Brian D. Fabes

The relationship between indentation and uniaxial creep in amorphous selenium

Ultralow load indentation techniques can be used to obtain time-dependent mechanical properties, termed indentation creep, of materials. However, the comparison of indentation creep data to that obtained during conventional creep testing is difficult, mainly due to the determination of the strain rate experienced by the material during indentation. Using the power-law creep equation and the equation for Newtonian viscosity as a function of stress and strain rate, a relationship between indentation strain rate,{center_dot}{epsilon}{sub {ital l}}={ital @};Dh/{ital h}, and the effective strain rate occurring during the indentation creep process is obtained. Indentation creep measurements on amorphous selenium in the Newtonian viscous flow regime above the glass transition temperature were obtained. The data was then used to determine that the coefficient relating indentation strain rate to the effective strain rate is equal to 0.09, or{center_dot}{epsilon}=0.0{center_dot}{epsilon}{sub {ital l}}.

Porosity and composition effects in sol-gel derived interference filters

The effects of porosity and composition on the optical thickness of sol—gel films is analyzed using the Clausius-Mossotti relationship. The optical thickness is predicted to decrease with shrinkage approximately linearly. For high density films the predictions agree with experimental data from SiO2 and SiO2-TiO2-Al2O3 coatings. At low densities, however, the optical thicknesses are much higher than predicted by the Clausius-Mossotti analysis. This difference is attributed to residual species in the unfired films. Using an empirical value for the ratio of the electronic polarizability αr to the molecular weight Wr of the residual species (αr/Wr= 1.65 × 10−25cm3 mol g−1) the difference between experiment and theory is accounted for quantitatively for both the single-component (SiO2) and multicomponent (SiO2-TiO2-Al2O3) coatings. Approaches to fabricating multicolor dichroic filters, which require large changes in optical thickness on heating, are presented.

Laser processing of channel waveguide structures in sol-gel coatings

Sol-gel derived silica, siica-titania, and tantala coatings were covered with a thin metal film and translated across a Nd:YAG laser beam (1.06 jim). The laser energy was absorbed by the metal film, which heated the underlying sol-gel coating. This heating densified the sol-gel coatings, thereby increasing the index ofrefraction of the laser heated region, and forming channel waveguide structures in all three systems. The channels formed by this technique were etched, to remove the undensified regions, which resulted in ridged waveguide structures. The structures were also produced by depositing a metal pattern using photolithographic techniques, and rastering the laser across the entire sample. The refractive indicies of laser densifled and furnace densified silica coatings were similar. Large differences were observed in the indicies oflaser and furnace densified coatings for the siica-titania and tantala systems.

Boron loss in furnace- and laser-fired, sol-gel derived borosilicate glass films

Borosilicate glass films were made by the sol-gel method from tetraethoxysilane and trimethylborate precursors. The precursor or glass composition at each stage of processing was analyzed to determine the sources of boron loss. The films were heated in a furnace and with a laser to compare boron volatilization by the two heating methods. The films were characterized by infrared spectroscopy, ellipsometry, induction-charged plasma spectroscopy, and Auger microscopy. The highest losses of boron occurred during coating and low temperature ({lt}500{degree}C) furnace firing. Films with the highest boron concentrations were made by dip coating and rapid firing, either with a laser or by placing them into a hot furnace. Infrared spectroscopy revealed Si{endash}O{endash}B bonds, indicating incorporation of boron into the borosilicate glass structure for laser- and furnace-fired films. {copyright}{ital 1996 Materials Research Society.}

Waveguide formation by laser irradiation of sol-gel coatings

Sol-Gel derived Si02-Ti02 films were densified by scanning a C02 laser beam across the surface. The effects of laser power and scanning rate on the morphology and optical properties of the coatings were studied. At low laser powers a depression, with rough sidewalls and approximately gaussian profile was formed in the coatings. As the laser power was increased the channels became wider and smoother, and flattened out on the bottom. At high laser powers a pair of ridges were formed in the bottom of the channel, apparently due to melting. The refractive index of the laser densified films was higher than for films fired in a furnace. Most laser densified areas supported waveguide modes but exhibited high loss. By processing at powers just below the damage threshold loss was reduced to a measurable value, 12.4 dB/cm for the TM0 mode for one sample. Annealing the films after laser processing also reduced the loss.

Laser densification of optical films

Sol-gel derived SiO2-TiO2 and WO3 films were densified with a CO2 laser. Laser fired SiO2-TiO2 films had higher optical loss (6 dB/cm) than similar films fired in a furnace (< 1 dB/cm). For coating shrinkages up to about 40% the increase in refractive index during firing was similar for both laser and furnace fired SiO2-TiO2 coatings. At higher shrinkages both anatase and rutile were observed in the laser fired coatings. Loss measurements of laser fired samples in this and, likely, in previous studies were made in crystalline coatings. It is proposed that this, at least in part, is the cause of the high optical losses typically measured in laser densified silica-titania waveguides. The electrochromic coloring behavior of the WO3 films was examined using proton insertion. The coloring characteristics (< 20 s for 90% coloring) and bleaching characteristics (< 3 s for 90% bleaching) of laser fired WO3 films were comparable to the coloring and bleaching characteristics of high-quality sol-gel derived films fired in a furnace. Optimal coloring was obtained using moderate laser energy densities (approximately 50 - 100 J/cm2). This optimum occurred with less shrinkage than for furnace fired electrochromic films with similar coloring behavior.

Laser processing of chemically derived dichroic filters

Optical interference filters were fabricated using multilayer stacks derived from sol-gel SiO 2 and SiO 2 -TiO 2 coatings. Laser processing was then used to modify the spectral properties (color) of local regions of these stacks. Changes in color were analyzed with respect to changes in the individual film thicknesses and refractive indices. Design considerations and some basic limitations of laser firing for tuning interference filter colors are also discussed.

Laser processing of sol-gel-derived multilayer interference filters

Optical interference filters were fabricated using multilayers derived from sol-gel SiO2 and SiO2-TiO2 thin films. Laser processing was then used to modify the spectral properties of local regions of these stacks. The feasibility of using laser processing for selectively changing the optical properties of thin film devices was thus demonstrated. Design considerations and some basic limitations of this technique for tuning interference filter colors are then discussed.

Temperature range of electrochromic effects in sol-gel WO 3 films under laser irradiation

Electrochromic WO3 films were begun to find a wide application for new devices of integrated optics such as displays, adaptive mirrors, elements with controlled transparency, etc. Laser technology would be a perspective method for patterning of those films and their local structure modification, so the laser radiation action on WO3 films begins to attract the attention of various investigators. In the paper some properties of these films and regimes of laser firing are stated. The alterations in coloring effect in the case of furnace and laser firing are founded. For the purpose of laser action optimization it is necessary to measure an actual temperature kinetics in the laser treatment zone and to investigate the structural alterations in WO3 films in such cases. It is a sophisticated task because the films thickness was about 0.2 - 0.5 micrometers .

Laser Processing of Multilayer Interference Filters

The use of lasers to modify the optical properties of multilayer interference filters is examined. The effects of laser firing on the optical properties of sol-gel deposited and electron-beam evaporated filters are compared experimentally. Filters made from sol-gel films are found to have tunability of at least 15%, while electron-beam evaporated filters are tunable by laser firing to less than 1%. This difference is explained by applying the Lorentz-Lorentz equation, which describes the effects of residual organics, water, and porosity in the sol-gel films on their optical properties. Possible routes to tailoring film microstructure and composition to optimize filter tunability are also discussed.