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Journal of Physics B Atomic Molecular and Optical Physics

Population trapping of excited atoms in strong chirped laser pulses

A. M. Popov, Olga V. Tikhonova, Ekaterina A. Volkova

Published Oct 8, 2014• 9 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/47/20/204012

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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.
We have observed trapping of population in Rydberg states of atoms and ions exposed to intense laser radiation. The trapping occurs in nonstationary ``wave-packet'' superposition states, created, via ac Stark-shifted multiphoton resonances during the laser pulse. The formation of these highly excited bound states requires the nonresonant absorption of up to 20 or more photons; some of the ionic Rydberg states lie nearly 32 eV above the ionic ground state. All of the trapped states having binding energies much less than the photon energy but remain stable against ionization even when the multiphoton ionization probability of the atom is saturated.
We present a scheme of population transfer between two metastable (ground) states of the $\ensuremath{\Lambda}$ atom without considerable excitation of the atom using single frequency-chirped laser pulses. The physics of the process is generation of the ``trapped'' superposition of the ground states by the laser pulse at sufficiently high laser peak intensity. The main conditions for realization of this regime are the following: The width of the transform-limited laser pulse envelope frequency spectrum (without chirp) must be smaller and the peak Rabi frequency of the pulse must be larger than the frequency interval between the two ground states of the $\ensuremath{\Lambda}$ atom. During the frequency chirp, the laser pulse must first come into resonance with the transition from the initially occupied ground state to the excited state and after that with the transition between the excited and second initially empty ground states. In the case when the envelope frequency spectrum width (without chirp) of the pulse exceeds the frequency interval between the two ground states, we show a possibility of controllable generation of superposition of the ground states with a controllable excitation of the $\ensuremath{\Lambda}$ atom.
The dynamics of hydrogen atom in the presence of a strong radiation field of the titanium-sapphire laser is studied for the Keldysh parameters γ ≥ 1 and γ ≤ 1. It is demonstrated that the ionization is supplemented with the effective population of the excited states with the principal quantum numbers n = 5–10 in the entire range of variation in the Keldysh parameter. The population of the excited Rydberg states can be interpreted as a consequence of the multiphoton resonance involving the initial 1 s state and a group of excited states in the vicinity of the continuum boundary with the simultaneous repopulation of these states by Λ-type Raman transitions under the action of the laser field. The resulting coherent Rydberg packet appears to be stable with respect to ionization, so that the ionization of the atomic system in the presence of strong electromagnetic field is suppressed. Physical reasons for the stabilization are discussed. An interpretation of the effective population of the Rydberg states in the recent experiments on the ionization of atomic helium by the titanium-sapphire laser is proposed.
We have observed suppression of ionization in Rydberg atoms irradiated by an intense short-pulse laser field. We find that single-photon ionization of the outer electron in a Ba 6snk Stark state can be severely suppressed relative to high-order (\ensuremath{\ge}3 photon) nonresonant multiphoton ionization of the tightly bound 6s core electron. This population trapping occurs in intense short laser pulses, where the bandwidth of the ionizing laser is greater than the spacings between adjacent Stark states, and may be due to a substantial modification of the initial Stark state via stimulated Raman processes through the continuum. The Ba+∗{\mathrm{Ba}}^{+\mathrm{*}}Ba+∗ ion should be left in a higly excited superposition state, or wave packet.