The sol-gel synthesis of materials consists of preparation of a solution homogeneous at the molecular level and solution-to-gel conversion of the solution near room temperature. This assures low temperature processing of optical materials of new compositions and various microstructures in a variety of shapes. Due to these advantages, the sol-gel method is suited for the creation and development of high technology optical solids. This paper reviews the sol-gel processing of optical materials and describes the optical solids prepared so far.

Sol-gel processing has been used to make optically transparent gel-silica matrices that have interconnected porosity in the 12 to 14 A scale. Optically active organics have been impregnated into the porous matrices resulting in optical composites with unique properties. However, for certain organic phases, such as required for solid state dye lasers or in-situ photo-polymerization, matrices with a large pore diameter and larger volume fraction of porosity are required. A new processing method that controls aging of gel-silica monoliths in an alkaline NH4OH environment followed by drying and stabilization from 600-1150°C results in optically transparent gel-silica matrices. The surface areas of the matrices range from 770 to 200 m2/g with total pore volumes from 0.5to 1.15 cm3/g, and mean pore radii from 20 A to 135 A. Very narrow pore size distributions are achieved and the pore configuration can be changed from ink bottle to a cylindrical cross section. Optical applications of matrices with these new textures are described, including performance of a solid state dye laser made by impregnating large pore gel-silica matrices (90 A average radius pores and 1.0 cm3/g pore volume) with 4 PyPO-MePTS lasing dye.

Microwave processing has achieved widespread interest in recent years as a rapid and efficient means of processing glass and ceramic materials. Unfortunately, not all materials are equally well-suited for microwave sintering due to wide variations in microwave susceptibilities for different materials. Silica and high silicates, for example, are relatively poor microwave susceptors. Employing a recently disclosed process, called secondary microwave heating, porous silica and doped silica gels have been successfully sintered into dense glasses. This method of microwave sintering relies upon a highly efficient microwave susceptor, known as a microwave suscepting ceramic composite (MSCC), to transfer energy to the sample being processed. Typical sintering times to achieve fully dense glasses from sol-gel are approximately 10 minutes. In this paper, the optical properties of conventionally sintered and microwave sintered silica and doped silica glasses will be compared. Dopants include chromium, copper, neodymium, erbium, and europium. Key properties to be examined include light scattering, UV-vis-nIR spectrophotometry, and fluorescence. In addition, the potential application of this process to small, sol-gel derived specialty shapes, such as lens, etc, will be discussed.

We describe a new coating method "Laminar Flow Coating"* (LFC) technique developed to obtain highly reflective (HR) laser damage resistant sol-gel multidielectric coatings. Such coatings are used in highpower lasers for inertial confinement fusion experiments (ICF). This technique uses substrates in an upsidedown position and a travelling wave of coating solution is laminary transported under the substrate surface with a tubular dispense unit. This creates a thin-film coating by the solvent evaporation. Satisfactory results have been obtained onto 20-cm square glass substrates regarding the optical performances, the tickness uniformity, the edge-effects and the laser damage resistance. This deposition technique combines the advantages of both classical techniques : the substrate non-exclusive geometry such as in dip-coating and the small solution consumption such as in spin-coating. The association of sol-gel colloidal suspensions and LFC coating process has been demonstrated as a promising way to produce cheap specific optical coatings1.

Eu3+ and U022+ have been used as optical probes iii sol-gel silica. Optical fluorescence and decay times were monitored as a function of processing parameters such as pH and densification temperature, and the optical data were correlated with structural changes in the materials. Fluorescence and decay times of Eu3+ ljO,2+ co-doped materials were studied and energy transfer effects were interpreted in terms of the pore morphology.

Ti02 sol—gel coatings, about 100 nm in thickness, were prepared by hydrolysis of Ti—alkoxides in alcoholic solutions and dip—coating. X—ray photoelectron spectra (XPS), thin film X—ray diffraction, dynamic microhardness (DH) and refractive indices of the coatings on silica glass substrates were measured after drying and heat treating at temperatures in the range from 100 to 900°C. The crystallization temperature of thin coatings was more than 100°C higher than that of the bulk gels. Ols spectra of XPS and the depth profile of the microhardness of the coatings show the structural changes in the depth direction in the coatings. The surface layer with high microhardness is considered to have an Anatase—like structure or to crystallize rapidly. The inner layer with relatively low microhardness contain a certain amount of water after heat treating at 500°C. Ols spectra of the inner layer near the surface of the substrate were different from those of the other parts of the coatings. Significant increase in the microhardness of the glass substrates was observed near the interface between the coating and the substrate was observed. This result suggests that stress in the surface layer of the substrate is induced by the coating. This result also shows good adhesion of the coatings on glass substrates. The structural changes of the coatings during drying and heat treating are also discussed.

For applications such as integrated optical devices, it is desirable to fabricate thick optical films, e.g. by the sol-gel process. while avoiding some of the problems associated with multilayer deposits. In the present work, alkoxide-derived 5i02 films were deposited by spin-coating onto single crystal silicon wafers, while varying independently several experimental parameters, namely the water/tetraethoxysilane (TEOS) molar ratio R, the volume percentage of TEOS, the aging period of the solutions and the rotational speed of the substrate. It was concluded that the thickness of the as-deposited porous silica gel films increased with decreasing R, as well as with increasing TEOS volume percentage or aging period of the solution (up to a certain point), whereas it showed a maximum when plotted as a funtion of the substrate rotational speed. Maximum thicknesses in excess of 370 urn were obtained for porous films, for which densification did not cause cra.cking,while porous undensified films up to 1.2 pm thick were also prepared. The conditions under which maxiimiin film thickness is achieved are discussed.

