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Journal of Modern Optics

Inversion-polarization Filaments under Weak Pumping

Vyacheslav I. Yukalov

Published Aug 1, 1990• 13 citations
Publisher

Abstract

It is shown that inversion-polarization filaments in laser media arise not only under strong but also under weak pumping. In the system with these filaments a coherent component of radiation appears, while a uniform system could radiate only incoherently. The coherent radiation is sharply anisotropic and is strongest in the direction of the filaments. Notwithstanding the weakness of pumping, in the system with inversion-polariton filaments a super-radiating regime is possible.

DOI
10.1080/09500349014551501

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It is shown that inversion-polarization filaments in laser media arise not only under strong but also under weak pumping. In the system with these filaments a coherent component of radiation appears, while a uniform system could radiate only incoherently. The coherent radiation is sharply anisotropic and is strongest in the direction of the filaments. Notwithstanding the weakness of pumping, in the system with inversion-polariton filaments a super-radiating regime is possible.
A three-level system which radiates at both intermediate transition frequencies is considered to be coherently pumped at a frequency near the two-photon resonance frequency. A sharply resonant rise in the second- and third-level population is found for slight detuning of the pump from two-photon resonance. Population inversion in either transition is shown to be possible, depending on the pertinent parameters. This effect furnishes a novel method of population inversion at a frequency higher than the pump frequency. Keywords include: Laser, Laser-type devices, Coherent pumping, Three-level systems, and Quantum electronics.
The emergence of laser action from an atomic system without population inversion is beginning to attract theoretical and experimental attention as a possible way of generating useful coherent radiation at unusual wavelengths. The goal of this communication is to clarify the physical origin of this phenomenon using a four-level system whose ground state consists of two quasidegenerate levels that are coupled to a higher-lying state by a Raman-like transition. In the presence of the ground-state coherence induced by the Raman field, we calculate the absorption spectrum of a weak probe whose tunable frequency scans across the transition frequencies between the two ground-state levels and another higher-lying state that may or may not be weakly excited by an incoherent pump mechanism. Our calculation, based on a dressed atomic state approach, clarifies the conditions that must be satisfied to produce gain, identifies the frequency (or frequencies) at which the gain is maximum, and elucidates the functions of the incoherent pump, the Raman field, and the coherence between the ground-state levels in this process. An appropriate set of Maxwell- Bloch equations for a ring-cavity model of this system confirms the appearance of laser action when the small-signal gain conditions are satisfied.