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Dependence on frequency of strong-field atomic stabilization

Howard R. Reiss

Published Jan 15, 2001• 24 citations
Publisher

Abstract

It is shown that strong-field atomic stabilization can occur at any frequency, that analytical methods exist that can describe all essential features of stabilization, that relativistic effects enhance the stabilization phenomenon, and that a simple physical picture exists that explains these properties. A necessary prelude is to show that the frequency properties of the three methods often conjoined by the KFR (Keldysh-Faisal-Reiss) label are quite different. Applicability of the SFA (Strong-Field Approximation) to stabilization at any frequency is shown, and verified by exhibiting close correspondence to numerical predictions by Popov et al. that also span both low and high frequencies.

DOI
10.1364/oe.8.000099

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It is shown that strong-field atomic stabilization can occur at any frequency, that analytical methods exist that can describe all essential features of stabilization, that relativistic effects enhance the stabilization phenomenon, and that a simple physical picture exists that explains these properties. A necessary prelude is to show that the frequency properties of the three methods often conjoined by the KFR (Keldysh-Faisal-Reiss) label are quite different. Applicability of the SFA (Strong-Field Approximation) to stabilization at any frequency is shown, and verified by exhibiting close correspondence to numerical predictions by Popov et al. that also span both low and high frequencies.
Quantitative results are presented for stabilization of atoms against ionization by strong laser fields over a wide range of frequencies for circular and linear polarization. With circular polarization, stabilization occurs for all frequencies. At high frequency (\ensuremath{\omega}>1 a.u.), the stabilization intensity is predicted by the occurrence of photon thresholds. This mechanism gives excellent numerical agreement both with present calculations and the results of Pont and Gavrila [Phys. Rev. Lett. 65, 2362 (1990)]. A different mechanism operates for \ensuremath{\omega}1 a.u. Linear polarization differs strongly from circular polarization in many respects.
Based on the field quantization, the angular distribution of the ionization rate of hydrogen atoms with the effect of a finite number of photons is computed in this letter. We find that the angular distribution of the ionization rate will fluctuate with the initial photon number of the field, and have a peak value higher than that predicted by Keldysh-Faisal-Reiss theory. This ionization peak is the embodiment of quantum effect. Furthermore, our calculations show that the phenomenon of ionization peak also exists when the initial field state is a coherent state.
An investigation of the foundations of strong-field theories of the Keldysh type is undertaken. It is found first that the Volkov solution, upon which the Keldysh approximation depends, exists unambiguously only in the relativistic case. Accordingly, a fully relativistic theory of atomic photoionization is formulated from first principles. A strong-field approximation (SFA) is expressed, based only on the proposition that the photoionizing field dominates the atomic potential in the final state. The nonrelativistic limit of the SFA gives exactly one of the known Keldysh-type theories, but the original Keldysh theory itself is found to neglect some fraction of the strong-field effects. It is shown on a variety of grounds that A2{\mathbf{A}}^{2}A2 terms must be fully retained in Keldysh-type theories. Important in this matter are the proper application of asymptotic conditions, the interchangeability of A\ensuremath{\cdot}p and A2{\mathbf{A}}^{2}A2 terms, and the consistent reduction to the nonrelativistic limit.
We present a nonequilibrium nonperturbative field theory for the Kondo effect in strongly interacting quantum dots at finite temperatures. Unifying the slave-boson representation with the Keldysh field integral, an effective Keldysh action is derived and explored in the vicinity of the zero slave-bosonic field configuration. The theory properly reflects the essential features of the Kondo physics and at the same time significantly simplifies a field-theoretic treatment of the phenomenon, avoiding complicated saddle-point analysis or 1/N1/N1/N expansions, used so far. Importantly, our theory admits a closed analytical solution which explains the mechanism of the Kondo effect in terms of an interplay between the real and the imaginary parts of the slave-bosonic self-energy. It thus provides a convenient nonperturbative building block, playing the role of a ``free propagator,'' for more advanced theories. We finally demonstrate that already this simplest possible field theory is able to correctly reproduce experimental data on the Kondo peak observed in the differential conductance, correctly predicts the Kondo temperature, and, within its applicability range, has the same universal temperature dependence of the conductance as the one obtained in numerical renormalization group calculations.
The key subject of these lectures is Interference Stabilization (IS) of atoms in a strong laser field. Physics of the phenomenon, its various manifestations, as well as many related subtopics will be discussed. Generalizations to the case of two-colour interference stabilization will be briefly described. As it will be shown, owing to the achievable in this case very high degree of stabilization, the phenomenon appears to be very promising for making progress in such an important field as propagation of laser pulses in a gas medium. Since the first work on IS (Fedorov and Movsesian, 1988), there have been a lot of publications on this subject. I am not going to mention here all the relevant papers.