Due to the difficulties in geometrical arrangement, up to now the polarization arrangement for waveguide Raman spectroscopy (WRS) was limited to only a few simple configurations. In this report, we present the experiments in extending polarization configurations for WRS. The resulted polarized WRS spectra of thorium phosphate xerogel thin films will be presented. In the region of Raman shift from 850 to 1200 cm-1 ,two components with different polarization behavior exist. Based on the symmetry arguments, these components are then identified to the different vibration modes of P043 ions. From the spectra it is also concluded that the film is amorphous and isotropic and has chain-like structure.

Sol-gel processing of tetramethyl orthosilicate with HNO₃ as a catalyst has been used to make optically transparent silica matrices with interconnected porosity of 1.2 to 1.4 nm radii. However, large pores are often needed for impregnation of the matrices with optically active organics. Larger pore volumes are also desirable for many applications of these optical composites. Two methods for producing larger pore matrices are compared: (1) Catalysis with dilute HF, and (2) aging in a basic NH₄OH solution. Pore radii of matrices made by the HF method are 5.0 nm after thermal stabilization at 900 degree(s)C and 4.4 nm after 1000 degree(s)C. Pore volumes are 0.9 cm³/g at 900 degree(s)C and 0.7 cm³/g at 1000 degree(s)C. The ammonia aging process yields 9.0 nm radius pores at 900 degree(s)C and 8.7 nm pores at 1000 degree(s)C. Pore volumes are 1.0 cm³/g at 1000 degree(s)C. Optical properties (including UV cut-off, UV-vis-NIR transmission, IR absorption, index of refraction), bulk and structural densities of the matrices made by both methods (900 degree(s)C and 1000 degree(s)C stabilization) are compared with the 1.4 nm pore radius matrices.

Fine patterning, pregrooved layer coatings, on glass disks by the sol-gel method has been successfully achieved. The pregrooving process, formation of a sol-gel film on a glass substrate, stamping the groove pattern on the gel film and heat treatment, is described. The specifications of the pregrooved glass disk of 130 mm in diameter are satisfactory for the requirements as an optical disk substrate in terms of dimensional and physical characteristics, mechanical characteristics, signals from grooves. It is noted that the sol-gel pregrooved optical disks substrates have such excellent features as small dynamic radial runout, very small tilt and much long life comparing with those of conventional plastic disk substrates. Cost- performance characteristics are discussed for suitable applications of the sol-gel pregrooved optical memory disks.

Fracture during drying has been the key hurdle in fabrication of large monolithic silica glass from alkoxide gels. Although existing literature suggests pore enlargement, aging, chemical additives, supercritical drying and freeze drying as helpful in avoiding fracture during drying, successful accelerated sub-critical drying of large silica monoliths from alkoxide gels has not yet been reported. In the present approach, acid catalyzed sols of TEOS, ethanol and water (pH equals 2) were cast as cylindrical rods in plastic molds of 8.0 and 10.0 cm diameter with volumes of 2000 cc and 3000 cc respectively. The resultant gels were aged for about 7 days and dried in a specially designed chamber under sub-critical conditions of the pore field. We have obtained monolithic dry gels in drying times of 3 - 7 days for sizes of 2000 - 3000 cc. The dry gels have narrow unimodal pore size distributions, with average pore radius of about 20 angstroms as measured by BET. Although capillary stress during drying increases with reduction of pore size, it was found that in this approach it is easier to dry gels of smaller pore size.

The preparation of crackfree and transparent SiO₂ coatings on soda lime glass with thicknesses of about 8 micrometers after densification at 500 degree(s)C is presented. The high thickness can be obtained by using an 80:20 mixture of methyltriethoxysilane and tetraethyl orthosilicate as alkoxide precursors in combination with an aqueous colloidal SiO₂ sol with particle sizes of about 7 nm. This principle of synthesis is also applied to ZrO₂ containing coatings yielding to thicknesses of about 3 micrometers . Refractive index measurements indicate that the coatings are nearly completely densified. At higher temperatures tensile stresses appear within the layers and are transmitted to the substrate, increasing its thermal stability.

A novel quarterwave-thick narrow-bandwidth antireflective coating has been developed for both plastic and vitreous substrates by a sol-gel route. This coating has revealed pronounced scratch- and climatic-resistances under adverse conditions. The single-layer coating consists basically of a composite material made of silica as the discontinuous phase and of a polytetrafluoroethylene-derived (Teflon^TM) organic polymer as the continuous phase. This leads to a fluorine-containing silica-based product so-called Fluosil-coating. The coating is applied by spin or dip from specific solutions at room temperature followed by a mild and short heat treatment. In addition to remarkable abrasion and environmental resistance properties, such coatings have displayed excellent laser-induced damage threshold levels surpassing uncoated substrates. We hope such a product might open new perspectives concerning household articles, architectural optical thin-films, ophthalmic uses and so on.

Transparent ZnO films having more than 1 micrometers in thickness were prepared by one dipping-pyrolysis process. The starting solution, produced by dissolving zinc 2- ethylhexanoate, dehydrated caster oil fatty acid and dimethyl silicone oil as a leveling agent into an organic solvent, was coated on the plate glass and cured at 200 degree(s)C and heated up to 500 degree(s)C. The thick ZnO films covered with SiO₂ films are very useful for UV cut-offs at about 380 nm and for high transparency in the visible region. The ZnO films with the durable films have a possible application to automobile windows and the like. Characterizations are also reported by scanning electron microscopy (SEM), X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), optical spectroscopy and Taber abrasion test.

