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Soviet Journal of Quantum Electronics

Generation of stimulated noncoherent radiation in light-scattering media exhibiting chemical reactions

I. A. Izmaǐlov, V. A. Kochelap, L Yu Mel'nikov

Published May 31, 1982• 0 citations
Publisher

Abstract

It is proposed that a feedback resulting from scattering be used to achieve lasing in a disperse reactive medium. The example of a simple two-level system shows that under advanced lasing conditions the quantum efficiency of the radiation emission approaches the quantum efficiency of the excitation of the upper level, and the emission spectrum becomes much narrower. Feasibility of chemical pumping of such a laser is estimated on the basis of calculations of heterophase burning of a drop of a fuel in an oxidizing atmosphere. The growth increment of light is calculated and the threshold conditions for the excitation of lasing are found. Examples are given to illustrate the feasibility of purely chemical pumping of a laser with a nonresonant feedback. It is shown that dense reactive media can be used in such lasers.

DOI
10.1070/qe1982v012n05abeh012377

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Physical Review A•Oct 1, 1996•190 citations
It is proposed that a feedback resulting from scattering be used to achieve lasing in a disperse reactive medium. The example of a simple two-level system shows that under advanced lasing conditions the quantum efficiency of the radiation emission approaches the quantum efficiency of the excitation of the upper level, and the emission spectrum becomes much narrower. Feasibility of chemical pumping of such a laser is estimated on the basis of calculations of heterophase burning of a drop of a fuel in an oxidizing atmosphere. The growth increment of light is calculated and the threshold conditions for the excitation of lasing are found. Examples are given to illustrate the feasibility of purely chemical pumping of a laser with a nonresonant feedback. It is shown that dense reactive media can be used in such lasers.
A theoretical study is made of the influence of changes in the relaxation of a three-level system in a resonance field on the dispersion, absorption, and difference between the populations of levels activated by noncoherent pumping. This change in the relaxation is due to a tendency of a thermal reservoir to create an additional difference between the populations of quasienergy states that appear under the influence of the resonance field. It is shown that as a consequence there is a considerable change in the nature of the interaction with the radiation field in laser crystals with three-level active centers when the pump power is close to the threshold. The lasing conditions are found subject to such changes. It is shown that the threshold pump power can be reduced in this case to a value at which the difference between the populations of the active levels vanishes and lasing appears because of an inversion of the populations in a system of quasienergy levels. Under transient conditions the lasing frequency near the threshold should then depend on the pump
We report a theoretical study on the possibility of amplification and generation of radiation by an inversionless two-level system in the ‘red’ wing of its spectral line under resonant diode pumping. The two-level system is used to simulate atoms of an active gas, while it is in the atmosphere of a high-pressure buffer gas. The effect results from the fact that the probability of stimulated emission in the ‘red’ wing of the spectral line exceeds the probability of absorption if the homogeneous broadening due to the interaction of particles with the buffer gas significantly exceeds the natural one (at high pressures of the buffer gas). It is found that the higher the buffer gas pressure and the higher the pump radiation intensity, the greater the inversionless amplification. It is shown that the gain in a two-level system in the ‘red’ wing of its spectral line can reach 0.011 cm −1 when the radiation intensity and the width of the radiation spectrum of pump diodes are 5 kW cm −2 and 4 cm −1 , respectively. The use of transverse diode pumping of an active medium placed in an optical cavity will make it possible to obtain lasing with frequency tuning and thereby solve the problem of converting incoherent broadband radiation into a coherent laser light in a gas of two-level active particles.
A new version of obtaining short-wave lasing on transition between highly excited states and the ground state of active atoms in the buffer gas atmosphere is studied theoretically. The mechanism of obtaining population inversion on such a transition is associated with the establishment of the local Boltzmann distribution of populations in a group of highly excited levels due to frequent collisions. If the excitation of the upper-lying level is performed by two laser radiation sources with frequencies ω 1 and ω 2 , short-wave lasing can be obtained at a frequency close to the total frequency ω 1 + ω 2 . The conditions for the emergence of population inversion are analyzed and simple analytic formulas are derived. It is shown that collision-induced enhancement of short-wave radiation can occur for pumping intensities on the order of 100 W/cm 2 . For pumping intensities on the order of 1000 W/cm 2 , the amplification factor for short-wave radiation may attain values of 3 cm −1 (for an active atom concentration of N ∼ 10 15 cm −3 ), which is sufficient for the development of lasing per path through an active medium (superradiance condition) for a length of the active medium on the order of 10 cm.
A theory is given of the stimulated scattering of light as a result of interaction of a strong electromagnetic wave with a dynamic grating (wave) of the population N of an excited state if N affects the refractive index of the medium. The medium is regarded as an assembly of two-level systems and the dependence of the permittivity ε on N is assumed to be linearized. If δε / δ N <0, the amplification due to the stimulated scattering is expected in the anti-Stokes region on illumination of an unexcited medium. Under lasing conditions the amplification should occur in the Stokes region of the strong electromagnetic field (wave). This mechanism may be the cause of the mutual coupling between laser modes.
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.