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Journal of Physics B Atomic Molecular and Optical Physics

Time-dependent pulse and frequency effects in population trapping via the continuum

Emmanuel Paspalakis, M. Protopapas, P. L. Knight

Published Feb 28, 1998• 31 citations
Publisher

Abstract

We investigate the effects of smooth laser pulses in both an autoionization system and in the laser-induced continuum structure scheme. We are particularly interested in the phenomenon of population trapping in these systems and discuss the effects of time-dependent pulses on this trapping. We present analytical solutions to both systems which hold for any pulse shape, provided the adiabatic approximation holds. We also pay special attention to the effects of large pulse energies on the trapped population, which we find to be detrimental and propose a possible solution to this problem by using chirped (time-dependent) frequencies.

DOI
10.1088/0953-4075/31/4/024

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We investigate the effects of smooth laser pulses in both an autoionization system and in the laser-induced continuum structure scheme. We are particularly interested in the phenomenon of population trapping in these systems and discuss the effects of time-dependent pulses on this trapping. We present analytical solutions to both systems which hold for any pulse shape, provided the adiabatic approximation holds. We also pay special attention to the effects of large pulse energies on the trapped population, which we find to be detrimental and propose a possible solution to this problem by using chirped (time-dependent) frequencies.
Summary form only given. We study two fundamental systems that involve continua: a system with a single autoionizing state and the laser induced continuum structure scheme (LICS), both shown. We pay special attention to the response of these systems to time-dependent pulse interactions, which we find to differ from that of square pulse interactions. Such a study is particularly useful from the point of view of the many applications associated with these systems. For the theoretical analysis we perform appropriate transformations that allow us to study both systems simultaneously. From an analysis of the resulting Hamiltonian of these systems, we determine the important conditions for efficient population trapping.
A theoretical study of laser-induced autoionization of an atomic system in interaction with a time-dependent excitation is presented. Interference effects between the two ionization channels of the atom are found to be responsible for coherent phenomena, such as the enhancement of the photon yield, for a pulsed excitation. An analysis of the pulse-shape effects shows that pumping mechanisms of the proper dressed state, occurring during the rise time of flat-top pulses, can result in an enhancement of the photon yield. An adiabatic approximation based on the use of time-dependent complex dressed states is shown to be adequate to describe the interaction with smooth pulses.
We study the possibility of transferring population between two bound atomic or molecular states via an autoionizing state. Conditions for efficient transfer are given and the effects of the AC Stark shifts, the Fano parameters and the incoherent channels of ionization are explored. An adiabatic analytic solution which helps in the understanding of the behaviour of the system is also given. Efficient transfer is realized by using both a counterintuitive pulse sequence and concurrent laser pulses.
Two laser pulses that overlap in time have been successfully used for coherent excitation of a molecule or atom to a desired state. An intermediate state that is at or near resonance with the laser frequencies has been used in past experiments and calculations. Here we replace this intermediate state by a continuum of intermediate states. An analytic solution of a simple model suggests that the continuum can be used in such an excitation scheme, provided that the laser pulses are arranged in the so-called counterintuitive order. We use a special case of this solution to relate this work to ionization suppression and coherent population trapping.
The population trapping in highly excited atomic states under intense femtosecond Ti:sapphire transform limited and chirped laser pulse action is studied. It is demonstrated that trapping appears as a result of the multiphoton resonance between the initial ground state and a set of excited states near the continuum boundary accompanied by repopulation of the Rydberg states by the Raman Λ-type transitions via the continuum and the interference stabilization. The specific features of the population trapping in both positively and negatively chirped pulses are analyzed and discussed.
The authors show that the population trapping phenomenon found by Deng and Eberly (1986, 1988) for free-free transitions between structureless continua occurs under much more general conditions. They solve the time-dependent models exactly and show for asymptotically long times that population trapping can occur for two continua with energy-dependent matrix elements, and for an infinite number of structureless continua with coupled bands. Furthermore, they show that the trapping persists for two continua coupled by a noisy laser. The amount of trapping depends on the initial conditions.