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Quantum Electronics

Christiansen effect in disperse systems with resonant absorption

Dmitry A. Zimnyakov, Elmira A Isaeva, Anna A. Isaeva

Published Jan 31, 2012• 0 citations
Publisher

Abstract

We discuss the results of experimental studies of competition of absorption and scattering of laser radiation propagating in dispersive media with resonant absorption. As media under study, use is made of a suspension of polystyrene particles in solutions of rhodamine 6G in ethylene glycol probed by laser light with a wavelength of 532 nm. It is found that an increase in the dye concentration leads to an increase in optical transmittance of suspensions and an increase in speckle modulation of the forward-scattered radiation. We interpret these features as a manifestation of Christiansen effect in disperse systems with resonance absorption.

DOI
10.1070/qe2012v042n01abeh014703

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We discuss the results of experimental studies of competition of absorption and scattering of laser radiation propagating in dispersive media with resonant absorption. As media under study, use is made of a suspension of polystyrene particles in solutions of rhodamine 6G in ethylene glycol probed by laser light with a wavelength of 532 nm. It is found that an increase in the dye concentration leads to an increase in optical transmittance of suspensions and an increase in speckle modulation of the forward-scattered radiation. We interpret these features as a manifestation of Christiansen effect in disperse systems with resonance absorption.
The interaction of polychromatic radiation of a dye laser with a neon gas-discharge plasma in shielded cylindrical tubes at medium gas pressures (1-30 Torr) near atomic spectral absorption lines of NeI atoms with wavelengths 650.6 and 656.2 nm was experimentally studied. Along the direction of propagation of the laser radiation, a shift in the central frequency of absorption is observed in the presence of inhomogeneities in the plasma column. The character of the optical transmission spectrum near narrow resonances was empirically shown to substantially depend on the density of absorbing atoms in a plasma; if this density exceeds 1012 cm-3, the contour of the transmission spectrum near the absorption resonance acquires a dispersive form. The general laws of the formation of nonstationary plasma transmission spectra depending on the laser power, the density of absorbing atoms, the nature of plasma inhomogeneities along the direction of propagation of laser radiation are examined in detail. The role of nonlinear and coherent effects and phase self-modulation during resonant interaction of short pulses of polychromatic laser radiation with gas-discharge plasma in narrow shielded tubes is discussed.
In several recent experiments, isotropic lasing action was observed in paints that contain rhodamine 640 dye molecules in methanol solution as gain media and titania particles as optical scatterers. These so-called paint-on laser systems are extraordinary because they are highly disordered systems. The microscopic mechanism for laser activity and the coherence properties of light emission in this multiple-light-scattering medium have not yet been elucidated. In this paper we derive the emission intensity properties of a model dye system with excited singlet and triplet electronic energy levels, which is immersed in a multiple-scattering medium with transport mean free path l*. Using physically reasonable estimates for the absorption and emission cross section for the singlet and triplet manifolds, and the singlet-triplet intersystem crossing rate, we solve the nonlinear laser rate equations for the dye molecules. This leads to a diffusion equation for the light intensity in the medium with a nonlinear intensity-dependent gain coefficient. Using this model we are able to account for nearly all of the experimentally observed properties of laser paint reported so far when l*\ensuremath{\gg}${\ensuremath{\lambda}}_{0}$, the emission wavelength. This includes the dependence of the peak intensity of amplified emission on the mean free path l*, the dye concentration \ensuremath{\rho}, and the pump intensity characteristics. Our model recaptures the collapse of the emission linewidth at a specific threshold pump intensity and describes how this threshold intensity varies with l*. In addition, our model predicts a dramatic increase in the peak intensity and a further lowering of the lasing threshold for the strong scattering limit l*\ensuremath{\rightarrow}${\ensuremath{\lambda}}_{0}$. This suggests a striking enhancement of the characteristics of laser paint near the photon localization threshold in a disordered medium. \textcopyright{} 1996 The American Physical Society.
The effect of the form change of the intensity correlation function of light scattered back by emulsion or suspension of the particles in liquids is obtained experimentally, when the exciting light intensity increase. The change consists in the appearing in the correlation function of the cosine component with frequency equal to inverse relaxation time of particles concentration fluctuations. The appearance of such cosine component tells about the appearance of the additional spectral line, shifted with respect to exciting light by the half-width of the spectral line of spontaneous polarized Rayleigh scattering by particles. The cosine is caused by beating between exciting radiation and this spectral line. This frequency shift is of about 25-250 Hz depending on particles sizes and characteristic of liquid, in which they are weighted. Together with the scattering relative intensity increase the cosine appearance confirms the obtaining of the stimulated light scattering at variations of the particles concentrations.
A review is given of recent theoretical investigations on coherent propagation and nonlinear interaction of multifrequency ultrashort radiation pulses in resonantly absorbing media. An analysis is made of frequency conversion by parametric interaction and stimulated Raman scattering of laser pulses in resonant three-level media. New ways of controlling the parameters of coherent pulses and the use of multifrequency pulses in the spectroscopy of multilevel media are discussed. The experimental capabilities of the investigated effects are considered.