Step index multimode optical fibers were successfully drawn from germania doped silica rods prepared by sol-gel process. The fiber, drawn using rod-in-tube technique, had a 100 micron core with a pure silica cladding of 140 micron. The numerical aperture of the fiber was 0.21. Initial experimental results indicate an attenuation of 20 dB/km at 850 nm wavelength. Precursors used for sol preparation were tetraethyl orthosilicate, Si(OC₂H₅)₄ and tetraethyl orthogermanate, Ge(OC₂H₅)₄. Clear wet gels were routinely produced without any problem of premature precipitation of germanium dioxide even at high dopant concentration levels. The gels were dried by supercritical drying technique. Dry gels were consolidated to clear glass samples routinely at a temperature of 1300 degree(s)C. Fiber was drawn from these rods at a temperature of 1800 degree(s)C. The sintering parameters, i.e., type of gas flow at different steps of the sintering operation, duration of such steps and temperature were optimized to eliminate reboil of the glass above 1800 degree(s)C, resulting in bubble-free glass fibers.

Recent works in our laboratories investigated the microstructure of titania films prepared from a colloidal solution and used as planar waveguides. The transmission electron microscopy including high resolution observations together with waveguide Raman spectroscopy, especially in the very low frequency range, showed a strong influence of the heat-treatment procedure on the films' morphology. In addition, atomic force microscopy provided valuable information on their surface roughness. At lower temperatures (450 degree(s)C), the films' structure consists of a mixture of amorphous TiO₂ and anatase nanocrystals (4.5 - 5 nm) and its surface is smooth. On the contrary, when increasing the annealing temperature, the nanocrystals grow (50 nm at 1000 degree(s)C), pores (5 - 20 nm) are clearly distinguished and the films' waveguiding properties disappear. Here, we discuss the optical behavior of these films and its relationship to microstructure. It is clearly shown that a high degree of crystallization induces an important porosity, a rough surface and thus dramatically affects the films' waveguiding properties. Finally, we report a relatively simple strategy to stabilize the amorphous phase and control the crystallization in order to improve the optical quality of the waveguides.

Sol-gel technology is providing a viable alternative path towards developing doped optical components via impregnation of Type VI gel silica using a vapor or liquid phase. Past work presented an optical technique for determining quantitative mass transport properties of Cr³⁺ ions within the water filled porous phase of Type VI silica. Ion influx is measured by integrating the strong absorption bands produced by the chromium in the visible region. Diffusion coefficients are determined for an array of pore properties (radius, volume, surface area) as well as solution concentrations. Diffusion coefficients are calculated to be 2.0 X 10^-8 cm²/sec for the most restricted case and approach 1.6 X 10^-6 cm²/sec, the bulk liquid diffusion coefficient, as the ratio of diffusing solute diameter to the pore diameter decreases. Final chromium distributions are determined using electron microprobe x-ray. Higher chromium distributions are found on surfaces of the gels from which solvent is restricted from evaporating. Sample geometries affect the percent change of concentration across the cross-section.

The following timeline summarizes the recent advancements made in the development of gel derived radial GRIN lenses in terms of two characteristic optical properties - the size of index of refraction gradient (An) and the diameter of the GRIN element.

A new technique is presented for controlling the shape and increasing the maximum index change of radial gradient-index profiles formed in sol-gel bodies. This is accomplished by modifying the relative rates of the reactions responsible for creation of the gradient. By altering the manner in which the leaching action is arrested, the index change can be maximized for a given shape to double that of the previous technique. Detailed composition analysis of the data reveals that dopant diffusion and precipitation are the dominant factors influencing the observed results.

Optical microlens arrays are important in many optical systems. A new method for producing arrays of negative lenses, geodesic lenses, and planar GRIN lenses is described. The lenses and arrays are produced using CO₂ laser densification of near fully densified gel-silica (Type VI) matrices. Critical processing conditions include initial density of the matrix, laser power, laser pulse width, focal length of the laser beam, and ambient atmosphere. The gradient refractive index (GRIN) region formed on the surface of the gel-silica matrix is capable of focusing an optical ray in a direction parallel or perpendicular to the surface. FTIR specular reflectance microscopy with a spot size of 0.1 mm and resolution of 0.5 cm^-1 at normal incidence is used to characterize laser densified spots with diameter of 1.50 mm. The wavenumber of the Si-O stretching mode is shifted toward a higher frequency from the periphery to the center of the densified spot. The increase in frequency of the structural molecular vibrations gives rise to the GRIN optics. A refractive index gradient of 0.09 to the microlens is achieved with a 300 micrometers diameter full density region. Focal lengths of the GRIN planar microlens on the gel-silica matrix range from 1.7' to 5.0'. Comparison of the materials, fabrication methods, and properties of different types of optical microlenses are summarized.

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.

PLZT thin film is an attractive materials due to its large electro-optic and nonlinear optical effect. PbTiO₃ and PLZT (10/0/100, 28/0/100) thin films on different substrates have been prepared by sol-gel process. (100) oriented PbTiO₃ film on SrTiO₃ (100) single crystal substrate and (111) oriented PLT films on sapphire (0001) substrate have been obtained. Transmission of the films is more than 80% in visible wavelength range. Some electric and nonlinear optical properties have been studied.

