Wet chemical processing of ceramics, glasses and inorganic-organic hybrids in the form of films has a large number of both proven and potential optical applications. The present review focuses on progress since 1990 in the areas of ferroelectric films, electrochromic and photochromic films, planar waveguides, and NLO films. Where appropriate, advances are illustrated by results obtained in our laboratories.

The cost of a large laser system is a strong function of the overall electrical to final photon efficiency. To improve the stored energy and therefore the pumping efficiency of sophisticated and costly laser-glass amplifiers, we have developed a novel two-layer broadband antireflective coating for the blast-shield component. The blast-shield is an optic placed between the flashlamps and the laser disk amplifiers to prevent damage of laser disks by possible explosion of a flashlamp. The sol-gel antireflective coating was dip-coated at room temperature onto 8-cm diameter glass samples. The coating basically consisted of a halfwave- thick high-index material such as ZrO₂-PVP (PolyVinyl Pyrrolidone) and a quarterwave- thick low-index material such as SiO₂-siloxane. To improve the abrasion resistance of the coated part, a lubricating and water-repellent material was applied as a very thin overcoat. In addition to a 6.5 to 7.2% transmission gain over the spectrum of interest, the coating was moderately abrasion resistant and chemically durable. Flashlamp-induced damage to the antireflective coating for 1000 glow discharges at 10 to 12 J/cm² were minimal and similar to uncoated parts.

The adhesion of sol-gel antireflective porous silica coatings on vitreous optical substrates has been dramatically improved by exposure to ammonia vapors or a dip in basic solutions. The approximately 70 to 270-nm thick coatings consisted of monolayers of spherical, 20-nm diameter amorphous silica particles deposited from ethanolic colloidal suspensions by conventional liquid coating techniques. Although, the as-deposited coatings had only low adhesion and were easily damaged when cleaned by standard drag-wiping procedures, coatings exposed over 5 hours to ammonia vapors passed both adhesive-tape and moderate abrasive- resistance tests. The increase in strength was accompanied by a roughly 20% shrinkage of the original coating thickness but the antireflective properties were retained. Our explanation of this chemical effect is a base-catalyzed phenomenon leading to surface silanol condensation and hydrogen-bonding of neighbor silica particles. In addition, since this basic treatment enhanced the laser damage resistance, such strengthened antireflective coatings have been successfully evaluated on flashlamps used on Phebus, Europe's most powerful laser. This allows an increase of the laser-disk pumping efficiency.

In order to densify and to improve the physical properties, TiO₂ sol-gel films, about 100 nm in thickness, on silica glass or silicon wafer were implanted with Ar⁺ or B⁺ ions. The refractive index of the as-dried films increased and the IR absorption band of OH disappeared after Ar⁺ implantation. Dehydration and densification of sol-gel films were enhanced by Ar⁺ implantation. On the other hand, the refractive index and the thickness of the films hardly changed by B⁺ implantation. However, IR absorption bands attributed to B-O bond were observed after B⁺ implantation. This suggests that sol-gel films could be chemically modified by ion implantation with reactive ion species.

A vapor-gel process is implemented to achieve deposition of stoichiometric LiTaO₃ on a variety of substrates using LiTa(OButⁿ)6 precursor. Such process involves the vapor- phase hydrolysis of volatile double alkoxides, and polycondensation of the partially hydrolyzed double alkoxide on heated substrates. It was demonstrated that an intrinsically stoichiometric vapor-deposition can be realized using heterometalorganic complexes and the hydrolysis- polycondensation reaction scheme. We report typical deposition and annealing conditions used in this study, x-ray diffraction, SEM and composition analyses of the films grown on fused silica, Pt, sapphire (1010) and LiNbO₃ (0006). Composition analysis achieved by measurement of lattice parameters confirmed that the vapor-gel method using double alkoxide can produce high quality, stoichiometric LiTaO₃. Mass spectrometric study of the precursor compound LiTa(OButⁿ)6 suggests that the volatile species responsible for the stoichiometric vapor deposition is Li₂Ta₂(OButⁿ)12. Rocking curve measurement indicated excellent epitaxial growth of LiTaO₃ on LiNbO₃ (0006).

