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Journal of Physics B Atomic Molecular and Optical Physics

Phase control through channels involving multiple continua and multiple thresholds

Takashi Nakajima, Jian Zhang, P. Lambropoulos

Published Feb 28, 1997• 33 citations
Publisher

Abstract

We present a theory of phase control which governs the time-dependent behaviour of a system involving one bound and one decaying discrete state embedded in several continua belonging to several ionization thresholds. Representative numerical results are shown, focusing on the behaviour of the branching ratio as a function of the relative phase of two lasers, detuning, and Rabi frequencies (or intensities). Specific quantitative results on the Ca atom are also reported.

DOI
10.1088/0953-4075/30/4/025

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We present a theory of phase control which governs the time-dependent behaviour of a system involving one bound and one decaying discrete state embedded in several continua belonging to several ionization thresholds. Representative numerical results are shown, focusing on the behaviour of the branching ratio as a function of the relative phase of two lasers, detuning, and Rabi frequencies (or intensities). Specific quantitative results on the Ca atom are also reported.
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A theoretical analysis of the phase control of a two-channel atomic ionization scheme is given. We report results on phase-selective excitation, both with square and smooth laser pulses. We show that the phenomenon of population trapping, which is known to occur in this system, can be greatly influenced by the phase difference of the lasers involved in the excitation. As a result, the time dependences of the ionization probability, the populations in the bound states and the form of the photoelectron energy spectra can all be controlled by varying the phase difference of the lasers. We also show that for specific atomic and phase parameters population can be entirely trapped within a superposition state, totally immune to decay, with no ionization whatsoever.
A theoretical analysis of the phase control of a two-channel atomic ionization scheme is given. We report results on phase-selective excitation, both with square and smooth laser pulses. We show that the phenomenon of population trapping, which is known to occur in this system, can be greatly influenced by the phase difference of the lasers involved in the excitation. As a result, the time dependences of the ionization probability, the populations in the bound states and the form of the photoelectron energy spectra can all be controlled by varying the phase difference of the lasers. We also show that for specific atomic and phase parameters population can be entirely trapped within a superposition state, totally immune to decay, with no ionization whatsoever.
We solve a model with one bound state and two continua.1 Two lasers are used to study the above threshold ionization process. The first laser couples the electron in a bound state to a continuum state. The photons are assumed to be energetic enough to promote the electron directly into the continuum far from the threshold; this avoids complications due to the Rydberg levels and the ponderomotive shifts of the threshold. The second laser mixes the electron wavefunction between two continuum states (C—C transitions); since we avoid the rotating wave approximation, multiple transitions between these continua are allowed. We find that the ionization rate is unaffected by the C—C transitions. Our model of hydrogenic atoms allows redistribution among the continua only during the ionization process and the photoelectron spectra exhibits peak suppression; i.e., the population is not the strongest at the energy of the ionization laser.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
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.