Synthesis and Fluorescence of Opal & Air-Sphere Photonic Crystals

In this thesis the preparation and characterization of opal-based photonic crystals are described with the aim to control the properties of internal light sources

Emission Spectra and Lifetimes of R6G Dye on Silica-Coated Titania Powder

Many molecular species adsorbed on semiconductor surfaces show photochemical reactivity. These photosensitization processes attract a lot of attention from a\ud practical point of view for uses such as solar energy conversion.1-3 The organic molecule is excited by light and injects an electron into the conduction band of the semiconductor. This allows conversion from visible light\ud to electric current. The inverse process, that is, electrons at the semiconductor surface are scavenged by the ubiquitously present molecular oxygen, results in the degradation of the dye.4 Since the electron transfer rate\ud ismuchfaster than the radiative lifetime, the fluorescence efficiency for emission is strongly reduced to a few percent. If a high fluorescence efficiency close to a semiconductor surface is required, the dye or the host solid must be shielded, for instance, by coating the semiconductor, such as titania (TiO2) in the form of photonic crystals, with an insulating layer

An experimental study of strongly modified emission in inverse opal photonic crystals

We present the first experiments that demonstrate strong angle-independent modification of spontaneous emission spectra from laser dyes in photonic crystals, made of inverse opals in titania. We show that both the fluorescence quantum efficiency and weak disorder play a key role in interpreting the experimental data. We compare the angle-independent emission spectra of dye in photonic crystals with spectra from such crystals with much smaller lattice spacings, for which emission is in the long wavelength limit. The ratio of emission power spectra shows inhibition of emission up to a factor 5 over a large bandwidth of 13 % of the first order Bragg resonance frequency. The inhibition shifts to increasing wavelength with the lattice parameter, confirming the photonic nature of the phenomenon. The center frequency and bandwidth of the inhibition agree with the calculated total density of states, but the measured inhibition of the vacuum fluctuations is much larger. This result is confirmed by experiments using different dyes. We likely probe the strongly modulated local photonic density of states, due to the spatially nonuniform distribution of dye molecules over the unit cell.