Colloidal suspensions of oxides have been used to prepare dielectric HR mirrors, specifically for high power fusion laser applications, on substrates up to 38 cm square using a meniscus coater. These coatings consist of porous quarterwave layers of alternating high and low refractive index oxides. Silica was used exclusively as the low index oxide and AlOOH, ZrO₂ or HfO₂ as the high index material. Coatings were weak because of low particle-to- particle adhesion. The use of organic polymer binders in the high index component in particular, was found to increase strength thereby improving the laser damage threshold and also reducing the number of layers required for 99% reflection due to increase in the refractive index.

This study was conducted on both inorganic silica and organically modified silica (ormosil) films. Sol-gel derived silica films were prepared by spin-coating either a sol or a colloidal silica (COLSI) suspension mixed with a polysiloxane solution onto glass substrates. The films were microscopically investigated to reveal their structure. The optical quality of the films was measured using a scatterometer which measured the intensity of light scattered from the films as a function of angle of scatter. Scatterometry allowed the quantitative study of the factors that affect film quality, particularly film composition and number of layers spun. The effect of heating the film in-between the application of successive coatings was also investigated. Various tools were used to inspect the films, these include interferometric profilometry, probe profilometry, phase-contrast and confocal optical microscopy, and electron microscopy. Laser dyes and also various molecules possessing significant nonlinear optical properties have been successfully incorporated into the films.

Temporal evolution during spin coating is interferometrically monitored, with close regard to gelation at the final stages of the process. Uniform silica films on silicon were produced from an established sol composition, from which slight deviations were taken to produce nonuniform, cracked films. A distinct evolution of the sol to gel transition could then be detected, at speed of rotations of 1000 - 3000 rpm and data acquisition at 100 Hz. Interpretation of a set of experimental results is discussed in terms of a two-phase system, in the light of, and as a test of, current theoretical models.

Photo-induced changes in structure of sol-gel derived SiO₂ films were studied in order to explore a possibility of densification of sol-gel films at room temperature using energetic photons. Gel films which were produced on silicon substrates by dip-coating of sols prepared through hydrolysis of tetraethoxysilane were subjected to synchrotron radiation at room temperature. Structural changes of the gel films were investigated by ellipsometry and infrared spectroscopy. The radiation resulted in an increase in refractive index, a decrease in thickness of the films and a loss of OH groups. Increase in temperature of the films was less than 50 degree(s)C during the irradiation. The results indicate that ultraviolet lights induce densification of gel films through electronic excitation.

The use of porous sol-gel films as hosts for semiconductor microcrystals, with applications in integrated optics, has been examined. Techniques are described for characterizing porosity using ellisometric measurements, and the use of this technique to investigate the effects of process parameters is reported. In particular, the effect on pore microstructure of water concentration, catalyst type and concentration and film annealing temperature are presented. A technique for doping the porous host film with CdS crystallites is then described. Absorption spectra for the doped films are presented which given an indication of the size and concentration of these crystallites, and the way in which these are affected by process parameters is also reported. The cadmium counter-ion is shown to strongly influence the doping concentration, and the film annealing temperature is shown to affect the crystallite size.

Acid catalyzed solutions of methyltriethoxysilane (MTES) mixed with tetraethoxysilane (TEOS) or titanbutilate (TiBut) were used to deposit silica and silica-titania coatings by dipping and spinning. Inorganic coatings free of cracks were obtained with a thickness up to 1.5 micrometers , after densification at 500 degree(s)C. The MTES derived coatings were used as matrices for metallic nanoparticles. Gold, silver and platinum nanoparticles were obtained in the coatings by thermal reduction. Gold particles were obtained at a temperature of 200 degree(s)C, while temperatures around 800 degree(s)C were necessary for silver and platinum. The effect of the composition of the matrix, the heat treatment temperature and the atmosphere on the formation of the particles was studied. The metallic particles were characterized by transmission electron microscopy, x-ray diffraction and UV-Vis absorption spectroscopy. The content of MTES and the heat treatment temperature were found to affect the dimension and the shape of the metal particles. The distribution in particle size may be tailored by choosing the heat treatment temperature and the amount of MTES.

