Stimulated emission of dyes mediated by plasmonic colloids

This thesis investigates Rhodamine B-based colloidal systems as model platforms for studying the evolution from amplified spontaneous emission toward feedback-assisted random-lasing-like behaviour. The work addresses the difficulty of distinguishing these regimes experimentally, particularly in dye solutions, where spectral narrowing and superlinear intensity growth may arise even in the absence of a clearly defined optical feedback mechanism. To explore this issue, Rhodamine B was combined with plasmonic nanoparticles differing in material and morphology, including gold and silver nanospheres and gold nanostars. Their comparison allowed the roles of spectral overlap, scattering, plasmonic enhancement, and morphology-dependent field localization to be examined. Rhodamine B solutions without metallic nanoparticles and systems containing dielectric SiO₂ particles were used as reference samples. The samples were optically pumped at 532 nm, and their emission spectra were recorded as a function of pump fluence. The onset and evolution of stimulated emission were evaluated through the appearance of a narrow spectral contribution, the intensity measured at the stimulated-emission maximum, and the ratio between the fitted stimulated and spontaneous emission components. The onset assignment was further supported by monitoring the evolution of spectral narrowing. The results reveal a marked dependence of the stimulated-emission response on nanoparticle material, morphology, and concentration. While the gold-based systems predominantly showed plasmon-assisted ASE-like behaviour, the highest-concentration silver nanoparticle sample exhibited the strongest spectral narrowing and the clearest indications of scattering-assisted feedback. However, the available spectral and transport-related evidence does not support an unambiguous assignment to random lasing. The observed response is therefore more cautiously described as feedback-assisted ASE or random-lasing-like stimulated emission.