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Physical Review A

Multilevel light-induced continuum structure: Strong-probe and stabilization effects

R. Parzyński, Antoni Wójcik, J. Schmidt

Published Oct 1, 1994• 4 citations

Abstract

We present an analytical solution to a multilevel model of light-induced continuum structure, in which, instead of a single level, a quasicontinuum of excited levels is laser embedded into a previously structureless atomic continuum. The found solution is fully time dependent and is valid for probing strengths of the structured continuum from the ground level by another laser, which can vary from being much weaker than the embedding one to being as strong as the embedding one. For three different embeddings (weak, moderate, and strong), each one determined by a different relation between the ionization rate \ensuremath{\gamma} and the spacing \ensuremath{\Delta} of the quasicontinuum levels (\ensuremath{\Elzxh}\ensuremath{\gamma}/\ensuremath{\Delta}\ensuremath{\ll}1, \ensuremath{\Elzxh}\ensuremath{\gamma}/\ensuremath{\Delta}\ensuremath{\simeq}1, and \ensuremath{\Elzxh}\ensuremath{\gamma}/\ensuremath{\Delta}\ensuremath{\gg}1), we study the evolution of the ionization spectrum with increasing probing strength. We observe and interpret such strong-field effects in the spectra as (i) conversion of a Fano-like train of peaks into a train of dips; (ii) peak moving, narrowing, and lowering; and (iii) saturation, population trapping, and stabilization against ionization.

DOI: 10.1103/physreva.50.3285

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We present an analytical solution to a multilevel model of light-induced continuum structure, in which, instead of a single level, a quasicontinuum of excited levels is laser embedded into a previously structureless atomic continuum. The found solution is fully time dependent and is valid for probing strengths of the structured continuum from the ground level by another laser, which can vary from being much weaker than the embedding one to being as strong as the embedding one. For three different embeddings (weak, moderate, and strong), each one determined by a different relation between the ionization rate \ensuremath{\gamma} and the spacing \ensuremath{\Delta} of the quasicontinuum levels (\ensuremath{\Elzxh}\ensuremath{\gamma}/\ensuremath{\Delta}\ensuremath{\ll}1, \ensuremath{\Elzxh}\ensuremath{\gamma}/\ensuremath{\Delta}\ensuremath{\simeq}1, and \ensuremath{\Elzxh}\ensuremath{\gamma}/\ensuremath{\Delta}\ensuremath{\gg}1), we study the evolution of the ionization spectrum with increasing probing strength. We observe and interpret such strong-field effects in the spectra as (i) conversion of a Fano-like train of peaks into a train of dips; (ii) peak moving, narrowing, and lowering; and (iii) saturation, population trapping, and stabilization against ionization.

An analytical, time-dependent, fully nonperturbative solution of a general model of a light-induced continuum structure, in which a dense Rydberg-like quasi-continuum is laser-embedded into a previously structureless atomic continuum, is reported. On the basis of this solution we point to the limitations of Fano theory, which was originally formulated for an analogous problem of a number of discrete states and one continuum with configuration interaction, when this theory is adapted to the problem of multilevel light-induced continuum structures.

Laser induced continuum structures (LICS) can be produced by a strong laser embedding an excited atomic bound state into a flat atomic continuum, leading to a tunable resonance of adjustable width that can be probed by a second laser. If this second laser is of similar intensity to the embedding laser then the distinction between the two becomes artificial and saturation effects become important. In this paper the effect on LICS of transitions caused by both lasers between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods and allowing for the ionization of each atomic state by both laser fields. Formal results correct to all orders are also given in terms of a T-matrix approach. Numerical calculations are presented showing the effects on the LICS resonant structures of continuum-continuum coupling processes. Significant changes to the Fano profiles (second laser weak) and to coherence holes (second laser strong) occur. Analytical results are also given.

We have investigated the atomic coherent population trapping in a multilevel laser-induced continuum structure system including cascade two-photon processes, in which two sets of quasicontinua of excited levels to be modeled as the Bixon-Jortner quasicontinuum are laser-embedded into a previously structureless atomic continuum. The condition leading to the atomic coherent population trapping and the coherent population trapping state are given explicitly. And the effects of the Fano factors, the laser intensities and the detunings on the amount of population trapped in the atomic bound states and the populations distributed in the two sets of quasicontinua are also examined.

We present experimental and theoretical results of a detailed study of laser-induced continuum structures (LICS's) in the photoionization continuum of helium out of the metastable state

2s{}^{1}{S}_{0}.

The continuum dressing, with a 1064-nm laser, couples the same region of the continuum to the

4s{}^{1}{S}_{0}

state. The experimental data, presented for a range of intensities, show pronounced ionization suppression (by as much as 70% with respect to the far-from-resonance value) as well as enhancement, in a Beutler-Fano resonance profile. This ionization suppression is a clear indication of population trapping mediated by coupling to a continuum. We present experimental results demonstrating the effect of pulse delay upon the LICS's, and for the behavior of LICS's for both weak and strong probe pulses. Simulations based upon numerical solution of the Schr\"odinger equation model the experimental results. The atomic parameters (Rabi frequencies and Stark shifts) are calculated using a simple model-potential method for the computation of the needed wave functions. The simulations of the LICS profiles are in excellent agreement with experiment. We also present an analytic formulation of pulsed LICS's. We show that in the case of a probe pulse shorter than the dressing one, the LICS profile is the convolution of the power spectra of the probe pulse with the usual Fano profile of stationary LICS's. We discuss some consequences of deviation from steady-state theory.

Laser-induced continuum structures (LICS) can be produced by a strong laser embedding a low-lying atomic state into a flat atomic continuum, which leads to a tunable asymmetric Fano resonance of adjustable width that can be probed by a second weak laser. In this paper the effects on LICS of transitions caused by the embedding laser between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods. Formal results correct to all orders are presented in terms of a T-matrix approach. Numerical calculations are presented which include continuum-continuum coupling correct to the lowest power of the embedding laser intensity. The time evolution of the pure LICS Fano profile is also shown.

We report the observation of laser-induced structure (LICS) in the flat photoionization continuum of helium. The structure is a strong and spectrally sharp resonance, showing both enhanced and diminished ionization (a Beutler-Fano profile). Our observations show, in an otherwise unstructured continuum, pronounced ionization suppression (as much as 70%), which is due to population trapping associated with LICS. We also show the effect of dynamic Stark shifts upon the LICS profile. A discussed theoretical model, incorporating significant dynamic Stark shifts, gives a quantitatively accurate description of the line profile for a wide range of pulse intensities.