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.
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.
The orientational ordering of liquid crystal (LC) materials directly determines their optical properties. Controlling the orientational order allows the optical properties to be engineered for display and switching applications. Recent advances in LC ordering with LCs embedded in porous networks have resulted in materials with exciting new properties, enabling new display and switching technologies. A new technique called Glancing Angle Deposition (GLAD), based on conventional thin film fabrication, allows engineering of porous structures of inorganic materials in three dimensions on a nanometer scale. By impregnating the void spaces in these porous inorganics with various polymers and LCs, we have created a new type of hybrid material where the orientational order of the impregnate is controlled by the inorganic backbone structure. Optical measurements of GLAD materials with various impregnates demonstrate that simple rodlike liquid crystalline materials (nematics) are oriented by a helical inorganic backbone to form a phase similar to the chiral nematic phase seen in other (cholesteric) liquid crystals. This new hybrid material appears promising for optical switching and display applications.
Unique thin film microstructures have been fabricated with the Glancing Angle Deposition (GLAD) technique. These porous, thin films can be engineered with a variety of different morphologies to sub-micron dimensions, including helical, post, and chevron or zigzag microstructures. This paper reports some recent results in study and application of films deposited using GLAD, namely: the use of low pressure, long throw sputtering to produce porous titanium films; deposition of porous, structured ZrO2 films for use as thermal barriers; and measurement of the mechanical response of chiral or `microspring' thin films.
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