A series of sol-gel derived PT-based films, including PT, PZ, PZT, PLT, PLZ and PLZT, was prepared on platinized Si, fused SiO₂ and Corning 7059 substrates. These films were fired at 400 - 700 C for 30 mins. The phase assembly and development were dependent on the precursor chemistries, processing and choice of substrates. The presence of Zr impacted significantly on the crystallization behavior, PbO loss and cracking behavior of the films. Crystallization was severely retarded, especially in Zr-containing PZT films when deposited on amorphous substrates compared to crystalline Pt substrates. Amorphous and crystalline PZT films can be utilized for passive and active optical applications. Waveguiding was achieved in an amorphous PZT 53/47 and a crystalline PLT 28 films and gave attenuation losses of 1.0 and 1.4 dB/cm respectively which represent the lowest values reported to date. The optical properties of the films were investigated using ellipsometry, UV-VIS transmission spectroscopy and waveguide loss measurements. Depending on composition and processing conditions, PZT films (2500 A thick) with refractive indices of 1.60 to 2.33 and absorption edges of 2900 - 3100 A can be obtained. It was ascertained that the resulting interfacial reaction layers between the films and substrates affected considerably the optical properties of thinner films (< 2000 A).

Perovskite Pb(Fe_1/2Nb_1/2)O₃ (PFN) coating films have been prepared on Si(100) and MgO(100) single crystal, and silica glass substrates by the sol-gel method using Nb(OC₂H₅)₅, Fe(NO₃)₃(DOT)9H₂O, and Pb(OC₃H₇ⁱ)₂ and Pb(CH₃COO)₂(DOT)3H₂O as starting materials. Five solutions with different preparation routes were used for dip-coating. Starting materials, mixing sequence of starting materials and substrate materials influenced the formation of PFN perovskite in the resultant coating films to a great extent. The use of Pb(CH₃COO)₂(DOT)3H₂O as lead sources and MgO(100) single crystal substrate lead to the direct formation of PFN coating films of single-phase perovskite. The results obtained are discussed in terms of macromolecules formed in coating solutions.

Amorphous thin films of ferroelectric LiNbO₃ and Pb(Zr_0.52Ti_0.48)O₃ were prepared on a variety of substrates. Their properties were characterized in comparison with typical ferroelectric crystal behavior. It was found that these amorphous thin films show ferroelectric-like behaviors well before being crystallized. These findings suggest possible practical applications in areas where ferroelectric crystals are routinely in use. A structural model is proposed to account for the ferroelectric-like behaviors in the amorphous phases.

Nonlinear optical materials have been synthesized via the sol-gel route. A chemical control of the reactivity of molecular precursors toward hydrolysis and condensation allows the deposition of transparent thin or thick films. Potassium titanyl phosphate (KTP) can be better synthesized from phosphoester precursors [PO(OH)_3-x(OR)_x] rather than phosphoric acid [PO(OH)₃] or phosphate esters [PO(OR)₃]. These precursors lead to crystalline transparent KTP thin films around 600 degree(s)C. Organic molecules for nonlinear optics can be embedded into an oxide sol-gel matrix. Second harmonic generation has been obtained after poling with thick films in which organic chromophores are chemically bonded to the oxide backbone via the use of functionalized silicon alkoxide precursors.

(beta) -BaB₂O₄((beta) -BBO) is known as one of the best inorganic nonlinear crystals. It is now widely used for frequency doubling, frequency mixing in the UV range and optical parametric oscillation in the UV and near infrared regions. To put this material into thin film form would make it very attractive for integrated optical devices. Vacuum deposition techniques are not suitable for the fabrication of borates thin films due to the difficult evaporation or sputtering of oxides based on light elements. The low processing temperature of the sol-gel method allows the formation of stoichiometric and expected oxygen coordination of BaB₂O₄ in liquid phase. (beta) -BBO thin films has been successfully prepared by the sol-gel method through hydrolysis of barium and boron alkoxides. The accomplishment of hydrolysis is found to be essential for the removal of residual organic at high temperatures. The choice of precursors, hydrolysis ratio, and thermal treatments have critical influences on the formation, the morphology and the nucleation of (beta) -BBO phase of the film. The undesirable interaction between the film and the substrate limits the formation of (beta) -BBO polycrystalline film to only a few selected substrates.

Sol-gel routes were developed to Ba₂NaNb₅O₁₅, BaTiO₃, Ba_0.8Pb_0.2TiO₃, Ba_0.8Sr_0.2TiO₃, Bi₄Ti₃O₁₂, LiNbO₃, LiTaO₃, Pb₅Ge₃O₁₁, PbTiO₃, Pb_0.85La_0.15Ti_0.96O₃, PbZr_0.2Ti_0.8O₃, WO₃ and ZnO thin films. Second harmonic generation (SHG) from these films deposited on fused SiO₂ was investigated as a function of processing temperature. The SHG values were found to be related to the second-order nonlinear optical susceptibilities of the bulk materials, and dependent on the crystal size and crystalline orientation of the film. The highest SHG observed was from PbTiO₃ films which is consistent with the fact that the d-coefficient of PbTiO₃ is the highest among that of any room temperature ferroelectric. The poor SHG activity of BaTiO₃-based films was due to the very small grain size of the sol-gel derived material.

The synthesis temperature due to bismuth and bismuth, aluminum-substituted yttrium iron garnet (Bi:YIG, Ya_xBi_xFe₅O₁₂, Bi, Al:YIG, Y_3-xBi_xFe_5-yAl_yO₁₂) films prepared by sol-gel process were investigated. The synthesis temperature of garnets tends to decrease with increasing Bi content. The maximum Bi substitution x and Al substitution y are about 2.0 and 0.8. The Faraday rotation angle ((theta) _F) of the films prepared by dip-coating were found to be very large values 50 deg/micrometers for YBi₂Fe₅O₁₂ and 57 deg/micrometers for YBi₂Fe_4.6Al_0.4O₁₂.

