Fluorescence and random lasing in disordered media

Random lasers have fascinating and useful properties, and may be applied in fields such as sensing, imaging, spectroscopy, material processing, optical displays and new lighting systems. This thesis, ''Fluorescence and random lasing in disordered media" aims to provide an experimental study of incoherent random lasers and to demonstrate a new sensing application for these lasers. In this thesis, we study the spectral and coherence signatures of threshold in random lasers using alumina and Rhodamine 6G under nanosecond-pulsewidth pumping. We compare coherence properties (temporal and spatial coherence) with emission spectra and study the effects of particle concentration and scattering length for the weakly scattering and diffusive random laser regimes. The threshold, as defined by changes in the visibility of the interference fringes, is consistent with that defined by changes in the emission peak intensity or in the emission linewidth. We investigate radiative and non-radiative energy transfer processes from Rhodamine 6G (donor) to Methylene blue (acceptor) to fabricate efficient infra-red random dye lasers (> 700 nm). The energy transfer efficiency is influenced by acceptor concentration, pump energy density and spectral overlap, and the optimum random laser performance is achieved when the concentration of Methylene blue is 6 times the concentration of Rhodamine 6G. A Stern-Volmer analysis is used to quantify the radiative and non-radiative energy transfer. Colloidal-solution and solid-state random dye lasers (Rhodamine 6G and Rhodamine 640 with alumina and gold nanoparticles) are characterized based on the wavelength shift of the emission peak, local field enhancement, fluorescence quenching, absorption and scattering properties. The improved performance of metal-based random lasers is attributed to localized surface plasmons. We also investigate the effects of metal-fluorophore distance and surface roughness of nanoparticles on the characteristics of random lasers. The surface plasmon resonance of silver changes with increased silver-fluorophore distance, and is also affected by the surface roughness of silver-gold bimetallic nanoparticles. Increased silver-fluorophore distance increases the lasing threshold and slightly blue-shifts the emission peak due to self-absorption. Random lasing for Rhodamine 640 / lumpy bimetallic (silver and gold) nanoparticles is compared to random lasing for Rhodamine 640 / silver nanoparticles. We investigate the possibilities of random lasing from active nanoscatterers, ruby and zinc oxide, but these systems operate below threshold and emit fluorescence and amplified spontaneous emission only. Finally, we demonstrate a new dopamine measurement technique using threshold and spectral measurements in gold/dye random lasers. The presence and concentration of dopamine is detected through the random laser emission peak shift, emission linewidth, emission peak-to-background ratio and random lasing threshold. Considering these parameters, dopamine is most sensitively detected by a change in the emission linewidth with a limit of detection of 1 × 10⁻⁷ M. Potential future work is to develop the random laser measurement technique to be suitable for clinical use.