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Optics and Spectroscopy

Parametric collective phenomena during the propagation of polychromatic laser pulses in an optically dense resonant medium without population inversion

Sergei N Bagaev, V. S. Egorov, Igor B. Mekhov, Peter Moroshkin, Чехонин Игорь Анатольевич

Published Dec 1, 2002• 3 citations
Publisher

Abstract

We experimentally and theoretically study the interaction of broadband polychromatic laser pulses with an optically dense resonant extended medium without population inversion. Experimental (probe field-pumping beam) measurements of the transmission and amplification spectra were carried out in the plasma of a positive neon glow-discharge column containing a large number of metastable atoms. The strong coupling in the field-matter system and the collective behavior of the atomic system in a resonant field were attributable to a high (∼10 12 cm −3 ) density of atoms at the lower (metastable) level of the optical transitions under consideration and to a relatively low intensity of the interacting laser beams. We observed a broadband weakening of the probe field in the absence of pumping and its strengthening in the line wings in the presence of a strong field. We develop a theoretical model for the parametric amplification of collective interactions in dense extended media based on the solution of the semiclassical Maxwell-Bloch equations for conditions under which the pumping field does not destroy the dipole interaction between atoms through probe-field photons.

DOI
10.1134/1.1531723

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•Jul 20, 2003•0 citations
We experimentally and theoretically study the interaction of broadband polychromatic laser pulses with an optically dense resonant extended medium without population inversion. Experimental (probe field-pumping beam) measurements of the transmission and amplification spectra were carried out in the plasma of a positive neon glow-discharge column containing a large number of metastable atoms. The strong coupling in the field-matter system and the collective behavior of the atomic system in a resonant field were attributable to a high (∼10 12 cm −3 ) density of atoms at the lower (metastable) level of the optical transitions under consideration and to a relatively low intensity of the interacting laser beams. We observed a broadband weakening of the probe field in the absence of pumping and its strengthening in the line wings in the presence of a strong field. We develop a theoretical model for the parametric amplification of collective interactions in dense extended media based on the solution of the semiclassical Maxwell-Bloch equations for conditions under which the pumping field does not destroy the dipole interaction between atoms through probe-field photons.
Experimental and theoretical investigation of the coherent propagation and amplification of polychromatic broadband laser pulses in optically dense resonant media without population inversion was carried out for the cases of plasma in a Ne glow discharge and a pulse discharge in Hg-Ar mixture. High density of Ne and Hg atoms in the metastable state (about 1012 cm-3), which was chosen to be the ground state of the two-level system under consideration, leads to the collective behavior of the atomic system in resonant electromagnetic field. The broadband probe field attenuation and the sideband amplification of the probe under the action of the coherent pump were observed. The amplification dependence on the pump intensity was obtained. The model of the parametrically enhanced collective interactions was developed and comparison the experimental data was made.
Experimental and numerical investigation of single-beam and pump-probe interaction with a resonantly absorbing dense extended medium under strong and weak field-matter coupling is presented. Significant probe beam amplification and conical emission were observed. Under relatively weak pumping and high medium density, when the condition of strong coupling between field and resonant matter is fulfilled, the probe amplification spectrum has a form of spectral doublet. Stronger pumping leads to the appearance of a single peak of the probe beam amplification at the transition frequency. The greater probe intensity results in an asymmetrical transmission spectrum with amplification at the blue wing of the absorption line and attenuation at the red one. Under high medium density, a broadband of amplification appears. The theoretical model is based on the solution of the Maxwell-Bloch equations for a two-level system. Different types of probe transmission spectra obtained are attributed to complex dynamics of a coherent medium response to broadband polychromatic radiation of a multimode dye laser.
An experimental and theoretical study of the two-wave interaction (of a probe wave and a pumping wave) of wide-band multimode laser pulses with an optically dense extended resonant medium (metastable neon atoms in a positive glow-discharge column) is presented. Depending on the pumping power and characteristics of the medium, various regimes of interaction between the field and the matter, leading to qualitatively different types of amplification spectra, are realized: a cooperative interaction regime at low pumping powers corresponds to amplification spectra consisting of two peaks, which lie on both sides of the absorption line; in the regime of nonstationary Rabi oscillations (Rabi flopping), the amplification appears in the form of an isolated peak at a frequency close to the resonance in the substance; under certain interaction conditions, a dispersive shape of amplification spectra is observed. A significant gain in the probe field (by several times at a length of 10 cm) is obtained as a result of a nonstationary population inversion upon rotation of the Bloch vector of the resonant medium under the action of a strong pumping field in the course of propagation of the two waves.
The nonstationary pump-probe interaction between short laser pulses propagating in a resonant optically dense coherent medium is considered. Special attention is paid to the case where the density of two-level particles is high enough that a considerable part of the energy of relatively weak external laser fields can be coherently absorbed and reemitted by the medium. Thus, the field of the medium reaction plays a key role in the interaction processes, which leads to collective behavior of an atomic ensemble in the strongly coupled light-matter system. Such behavior results in fast excitation interchanges between the field and a medium in the form of optical ringing, which is analogous to polariton beating in solid-state optics. This collective oscillating response, which can be treated as successive beats between light wave packets of different group velocities, is shown to significantly affect the propagation and amplification of the probe field under its nonlinear interaction with a nearly copropagating pump pulse. Depending on the probe-pump time delay, the probe transmission spectra show the appearance of either a specific doublet or coherent dip. The widths of these features are determined by the density-dependent field-matter coupling coefficient and increase during the propagation. Besides that, the widths of the coherent features, which appear close to resonance in the broadband probe spectrum, exceed the absorption-line width, since under the strong-coupling regime, the frequency of optical ringing exceeds the rate of incoherent relaxation. Contrary to stationary strong-field effects, the density- and coordinate-dependent transmission spectra of the probe manifest the importance of collective oscillations and cannot be obtained in the framework of a single-atom model.