Electrochromic devices have potential applications in architectural and automotive fields to regulate the transmission and reflection of the radiant energy. Preliminary characteristics of an all sol-gel window with the configuration glass\ITO\WO₃\TiO₂\TiO₂-CeO₂\ITO\glass is presented, and compared with another window where WO₃ was deposited by evaporation. We also present preliminary data of a sol-gel Nb₂O₅ electrochromic layer as well as a thorough evaluation of the properties of a sol-gel TiO₂-CeO₂ ion storage coating where electrochemical properties are studied as a function of various parameters (thickness, heat treatment, etc.).

Solid state electrochromic (EC) devices are composed of multiple thin film layers with each coating having specific optical and electrical properties. Considering the applications of EC devices and the nature of the coatings employed, sol-gel offers several advantages in processing of these devices. These merits and the sol-gel activities in this area are discussed.

Amorphous and crystalline sol-gel derived tungsten oxide thin films were prepared. Their microstructure, electrochemical, and electrochromic properties were analyzed. Both crystalline and amorphous samples that exhibited fast kinetics and large optical modulation, and also those which had poor electrochromic properties could be made.

Photochromism is defined as a reversible light induced color change of a material. By encapsulating photochromic molecules in glasses prepared by the sol-gel technique, transparent photochromic glass has been made. The new optical materials have potential applications in the areas of information recording and optical switching. Both the reversible color changes and the corresponding rates of these changes depend in part on the environment of the molecule in the glass matrix. In this paper, the results of studies involving the incorporation of the photochromic molecule, 2,3-diphenylindenone oxide in a variety of sol- gel glasses (aluminosilicate, silicate, ORMOSIL) are reported. The unirradiated gel and glass are colorless. Upon exposure to light of wavelengths less than 400 nm, the material turns red. When the irradiation ceases, the material returns to its original colorless form. The photochromic and spectroscopic properties of these glasses are presented and the rates of the color transformations are discussed.

The rigidochromic molecule rhenium(I)chlorotricarbonyl-2,2'-bipyridine was used in fluorescence depolarization experiments to probe the gelation, aging, and drying of silica and aluminosilicate sol-gel derived materials. These studies indicate that the local environment of the probe is fluid until well after gelation has occurred. Aluminosilicate gels show an increase in local viscosity after gelation while silica gels show no increase until the drying stage is begun. These results are compared to previous studies in which the shift of the emission band was used to indicate the rigidity in the local environment of the probe.

A sol-gel derived photochromic glass coating of about 1.5 micrometers in thickness is synthesized by infiltration of Ag⁺ into a predried Na-Al-B-Si gel layer. The formation of small Ag colloids is initiated by a soft heat treatment and the colloids are converted by a HCl vapor treatment into AgCl crystallites of about 40 nm in diameter. The coating darkens by UV irradiation, forming Ag crystallites of about 5 nm in size. This process is completely reversible at 400 degree(s)C and no decay is observed in numerous cycles. An amplitude hologram which acts as a diffraction grating with a grating period of about 10 micrometers can be produced within this layer by a two-wave mixing experiment with laser light of 351 nm wavelength. The diffraction efficiency of this grating is determined to 0.11%. After thermal erasing the diffraction pattern can be rewritten and erased repeatedly without any loss of efficiency.

Small particle processing opens a variety of interesting aspects for optical applications. For the utilization, the sol-gel process offers a variety of routes, like colloid synthesis, incorporation into sol-gel derived matrices and in combination with photopolymerizable components patterning processes based on photolithography. In this paper, a summary over a new conception of colloid stabilization and colloid processing by multifunctional ligands and further processing routes for optical applications is given.

A new class of promising composite materials consisting of sol-gel processed inorganic oxides and organic polymers has been developed over the last several years. These materials have been shown to be homogeneous, mechanically stable and have excellent optical properties. Second-order nonlinear optical properties are dependent upon the active chromophore being aligned within the structure. We report here the studies on the second-order nonlinear optical properties of a new oxide:polymer composite. A second-order chromophore, (N,N- diethylamino-((Beta) )-nitrostyrene or DEANST) has been doped into a sol-gel/polymer composite comprised of polyvinylpyrrolidone (PVP) and silica. The removal of solvent from a simple sol-gel preparation causes a tremendous shrinkage of the pores of the matrix. In addition, this particular polymer is capable of being crosslinked at elevated temperatures. These two effects were used to retain the alignment of the chromophores within this unique host material. The second-harmonic generation technique was used as a function of time to ascertain the utility of this approach and to determine the precise roles of processing conditions and components necessary to bring about this effect. The results of this work indicate that this composite can preserve the alignment of DEANST to satisfactory levels, making it an excellent candidate for device applications.

Organically modified silicates (Ormosils) have been used as the matrix material in the preparation of several different kinds of nanocomposites for optical applications via the sol-gel method. Polydimethylsiloxane (PDMS) and dimethyl-phenylmethylsiloxane copolymer have been used as the organic components to modify SiO₂ network in the Ormosils. A broad range of optically active materials such as semiconductors, metals, oxides, organics, and ions have been incorporated into Ormosils. The preparation process for the photo-active material doped Ormosils is described. The dependency of Ormosil properties on processing parameters and the effects of organic modifications on the silica network are investigated. Some examples of using Ormosils derived from tetraethoxysilane and PDMS as matrices to prepare photo- active composite materials are reported. Ormosils proved to be a promising new class of materials for various optical applications.

