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arXiv:2101.02603 · Entropy

Multilevel Laser Induced Continuum Structure

Kaloyan N. Zlatanov, Nikolay Vitanov

Published Jul 13, 2021• 2 citations
arXivHTMLPDF

Abstract

Laser-induced-continuum-structure (LICS) allows for coherent control techniques to be applied in a Raman type system with an intermediate continuum state. The standard LICS problem involves two bound states coupled to one or more continua. In this paper, we discuss the simplest non-trivial multistate generalization of LICS which couples two bound levels, each composed of two degenerate states through a common continuum state. We reduce the complexity of the system by switching to a rotated basis of the bound states, in which different sub-systems of lower dimension evolve independently. We derive the trapping condition and explore the dynamics of the sub-systems under different initial conditions.

DOI
10.3390/e23070891

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Laser-induced-continuum-structure (LICS) allows for coherent control techniques to be applied in a Raman type system with an intermediate continuum state. The standard LICS problem involves two bound states coupled to one or more continua. In this paper, we discuss the simplest non-trivial multistate generalization of LICS which couples two bound levels, each composed of two degenerate states through a common continuum state. We reduce the complexity of the system by switching to a rotated basis of the bound states, in which different sub-systems of lower dimension evolve independently. We derive the trapping condition and explore the dynamics of the sub-systems under different initial conditions.
We present results from a study of the coherence properties of a system involving three discrete states coupled to each other by two-photon processes via a common continuum. This tripod linkage is an extension of the standard laser-induced continuum structure (LICS), which involves two discrete states and two lasers. We show that in the tripod scheme, there exist two population trapping conditions; in some cases these conditions are easier to satisfy than the single trapping condition in a two-state LICS. Depending on the pulse timing, various effects can be observed. We derive some basic properties of the tripod scheme, such as the solution for coincident pulses, the behavior of the system in the adiabatic limit for delayed pulses, the conditions for no ionization and for maximal ionization, and the optimal conditions for population transfer between the discrete states via the continuum. In the case when one of the discrete states is strongly coupled to the continuum, the population dynamics reduces to a standard two-state LICS problem (involving the other two states) with modified parameters; this provides the opportunity to customize the parameters of a given two-state LICS system.
In recent years our understanding of how laser fields interact with a continuum of levels has progressed substantially. No longer is the continuum viewed as a dissipative environment, best described by a set of kinetic equations. Recent theoretical* and experimental work has shown that strong laser fields involving a continuum of levels may often maintain perfeet coherence. One of the m ore interesting colterent phenomena involving the continuum is the dressing of a continuum by a bound state, termed laser-induced continuum structure (LICS). In the LICS phenomenon, a strong laser field, connecting an unpopulated bound state to the continuum mixes some bound- state character into the continuum. This optically induced resonance can be detected by a second (weaker) laser field, which induces a transition from a populated bound state to the structured continuum.
Atomic states may be embedded in the continuum by dressing the atom by an intense laser field. This is often referred to as a laser induced continuum structure (LICS) [1]. This continuum, in general, also has a natural structure due to the doubly excited states, or autoionizing states, of the atom. We have considered the situation in which a complex atom or ion is subjected to an intense laser field such that the LICS states and autoionizing states occur simultaneously. We have have found that, by tuning the frequency and intensity of the field, the states can become degenerate, i.e. have the same position and width.
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.
Structure in the continuum of an atomic system attributed to the coupling of a bound state by a strong laser field has been reported elsewhere (1-3). Such experiments have employed a second laser to probe the continuum from the ground state by measuring polarization rotation 0), third harmonic generation (2) or three photon ionization (3). We report here an experiment similar to reference 2 using two photon resonant, third harmonic generation in sodium, perturbed by a second laser which couples the 5s state to the continuum. Under no conditions was any laser induced continuum structure (LICS) observed in these experiments, in contrast to the results reported elsewhere (2).