Energy Transfer Networks: Quasicontinuum Photoluminescence Linked To High Densities Of Defects

In a series of studies related to laser-induced damage of optical materials and deposition of plastics, we discovered a broadly emitting photoluminescence with fast lifetimes that we termed quasicontinuum photoluminescence (QC-PL). Here, we suggest that a high density of optically active defects leads to QC-PL, where interactions between defects affect the temporal and spectral characteristics of both excitation and emission. We develop a model that predicts the temporal characteristics of QC-PL, based on energy transfer interactions between high densities of defects. Our model does not explain all spectral broadening and redshifts found in QC-PL, since we do not model spectral changes in defects due to proximity to other defects. However, we do provide an example of a well-defined system that exhibits the QC-PL characteristics of a distribution in shortened lifetimes and broadened, redshifted energy levels: an organic chromophore (fluorescein) that has been dried rapidly on a fused silica surface. Recently, we showed that regions of fused silica exposed to up to 1 billion high-fluence laser shots at 351 rm nm at subdamage fluences exhibit significant transmission losses at the surface. Here, we find that these laser-exposed regions also exhibit QC-PL. Increases in the density of induced defects on these laser-exposed surfaces, as measured by the local transmission loss, lead to decreases in the observed lifetime and redshifts in the spectrum of the QC-PL, consistent with our explanation for QC-PL. We have found QC-PL in an increasing variety of situations and materials, and we believe it is a phenomenon commonly found on surfaces and nanostructured materials.