Laser dyes were trapped in SiO₂ xerogel host matrices to obtain a solid state dye laser. The evolution of the mechanical properties of two kinds of matrices, 'classic' and 'sono' gels, was followed during drying. A new impregnation process was performed on these xerogels: impregnation with a 'sono' sol. The influence of this treatment on certain physical and mechanical properties of the resulting impregnated gels was studied. The results indicate that impregnation substantially improves hardness, elastic modulus and fracture stress. The samples can then be easily polished to obtain optical quality surfaces and be used in a laser cavity. Moreover, optical properties related to laser emission of these materials such as efficiency, lifetime and longevity are better when the laser dye doped xerogels are impregnated. The organic dye molecule used was sulforhodamine 640, and results were obtained six months after their synthesis, with a pump beam working at a 5 Hz repetition rate with 450 (mu) J/pulse energy level. With the first pump shot on a fixed point of the samples, tunability from 600 to 650 nm, 60 (mu) J threshold, 2600 pump shots lifetime and a 10.5% slope efficiency were achieved using an impregnated 'sono' gel matrix.

Low loss sol-gel derived polyceram optical waveguides have been prepared for the first time. Polyceram films were obtained by reacting (N-triethoxysilyl propyl) o-polyethylene oxide urethane with silicon and titanium alkoxides. The optical properties of the films were investigated using ellipsometry, UV-VIS transmission spectroscopy and waveguide loss measurements. Refractive index and attenuation loss measurements were carried out as a function of organic/inorganic content, different processing conditions and aging of solutions. Refractive indices as high as 1.685 and attenuation losses as low as 1.4 dB/cm were obtained. In addition, surface morphology, mechanical properties and thermal stability of the polyceram films were studied.

A composite materials based on organically modified Si alkoxides and modified alkoxides from Zr were prepared. The index of refraction of the system was tailored by the variation of the modified Zr component. Channel waveguides were prepared by a laserwriting technique based on the ability to photopolymerize this material. To decrease optical loss caused by effects of the waveguide surface and to be independent of the used substrate, a buffer layer and a protective layer of the same material of lower index of refraction was combined with a waveguiding film and optical losses were determined.

Organically-doped porous sol-gel matrices of optical grade have evolved in recent years into a wide class of materials with diverse applications. We review recent progress made in our laboratories in three domains of applications: the trapping of enzymes with the consequent design of (e.g. glucose) sensors; the development of chemical sensors; and the design of photoactive material for (solar) light energy conversion.

The sol-gel process has been successfully used for preparing amorphous and transparent oxide materials doped with organic molecules. This paper reports on the possibility of using sol-gel materials as host matrices for organic dyes in order to perform optical pH sensors. Different materials have been investigated so as to study the influence of some characteristics (porosity, hydrophilicity, hydrophobicity...) on the dye optical behavior. Results will be presented for microporous silica and heteropolysiloxanes which are organic/inorganic composites materials obtained from ormosil precursors known to provide versatile systems. Preliminary experiments have been carried out with sol-gel thin films doped with fluorescein as pH indicator and coated onto flat glass substrates. Dynamic range and response time curves are reported for the different sensors. Problems of signal losses (leachability, dye degradation...), sensor lifetime and dye stability within the different matrices are also discussed.

Recent improvements on the preparation of silica gel-glass dispersed liquid crystals (GDLC) are presented. In these materials, liquid crystal microdroplets are trapped into a silica matrix. The feasibility of using these devices as electrooptical shutters is studied. It has been found that GDLC optical transmission and switching can be improved by addition of titanium monomers to the starting reactants employed in GDLC preparation. On the other hand, switching times are reduced by addition of ethanol. The influence of these dopants on droplet size is also studied.

Nanocomposites of Ormosis containing metal oxides, such as niobates, titanates and zirconates, were prepared by sol-gel processing. The materials were hydrolyzed partially and were dried in air atmosphere for appropriate periods. Afterwards, the materials were heat- treated between 200 degree(s) and 450 degree(s)C for 2 days in pure oxygen. The final bulk samples are transparent in infrared and visible ranges. X-ray diffraction patterns showed that these samples did not have any crystalline phases after heating up to 200 degree(s)C. Using a Nd:YAG laser of 1.064 micrometers wavelength, second harmonic generation, of green light (0.532 micrometers ), was observed in these metal oxides/Ormosils nanocomposites. The refractive index and other optical properties of the metal oxides/Ormosils were also measured. The microstructures of these samples were examined by transmission electron microscopy.

Amorphous siliceous materials with molecular dispersions of organic molecules, TPPS, TMPyP and DAQ were prepared by a sol-gel process in which Si(OC₂H₅)₄ was hydrolyzed in weak acid solution. Effect of ultrasonic irradiation on the sol-gel process was studied for the TPPS/a-SiO₂ system in order to attain the further molecular dispersions and shorten the sol-gel reactions. Optical properties of TMPyP doped sol-gel thin films were observed. An amorphous silica thin film which was doped with TMPyP on the order of 1 X 10^-3 mol/mol SiO₂ showed photochemical hole burning at 20 K. the TMPyP/a-SiO₂ was heat-treated at various temperatures and the changes of optical spectrum were observed as a function of temperature. It was found that the sample was comparatively stable up to 200 degree(s)C. Apparent quantum yields of PHB of porphines and quinizarin in polymer or sol-gel film and bulk were discussed.

