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Physical Review A

Coherent population trapping via a continuum with a train of ultrashort pulses

Elena Kuznetsova, Roman L. Kolesov, Olga A. Kocharovskaya

Published Sep 8, 2006• 5 citations
Publisher

Abstract

We consider the coherent population trapping effect with a train of ultrashort pulses in a $\ensuremath{\Lambda}$-type system with two bound states and a continuum replacing the upper state. Population transfer to the continuum can be totally suppressed provided that the splitting between the bound states is a multiple of the pulse repetition rate. Compared to the traditional case with only two laser fields coupling the bound states to the continuum, CPT with a pulse train allows one to avoid incoherent population losses due to interaction of the fields with ``wrong'' transitions, which results in only partial suppression of ionization. A method of suppression of excited-state absorption of pumping radiation in laser crystals, with pumping in the form of the pulse train, is proposed. As a particular example pumping dynamics of a ${\mathrm{Ti}}^{3+}:{\mathrm{YAlO}}_{3}$ crystal using this technique is numerically analyzed.

DOI
10.1103/physreva.74.033804

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We consider the coherent population trapping effect with a train of ultrashort pulses in a \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ-type system with two bound states and a continuum replacing the upper state. Population transfer to the continuum can be totally suppressed provided that the splitting between the bound states is a multiple of the pulse repetition rate. Compared to the traditional case with only two laser fields coupling the bound states to the continuum, CPT with a pulse train allows one to avoid incoherent population losses due to interaction of the fields with ``wrong'' transitions, which results in only partial suppression of ionization. A method of suppression of excited-state absorption of pumping radiation in laser crystals, with pumping in the form of the pulse train, is proposed. As a particular example pumping dynamics of a Ti3+:YAlO3{\mathrm{Ti}}^{3+}:{\mathrm{YAlO}}_{3}Ti3+:YAlO3​ crystal using this technique is numerically analyzed.
We discuss the coherent population trapping (CPT) effect induced in a three-level Λ-type system by a train of short pulses. Specifically, we consider a modified Λ system with the upper level replaced by a continuum of states. We show that CPT with a train of pulses can be applied to the suppression of excited-state absorption of pump radiation in crystalline laser materials. The utility of the technique for pumping a promising yellow-orange solid-state medium Ti:YAlO 3 and overcoming excited-state pump absorption is analyzed.
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.
Here, I discuss the interaction of two matched ultrashort pulses with a \ensuremath{\Lambda}-like system in which the upper intermediate state is replaced by a manifold of discrete states. Many of these states are simultaneously accessed by the broad bandwidth of the two excitation pulses. The equations of motion for the probability amplitudes of the levels involved in the excitation process are solved analytically in the quasi-impulsive regime. In this regime, the excitation pulses are made shorter than the characteristic time of the excited manifold; population is then transferred to the manifold without Rabi cycling. Under the appropriate conditions, this ``multi-\ensuremath{\Lambda}'' system can exhibit coherence effects such as coherent population trapping and electromagnetically induced transparency of one of the excitation pulses.
In this paper, we investigate the evolutional dynamics of a Λ-type atomic system of which two branches of transitions are driven by two trains of ultrashort pulse respectively. The accumulating of atomic populations and coherences due to excitations of successive pulses are demonstrated by the numerical simulations. The realization of the coherent population trapping (CPT) effect is verified. When the spontaneous generated coherence (SGC) is considered, the system will evolve to the dark state even when the initial state is not the dark state if the degree of the SGC is not maximal and the degree of the SGC will influence the time needed to reach the atomic steady state. With the maximal degree of SGC, whether the system will evolve into the dark state depends on the initial state. This fact means that the maximal SGC spoils the dark state geometry, which a Λ-type atom driven by light field satisfies. Additionally, we found that the involvement of the phase of the driving field can counteract the destruction stem from the maximal degree of the SGC on the dark state geometry, i.e. the system can be trapped in the dark state even when the initial state is not the dark state. Meanwhile, it is found that the phase can adjust the time needed by the system to reach the steady state in a quite wide range.
We propose a mechanism to produce a superposition of atomic and molecular states by a train of ultrashort laser pulses combined with weak control fields. By adjusting the repetition rate of the pump pulses and the intensity of the coupling laser, one can suppress a transition while simultaneously enhancing the desired transitions. As an example, various superpositions of vibrational states of the K2{\mathrm{K}}_{2}K2​ molecule are shown.