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5The 30th International Conference on Advanced Laser Technologies ALT'23
LD-O-13
Fluorescence amplification in laser-pumped random and homogeneous fluorescent media: the fundamental limitations
D.A. Zimnyakov1, S.S. Volchkov1, I.V. Mikhailov1, D.V. Tsypin1, A.S. Tokarev1, V.I. Kochubey2
1- Yury Gagarin State Technical University of Saratov, 77 Polytechnicheskaya St., Saratov, 410054, Russia 2- Saratov State University, 83 Astrakhanskaya St., Saratov, 410012, Russia
Fundamental features of the transition from spontaneous fluorescence to dominating stimulated emission in laser-pumped dye-based homogeneous and random media are considered. This transition, usually interpreted as overcoming the random lasing threshold [1,2] is analyzed in terms of kinetic equations for the relative population f of the excited state of dye molecules and the fluence rate of the fluorescence response in the pumped layer of the medium [3,4]. In the stationary mode of fluorescent emission at high pump intensities, the extreme value of f depends on the ratio of the wavelength-averaged cross section (ast)A of stimulated
emission of fluorophore molecules and the average cross section (arad) of radiation losses in the pumped
layer. On the other hand, the effect of a significant narrowing of the fluorescence spectrum, which is characteristic of overcoming the random lasing threshold, is due to a significant increase in the dwell time rdw of fluorescence photons in the pumped layer in the threshold region. An analysis of the competitive influence of the key factors (pump intensity, concentration of dye molecules, their absorption and emission cross sections, transport mean free paths of pump and fluorescence propagation) that determine f and Tiw was carried out using the above mentioned stationary solutions of kinetic equations and consideration of radiation transport in the pumped layer.
Experimental studies of the influence of (arad), f and Tiw in the pumped volume as the extinction mean free path l^ of a pumping laser radiation (355 nm and 532 nm) decreases were carried out using 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) solutions in ethylene glycol with various concentrations (from 1.2x 10-3 M to 1.2 x 10-1 M). At high DCM concentrations, the optical transport properties of the studied system are largely controlled by the effect of homogeneous nucleation in a saturated DCM solution and, accordingly, by the formation of an ensemble of DCM crystallites as the scattering sites for the pump and fluorescence radiation. This feature made it possible to vary the pump and fluorescence extinction mean free paths over a wide range, with absorption dominating at low mass fractions of DCM in solutions and scattering at large values of the DCM mass fraction (more than 2.0x10-2 M). Structural and optical transport properties of randomly inhomogeneous fluorescent systems formed in this way were analyzed using spectroturbidimetry, scanning electron microscopy, and X-ray diffractometry. An analysis of the probability density functions of the dwell times of fluorescent photons in the pumped volume as a function of lra( was performed using Monte Carlo simulations of the pump and fluorescence transport in the pumped layers of dye solutions.
Comparison of the obtained experimental data on the extreme conditions for the excitation of the fluorescent response in the studied systems at various l values with the theoretical results based on the developed phenomenological model showed a fair agreement between them. This research was funded by the Russian Science Foundation, grant number 22-29-00612.
[1] V.S. Letokhov, Generation of light by a scattering medium with negative resonance absorption, Soviet Physics JETP, 26, 835-840 (1968).
[2] D.S. Wiersma, The physics and applications of random lasers, Nature Physics, 4, 359-367 (2008).
[3] D.A. Zimnyakov, S.S. Volchkov, L.A. Kochkurov, A.F. Dorogov, On the minimum length of fluorescence enhancement upon laser pumping of randomly inhomogeneous media, JETP Letters, 116, 71-76 (2022).
[4] D. Zimnyakov, S. Volchkov, L. Kochkurov, A. Dorogov, Saturated emission states in fluorescent nanostructured media: the role of competition between the stimulated emission and radiation losses in the local emitters of fluorescence, Nanomaterials, 12, 2450-1 - 2450-19 (2022).