Optical limiters are devices that have the ability to limit the intensity of light that passes through them. They function like 'optical surge protectors,' blocking a greater fraction of photons when the incident light intensity is very high than when the incident intensity is low. This might allow an electronic sensor or a human eye to function under ambient lighting conditions and yet be protected from being blinded by direct laser illumination. Phthalocyanine (Pc) dyes which exhibit nonlinear optical behavior known as reverse saturable absorption (RSA) have been demonstrated to be effective in solution-based optical limiters. There is considerable interest in the possibility of fabricating optical limiters based on solid-state materials containing phthalocyanine dyes. Sol-gel processing is a solution based technique for preparing porous metal oxide monoliths at low temperatures. By adding an organic dye to the precursor solution, one can trap these organic molecules in the pores of a silica host. These potentially thermally stable materials are of interest for the fabrication of monolithic lens/limiter optical components. Our study of sol-gel synthetic approaches to phthalocyanine doped glassy materials show that phthalocyanine dyes in silica hosts are stable and exhibit optical nonlinearities comparable to those of the corresponding dye solutions.

Solid state optical gain media based upon organic dyes are an attractive alternative to solvent based systems, however the poor thermal and photo-stability of these dyes in polymer hosts has limited their use to date. Organically modified silicates (ORMOSIL's) may offer a partial solution by providing a host matrix with superior qualities. Examples of some of these qualities are excellent optical clarity, low-temperature synthesis, high laser damage threshold, good mechanical strength, and excellent host/dye compatibility. We will report on progress at Lockheed and UCLA on the development of these materials for use as solid-state tunable lasers and amplifiers. Lasing and gain characteristics of common laser dyes (Rhodamine 6G and B) in terms of their optical conversion efficiency and lifetime will be discussed. Efficiencies as high as 39% and output energies > 10 mJ have been achieved from laser- pumped plate and rod ORMOSIL laser cavities.

A multi-step hydrolysis procedure was developed to prepare composite organic dye/sol-gel thin films with variable porosity for such applications as switches or sensors. Variation of acid and base catalyzed hydrolysis sequences of three sols prepared from tetraethoxysilane with identical H₂O/Si ratios, dilution factors, and pH resulted in considerably different silicate speciation. Under conditions where monomer was avoided, the refractive indices of as- deposited films could be varied by an aging step prior to film deposition. This general strategy, which relies on the aggregation of fractal polymeric clusters, is compatible with the low temperature and near neutral pH requirements of both organic dyes and biologically-active species such as enzymes.

Using the sol-gel process, we synthesized zirconia/silica matrices doped with different organic dyes (rhodamine 640, ...). These samples were used to perform optical Kerr effect experiments with sequences of ultrashort light pulses (100 fs, 620 nm, 1 (mu) J focused on 50 micrometers diameter) to induce refractive index changes. A permanent birefringence around 7 X 10^-5 was obtained. By changing the direction of the polarization of the excitation pulses, we were able to locally control the directions of the neutral axes. We thus demonstrated the possibility of using this media as an all optical memory matrix and such doped xerogels will subsequently be referred to as OPTOGELS. We interpret our results as the possibility of locally controlling the orientation of the doping molecules encaged in the solid host matrix. The memory effect is probably due to links of hydrogen bond type between the organic molecules and the pore surface which prevent thermal reorientation. The electric field of the optical excitation pulses exerts a torque on the molecules. If this torque is greater than the energy linking the molecules to the pore surface, the molecules are temporarily released and aligned in the direction of the pulse polarization. Based on this interpretation, we have developed a model to explain the evolution of the birefringence as a function of the number of excitation pulses.

We have studied the relaxation kinetics of malachite green molecules trapped in two types of xerogel matrices: inorganic, a zirconia-silica gel synthesized by slow hydrolysis of alkoxides and hybrid organic/inorganic, methyl or vinyl groups being attached to the silicon atoms in the zirconia/silica system. The addition of the organic group increases the 'apparent viscosity' of the xerogel. The two relaxation times for the malachite green molecules absorption recovery are 1.4 and 10 ps in the zirconia/silica matrix. With methyl modified matrices decay times are respectively 3.5 and 15 ps and with vinyl modified ones, 10 and 23 ps respectively. However, partially modified matrices (i.e. vinyl groups attached to a fraction of the silicon atoms) always give relaxation times of 3 and 10 ps, whatever the extent of modification. These results lead us to interesting conclusions regarding the spatial organization of the doping molecules depending on the matrix.

Nonlinear behavior of glasses and especially thin films prepared by the sol-gel method make these materials an interesting class for optoelectronic devices especially when connected with their wave guide properties. From theoretical point of view these materials provided models for three-dimensional quantum confinement and allow identification ofthe energy ofthe excited states. These energy states ofthe quantum dots are positioned between the discrete energy levels of the atoms or bands of molecules and the broad band of the condensed phase. We describe the preparation of glasses doped by fluorescein, acridine and methyl-orange and discuss the origin of their nonlinear properties. Preparation of glass films doped by CdS and CuC1 quantum dots are also presented and their properties analyzed. The relation between the energies of the electronic states and size of the particles are discussed.

Microcrystalline CdTe-doped glasses were prepared by heating the gels in H₂-N₂ atmosphere. When heated in H₂-N₂ gas, Te ions were reduced into Te^2-, which reacted with Cd²⁺, ions to form CdTe crystals. The size of CdTe crystals was about 3.5 nm in radius, which was controlled by changing the conditions of gel synthesis. The optical absorption edge energies were reciprocally proportional to the square of the crystal radius.

