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LASER DIAGNOSTICS AND SPECTROSCOPY
Quasi-equilibrium saturated states of fluorescence emission by laser-pumped fluorescent random media: the fundamental role of characteristic
scales of radiative transfer
D.A. Zimnyakov1'2. S.S. Volchkov1, L.A. Kochkurov1, A.F. Dorogov1
1- Yury Gagarin State Technical University of Saratov, 77Polytechnicheskaya St., Saratov, 410054, Russia 2- Institute for Precision Mechanics and Control Problems of the Russian Academy of Sciences, 24 Rabochaya St.,
Saratov, 410028, Russia zimnykov@mail.ru
High-intensity laser pumping of fluorescent random media leads to a significant narrowing of the fluorescence spectrum, which is usually interpreted as a manifestation of random lasing in the pumped medium. Typically, the pump intensity at which the fluorescence spectrum narrows by a factor of two compared to the spontaneous fluorescence spectrum is taken as the threshold for random lasing. A characteristic feature of the random lasing mode at pump intensities substantially exceeding the threshold value is the almost constant half-width of the fluorescence spectrum, which is independent of the pump intensity. The spectral quality improvement factor Qsp = AX I /AX I ^ under these overpumping conditions, as a rule, does not exceed 8 - 10 (here AX I is the halfwidth of the emission spectrum in the spontaneous emission mode and AX_, ^ is the similar value at large pump intensities). Moreover, various pumped systems with significantly different optical transport parameters of scattering matrices and emission characteristics of fluorophores exhibit close Qsp values at high pump levels. Despite the abundance of theoretical and experimental works on various aspects of random lasing, such an effect of saturation of the spectral quality of fluorescence above the threshold has not yet been considered in detail.
Previously, it was found [1] that speckle modulation of pump radiation in a fluorescent medium has a significant effect on the formation of the fluorescence response. The light emitting system can be considered as a stochastic ensemble of uncorrelated micro-emitters of fluorescence associated with pump field speckles. One of the key parameters controlling the efficiency of pump conversion into a fluorescence response is the cross section of radiation losses of local emitters averaged over the ensemble. Further consideration of the saturation criterion for the excited state of fluorophore molecules and the probabilistic model of fluorescence enhancement in the pumped volume made it possible to determine the extreme value of the characteristic scale of fluorescence amplification in the medium at high pump intensities. It was established that this extreme amplification length does not depend on the emission properties of the fluorophore and the optical transport properties of the scattering matrix, but is determined only by the characteristic size of local fluorescence emitters and the factor of radiation exchange between them. Comparison of theoretical results with experimental data for R6G- and DCM-saturated layers of close-packed anatase particles pumped by pulse-periodic laser radiation at 532 nm showed good agreement between these datasets. The results obtained are of interest for the further development of fluorescent diagnostics of randomly inhomogeneous media and composite materials.
This work is supported by the Russian Science Foundation (the project № 22-29-00612).
[1] D.A. Zimnyakov, S.S. Volchkov, L.A. Kochkurov, V.I. Kochubey, A.G. Melnikov, G.V. Melnikov, Speckle patterning of
a pumping laser light as a limiting factor for stimulated fluorescence emission in dense random media, Optics Express, vol. 29, pp.2309-2331 (2021).
ALT'22
