Ordered photonic microstructures

Abstract

This thesis examines novel photonic materials systems possessing order in the atomic, microscopic, and macroscopic dimensional regimes. In the atomic order regime, a structure-property investigation is done for Er203 in which the first report of room temperature photoluminescence (PL) is provided. Thin films of the rare earth oxide were deposited via reactive sputtering of Er metal in an Ar/02 ambient, and subsequently annealed to promote grain growth. Heat treatment consisting of a 650°C followed by 1000°C anneal produces maximum crystallinity as measured by glancing angle x-ray diffraction. These films show characteristic PL at [lambda]=1.54 [mu]m. In the microscopic order regime, omnidirectional reflectors and thin film microcavities are demonstrated using sol-gel and solid-state materials. A first demonstration of omnidirectional reflectivity in sol-gel structures was accomplished using a dielectric stack consisting of 12 spin-on Si02/Ti02 quarterwave sol-gel films. Similarly, solid-state dielectric stacks consisting of 6 Si/Si02 sputtered films were used to demonstrate the same principle. Microcavities were formed using sol-gel structures, producing a low quality factor Q=35 due to limitations in film thickness control and lossy interfaces from stress-induced cracks. The high index contrast Si/Si02 microcavities enabled Q ~1000 using 17 total layers following hydrogenation of dangling bonds within the amorphous Si films. Combining fabrication processes for the solid-state microcavity and Er20 3 films, a device was fabricated to demonstrate photoluminescence enhancement of an Er20 3 film embedded in a microcavity. The structure consisted of 3-bilayer mirrors on either side of an Si02/Er203/Si02 cavity. The Q~300 was near the theoretical value for such a structure. At room temperature, PL of Er20 3 was enhanced by a factor of 1000 in the microcavity compared to a single thin film. In the macroscopic order regime, self-assembly of micron-sized Si02 and polystyrene latex colloidal particles into 2D crystals is presented. The colloidal assemblies offer a relatively easy processing route for fabrication of photonic bandgap structures. Large (> 1 mm diameter) single crystal grains of colloids were formed using controlled evaporation and fluid flow techniques. A novel solution enabling postprocessing of the fragile ordered assemblies is presented in which polyelectrolyte multilayers serve as adsorption platforms that anchor the colloidal assemblies. Tailorability of the polyelectrolyte surface properties (charge density, morphology) enables tuning of the colloid adsorption behavior. The polyelectrolyte surface affects colloid adsorption by influencing its surface diffusion. Observations of colloid surface diffusion were made using optical microscopy. Use of polyelectrolytes patterned via rnicrocontact printing enables fabrication of colloid assemblies containing predesigned point and line defects. The patterned polyelectrolyte adsorption template allows placement of colloids in specific geometric arrangement, making possible the realization of sensors or functional photonic bandgap devices such as waveguides or photon traps. Three mechanisms were used to control· adsorption: (1) pH of the colloid suspension, which determines the ionization of the uppermost surface of the polyelectrolyte multilayer; (2) ionic strength of the suspension, which determines the extent of charge screening about the colloid and polyelectrolyte; and (3) concentration of added surfactant, which causes charge screening and introduces hydrophobic interactions between the surfactant and polyelectrolyte.