited by a two-colour laser field. Here the two colours or frequencies are the fundamental and its third harmonic. The atomic model consists of two bound states which are near-resonantly coupled by a three-photon excitation by the fundamental and a one-photon excitation by the third harmonic, and a structureless continuum. We investigate how the laser parameters (intensity and frequency) and the relative phase between the two colours affects the degree of phase modulation in the ionization signal. We compare our findings from this simple model atom with those from more realistic atomic models by carrying out numerical integration of the hydrogenic Schrodinger equation in both one and three dimensions.