The effect of radiation losses in localized fluorescence upon laser pumping of fluorescent random media

Background and Objectives: Excitation of fluorescence upon laser pumping of fluorescent multiple scattering media is a complex process that plays a key role in many applications of modern optics and laser physics, from optical biomedical diagnostics to controlling the parameters of functional materials for photonics and optoelectronics. The effect of radiation losses in local fluorescence emitters, which are associated with laser speckles in pumped random media, on the fluorescence output is considered. Materials and Methods: Within the framework of the concept of an ensemble of statistically independent local fluorescence emitters in a laser-pumped random medium, a model of radiation losses in an arbitrarily chosen local emitter is considered. The model takes into account both an extreme value of the cross section of radiation losses for a single emitter, which depends on its characteristic size and the concentration of fluorophore molecules in the medium, and the effect of radiation exchange with other emitters. The scattering properties of a laser-pumped fluorescent medium are considered as a suppressing factor for the ballistic radiation transfer between the local emitters. The modeled results are compared to the previously obtained empirical data on the effective cross section of radiation losses for Rhodamine-6G-saturated layers of close-packed anatase nanoparticles under pulsed laser pumping at the wavelength of 532 nm. Results: The relationship between the effective cross section of radiation losses and the characteristic size of the emitters, the concentration of fluorophore molecules, and the mean scattering free path of fluorescent radiation was obtained. It was found within the framework of the considered model that both the characteristic size of local emitters and the reduction factor to the cross section of radiation losses due to light transfer from other emitters are practically independent of the pump intensity, with the exception of a narrow interval of low intensities. On the other hand, the scattering length significantly affects these characteristics due to the “screening” effect in the case of expressed multiple scattering. Conclusion: Radiation exchange between local stochastic emitters of fluorescence radiation in laser-pumped random media is one of the key factors controlling the fluorescence response of the medium with increasing pump intensity. The considered phenomenological model makes it possible to adequately interpret the experimentally observed features of the behavior of the effective cross section of radiation losses for these systems. The obtained results can be used as a physical basis for the development of new approaches to laser probing of random media in terms of the interpretation of the experimental data.