CC BY
12LM-I-21
Optical Non-Linearity and Light Diffusion in Laser-Pumped Fluorescent Nanocomposites: From a Spontaneous Fluorescence Emission to a Random Lasing
Dmitry A. Zimnyakov1'2, Sergey S. Volchkov1, Leonid A. Kochkurov1, Alexander F. Dorogov1
1 — Yury Gagarin State Technical University of Saratov, 77 Polytechnicheskaya St., Saratov,
410054, Russia
2 — Institute for Problems of Precision Mechanics and Control, 24 Rabochaya St., Saratov,
410028, Russia
zimnykov@mail.ru
Fluorescence emission in multiple scattering random media under high-intensity pulse-periodic laser pumping is considered in terms of the localization of "hot" pumped zones in the laser-treated medium. This localization is caused by the granular structure (speckle patterning) of the pumping laser field due to a significant excess of the coherence length of laser radiation over the characteristic scales of radiation propagation in the medium. From this point of view, each laser speckle in the pumped volume is associated with a local low-finesse optical cavity with a characteristic size of the order of the pump wavelength. Speckle-caused confinement of the pumping in combination in combination with the absence of optical feedback between statistically independent laser speckles is a natural limiter of the spectral quality of the fluorescence response even at pump intensities significantly exceeding the threshold of random lasing in pumped fluorescent random media. This feature is observed in numerous experiments on the excitation of random lasing in multiple scattering random media with a high fluorescence yield and can be described in terms of the ratio of the stimulated emission component to the spontaneous component of the fluorescence output.
Modeling of the rise/fall kinetics of fluorescence in random fluorescent media under speckle-patterned laser pumping [1] has shown that the crucial influence on the ratio of the stimulated to the spontaneous component fluorescence at high pump intensities is exerted by the depletion of the ground state of the fluorophore and radiation losses in speckle-associated local microcavities. It was found that the extreme value of the effective cross-section of radiation losses in local microcavities in the absence of radiative transfer between neighboring cavities is inversely proportional to the characteristic size of cavities and the fluorophore concentration in the medium. The real value of the effective cross-section of radiation losses turns out to be significantly (at least one and a half to two decades) less than the extreme value due to the significant effect of radiation exchange between the speckle-associated microcavities.
The discussed concept was verified in experiments on pulse-periodic laser pumping of layers of close-packed titania and silica nanoparticles doped with aqueous solutions of Rhodamine 6G [1,2]. The experimentally obtained dependences of the ratio of the stimulated to the spontaneous fluorescence component on the pump parameters and the characteristics of the pumped samples are in good agreement with the simulation results within the framework of the concept. The results obtained may be of interest from the point of view of further development of fluorescence diagnostics of media with a complex structure.
[1] D.A. Zimnyakov, S.S. Volchkov, L.A. Kochkurov, V.I. Kochubey, A.G. Mel'nikov, G.V. Mel'nikov, Speckle patterning of a pumping laser light as a limiting factor for stimulated fluorescence emission in dense random media, Optics Express, 29, pp. 2309-2331, (2021).
[2] D.A. Zimnyakov, S.S. Volchkov, L.A. Kochkurov, A.F. Dorogov, Specific features of fluorescence transfer in multiply scattering randomly inhomogeneous layers under intense laser pumping, Quantum Electronics, 50, pp. 1007-1014, (2020).