CdS and PbS semiconductor nanoclusters (2 - 10 nm) are produced by using direct precipitation, (gamma) -radiolysis and reversed micelles. A chemical capping reaction is made at the cluster surface by thiolate complexes. The structure and the size of the capped clusters are determined by Optical Absorption, X-ray Diffraction, Small Angle X-ray Scattering and High Resolution Electron Microscopy. After grafting of a gel precursor at the cluster surface, PbS and CdS particles can be incorporated in optically clear and dense oxide gels prepared by hydrolysis of metal alkoxides in a wet atmosphere.

The preparation of semiconductor dots with high number density and narrow particle size distribution, which are essential for high nonlinearity susceptibility, has been studied on CdS dispersed soda-borosilicate glasses by the sol-gel process using tetramethoxysilane (TMOS), triethylborate (B(OEt)₃), sodium acetate (NaOAc) and cadmium acetate (Cd(OAc)₂) as the starting materials. Both number density and particle size distribution of the CdS crystallites were dependent upon the treatment conditions in the conversion of Cd(OAc)₂ to CdO and CdO to CdS inside the porous gel. The optimum heat-treatment temperature for the conversion of Cd(OAc)₂ to CdO was around 400 degree(s)C, and that for the conversion of CdO and CdS by exposing the gel to H₂S was around 120 degree(s)C. Glass containing about 10 wt% CdS crystallites 4 - 7 nm in diameter was obtained by sintering the gel at 590 degree(s)C under the above treatment conditions.

The formation process of glasses doped with cadmium and zinc sulfide or selenide microcrystallites has been studied, along with the characterization of the glasses obtained. Sulfide microcrystallite-doped SiO₂ glasses were prepared by the heat-treatment at around 400 degree(s)C of gels in which thiourea complexes were confined. The formation of thiourea complexes of Cd(NO₃)₂ and Zn(NO₃)₂ in the starting solutions to form gels was confirmed from IR and Raman spectra. The crystalline phase of microcrystallites doped was hexagonal for CdS and cubic for ZnS and their mean diameter varied with the amount of thiourea complexes added. The absorption edges of the semiconductor-doped glasses were found to be shifted to shorter wavelengths in reference to those of corresponding bulk semiconductors due to the quantum size embedded in gels only in the presence of L(+)- ascorbic acid as an antioxidant. Most of Se in the gels, however, was found to be lost within forming selenides of the heat-treatment, though the complexes themselves produced cubic CdSe or ZnSe by the heat-treatment at around 400 degree(s)C in air.

Nanosized ZnS and PbS crystallites were grown within the pores of amorphous multicomponent silicate films deposited by sol-gel processing. Variation of the pore size (<EQ 2 - 4 nm) and porosity (5 - 33%) of the film caused a corresponding variation in semiconductor crystallite size (<EQ 1 - 4.5 nm) and thin film optical absorbance. These results are consistent with crystal growth constrained within the porous thin film host. Due to the small crystallite sizes, the absorbance spectra of the semiconductors were blue shifted relative to the bulk crystals. Crystallite sizes based on the positions of the absorption edges either underpredicted or overpredicted particles sizes measured by TEM indicating possible inadequacies of the models for such small particle sizes. Constrained growth within controlled pore size films appears to be a general route to composites with tailored properties.

Ultrafine particle doped glass composites that show changes in optical property by gas atmosphere were developed by using the sol-gel process. The composites consist of silica glass matrix, functional ultrafine particles and micropores. Transition metal oxides or noble metals were used as the functional ultrafine particles in this study. Diameters of doped particles were confirmed to be in the order of nanometer by X-ray diffractometry and transmission electron microscopy. When the composites were exposed to NO gas flow (0.75 vol%) at 350 degree(s)C, the optical transmittance of the Au-doped composite largely decreased at 400 nm, while that of the Fe₂O₃- or the Co₃O₄-doped glass composites largely increased at 600 nm.

Optical extinction of Ormosils (Organically Modified Silicates) with colloidal Au and Pt particles was studied by the absorption spectroscopy in the visible range. Both bulk and thin film forms of the metal-Ormosil nanocomposites were prepared and analyzed. The correlation between the nanocomposites' resonance absorption position with particle size and volume fraction of the colloidal metal particle was investigated and discussed. No third order nonlinear optical response was detected in the nanosecond time scale. The metal particle size and size distribution was studied by the transmission electron microscopy. X-ray analysis showed the metal particles to be crystalline with FCC structure. The metal clusters were incorporated into the Ormosil matrices by dissolving metal salts into the Ormosil precursor solution prior to gelatin. Reduction of the metal ions within the Ormosil matrices was achieved either by the thermal treatment or by UV irradiation of the samples.

A novel sol-gel process has been used to prepare homogeneous, fully dense, cylindrical samples of neodymium-doped silica and neodymium-aluminum-doped silica. The absorption and emission spectra, fluorescent lifetimes and quantum yields of all samples were measured. The quantum yields of Nd₂O₃-SiO₂ samples were very low due to clustering of the Nd in the silica matrix. The incorporation of alumina eliminated the clustering and quantum yields up to 55% were obtained. We showed that these quantum yields of less than unity were caused by residual OH quenching of the Nd fluorescence. The fluorescent properties were unchanged when the OH was replaced with OD. One sample was demonstrated to laser under laser pumping.

A new route for fabricating large, transparent silica gel microspheres has been developed which readily permits the incorporation of a wide variety of optically active organic and inorganic species. The fluorescence behavior of several organic dye-doped silica gel microspheres as a function of varied liquid environments is examined. The fluorescence of europium and uranium in sol-gel silica microspheres is described. Uranium sensitization of europium in sol-gel silica is presented for the first time.

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.