William J. Gunning

Antireflection surfaces in silicon using binary optics technology

Binary optics processing methods were applied to a silicon substrate to generate an array of small pillars in order to enhance transmission. The volume fraction of the silicon in the pillars was chosen to simulate a single homogeneous antireflection layer, and the pillar height was targeted to be a quarter-wave thickness. A mask was generated, using a graphics computer-aided design system; reactive-ion etching was used to generate the pillars. An improvement in long-wavelength infrared transmission is observed, with diffraction and scattering dominating at shorter wavelengths.

Codeposition of continuous composition rugate filters.

Rugate filters are optical thin film interference structures with sinusoidal refractive index profiles. Two-wavelength reflection filters have been fabricated by codeposition of SiO(2) and TiO(2). Composition modulation was monitored and controlled using quartz crystal rate controllers. The resulting filters exhibited two well-defined stopbands. Microscopic examination revealed that the structure is glasslike without pronounced thin film microstructure.

High‐quality optical and epitaxial Ge films formed by laser evaporation

High‐quality thin films of Ge were deposited by pulsed laser evaporation of molten Ge. Films deposited on 300 °C substrates showed very smooth morphologies and single‐crystal grain structures. Energetic ions in the vapor stream, generated by the laser‐induced plasma, were observed to affect nucleation and bulk‐film growth. Films deposited on ambient temperature substrates by laser evaporation were denser, harder, and exhibited higher values of refractive index and lower levels of intrinsic stress than the films deposited by thermal evaporation.

Deposition of optical thin films by pulsed laser assisted evaporation.

Thin films of several refractory metal oxides and Ge were deposited by pulsed laser evaporation using a TEA CO(2) laser. Films deposited on ambient temperature substrates had a polycrystalline microstructure. Ge films deposited on 300 degrees C substrates were single crystalline. The refractive indices of these films were higher than indices of films deposited by conventional evaporation techniques and were bulk values for HfO(2) and ZrO(2). The crystalline microstructure and high packing density of the films were attributed to the effect of energetic ions in the laser-induced plasma.

Microstructure and composition of composite SiO2/TiO2 thin films

The microstructure and composition of the SiO2/TiO2 analog (codeposited) and digital (thin layer pairs) gradient‐index films were examined by transmission electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, x‐ray photoelectron spectroscopy, scatterometry, and in situ ellipsometry. Both analog and digital structures were amorphous as‐grown. The SiO2 was incompletely oxidized at some of the interfaces in the layered structures and in the codeposited films, indicated competition from the TiO2 for available oxygen during growth. Digital structures with thin (65 A) layers remained well defined after annealing at 900 °C, but their order was completely destroyed by 1100 °C. Structures with thick (500 A) layers remained intact up to 1100 °C, with the TiO2 crystallizing throughout the layer width. Both anatase and rutile TiO2 crystallites were present in the layered and codeposited films after a high‐temperature anneal (T≳650 °C), while only the anatase phase was observed for ...

Miniaturized micro-optical scanners

Optical beam scanners are critical components for airborne and space-based laser radar, on-machine-inspection systems, factory automation systems, and optical communication systems. We describe here a laser beam steering system based on dithering two complementary (positive and negative) microlens arrays. When the two microlens arrays are translated relative to one another in the plane parallel to their surfaces, the transmitted light beam is scanned in two directions. We have demonstrated scanning speeds up to 300 Hz with a pair of 6-mm-aperture microlens arrays designed for input from a HeNe laser. The output beam covers a discrete 16 x 16 spot scan pattern with about 3.6 mrad separation and only 400 μrad of beam divergence, in close agreement with design predictions. This demo system is relatively compact; less than 2 in. on a side. We also describe several near-term applications, some critical design trade-offs, and important fabrication and design issues.

Intrinsic stress and structural properties of mixed composition thin films

Intrinsic stress and microstructure of mixed composition films were investigated for several binary systems of IR optical materials. These properties were measured for the entire range of compositions and for mixing obtained by codeposition and by layering of alternate pure components. The variation of stress with composition was observed to be significantly different, depending on the method of mixing. Microstructural analysis revealed a corresponding difference in the grain structure of the films. Low compressive stress films were obtained by coevaporation of high tensile stress materials. These can be used to produce thick and mechanically stable gradient-index optical coatings.

Double-cavity electrooptic Fabry-Perot tunable filter.

The experimental development of an infrared multiple-cavity electrooptically tuned Fabry-Perot filter using LiNbO(3) is described. This filter, designed for operation in the 3-5-microm spectral band, was demonstrated by angle scanning using a He-Ne laser at 3.39 microm. The multiple-cavity configuration increased the free-spectral range of the 2-A bandwidth filter from 67 to 670 A as observed in the increased angular spacing of transmission peaks. Because of the large halfwave voltage of LiNbO(3) at these wavelengths, tuning was restricted to approximately 15% of the free-spectral range.

Fabrication of refractive microlens arrays

Fabrication issues of microlens arrays, made by first forming photoresist microlenses, by patterning and reflowing photoresist islands under temperature, and then transferring this into the substrate by a dry etch process, were studied. Photoresist microlenses were reliably fabricated within a range of aspect ratios. The desired sag of the microlenses in the substrate was controllably achieved by adjusting the etch selectivity. Etching behavior of fused silica in mixtures of fluoroform with oxygen or sulfur hexafluoride was studied in detail. High quality microlens arrays were fabricated in fused silica, silicon and germanium, and selected lenses were characterized.

Electro-Optically Tuned Spectral Filters: A Review

Electro-optically tunable spectral filters have been developed over the years and now include a rather diverse collection of devices. These devices, which are of increased importance for remote sensing applications, employ linear electro-optic (Pockets) materials such that tuning is accomplished via externally controlled voltages. A brief review of the properties of linear electro-optic materials is presented followed by descriptions of tunable birefringence interference filters, Fabry-Perot devices, and other novel filter structures. Recent developments in electro-optic tunable filters are included. Issues relating to the further development of infrared tunable spectral filters are discussed.