Recent progress in thin film optical coating technology has enabled more complex filter designs and better control of out of band interference. One of the most significant advances in optical filters has been the manufacture of rugate filter designs based on sinusoidal variation of refractive index. The realization of a rugate filter requires a means of depositing an optical material whose refractive index can be significantly varied over a wide range, while having precise control of the index. The Glancing Angle Deposition (GLAD) technique satisfies these requirements by allowing fabrication of films with nano-engineered morphology whose optical properties can be tailored. GLAD is based on thin film physical vapor deposition by evaporation and employs oblique angle flux and substrate motion to allow nanometer scale control of structure and optical properties. Silicon rugate filter prototypes were made according to design specifications using computer control of deposition parameters which influence the film optical response.
A modern challenge of materials science and physics is the creation and understanding of photonic crystals. Glancing Angle Deposition (GLAD) enables the growth of thin film materials with designable morphological structure on the scale of tens of nanometers, similar to proposed geometries of photonic crystals. Here we present recent progress toward the realization of photonic crystals with GLAD. Square spiral films of silicon were fabricated with GLAD, and were analyzed with scanning electron microscopy (SEM) and spectroscopic ellipsometry. The SEM images clearly show a periodic and spiral structure, similar to that recently predicted to have a robust three-dimensional bandgap. Ellipsometric analysis is ongoing, with as yet no distinct features that might suggest photonic bandgaps. To measure and control the in-plane ordering of silicon thin films, we have deposited and characterized pillar microstructures, producing an indirect measurement of film porosity with varying flux incidence. Two dimensional Fourier transforms were applied to plan view SEM images of porous pillar microstructures, showing no regular lattice but a broad ring that suggests a short range average spacing. In-plane periodicities were observed up to 100nm. Ongoing research is toward fabricating and analyzing photonic crystal structures at visible and infrared wavelengths.