The sol-gel process was used to form ferroelectric epitaxial PbTiO₃ and SrTiO₃ thin films on single crystal SrTiO₃ substrates. In this study, optically transparent multilayer n$CHI[SrTiO₃/PbTiO₃] thin films were grown successfully along the (001) direction on single crystal SrTiO₃ substrates. Lead titanium double alkoxide and strontium titanium double alkoxide were used as precursors. The layers of transparent PbTiO₃ films were obtained by heat treatment at 700 degree(s)C in PbO atmosphere, while the layers of SrTiO₃ were obtained at 800 degree(s)C in air. The average thickness of each PbTiO₃ or SrTiO₃ layer was measured by SEM analysis. The change of optical transmittance with wavelength for a sample with 12 (6$CHI[SrTiO₃/PbTiO₃]) layers film was measured. The refractive index of PbTiO₃ and SrTiO₃ films grown on Si substrates was measured with respect to film thickness. The epitaxial growth of the PbTiO₃ thin films and SrTiO₃ thin films was investigated using XRD, HRTEM, and SEM analysis. The structure, growth parameters, and optical properties of the films are discussed.

Noble metal nanoparticle-dispersed SiO₂ and TiO₂ coating films were prepared by heating in air or in reducing atmosphere the gel films obtained from homogeneous solutions of corresponding noble metal salts and silicon or titanium alkoxides. In the Au particle-dispersed SiO₂ coating films obtained from solutions of HAuCl₄ (DOT) 4H₂O and Si(OC₂H₅)₄, the size and shape of the Au particles were markedly affected by the sol aging time, the amount of H₂O for Si(OC₂H₅)₄ hydrolysis and the exposure of the gel film to an amine vapor prior to heat-treatment. This suggests the strong relation between the structure and nature of the gel matrix and the final microstructure of the composite films. In the dielectric matrix of TiO₂ with high refractive index the surface plasma resonance peak of Au particles was at longer wavelengths than in SiO₂ matrix as predicted by Mie theory. Pt/TiO₂, Pd/TiO₂ and Ag/TiO₂ nanocomposite films exhibited optical absorption in the visible range due to the high dielectric constant of the matrix.

A sol-gel method for the preparation of transparent copper nano particle-containing SiO₂ coatings on glass has been developed. The sol is synthesized from alkoxysilanes and tetra ethyl orthosilicate with copper ammine complexes, prepared from Cu²⁺ salts and amino alkoxy silanes. Glass substrates are coated by dipping and layers up to 1 micrometers in thickness are obtained after thermal densification at temperatures between 200 degree(s) - 500 degree(s)C. The Cu colloid formation can be achieved using a reducing atmosphere during densification. Thus reddish-brown colored coatings on glass with optical densities between 0.5 and 2 are obtained. Under ambient air the color turns from reddish-brown to dark green. This process is reversible and by re-heating under reducing conditions the reddish-brown color can be re- established. UV-VIS absorbance measurements and structural investigations by WAXS, TEM, ESCA and SNMS show that the green color is due to an oxide layer at the colloidal interface.

An organic-inorganic synthesis route to Au-colloid containing, transparent SiO₂ coatings has been developed, using four different types of functionalized silanes as stabilizing ligands for the Au. By variation of the kind and the concentration of the stabilizing silane in the sol the onset temperature for the colloid formation varies between 100 and 300 degree(s)C and the final colloid radii can be controlled in a range between 3 and 30 nm after densification of the composite coatings on glass at 500 degree(s)C. The third order polarizability (Chi) _m⁽³) in the metal particles is one order of magnitude higher than in glass composites and exhibits a strong dependence on the ligand.

A photochemical method of producing nanometer gold particles in optically transparent sol-gel silicate materials is described. Organometallic gold precursor compounds are dissolved in the sol and encapsulated in the growing silicate network. Irradiation of the doped monoliths with ultraviolet light causes the photodeposition of gold particles within the silica gel or xerogel. The particles are characterized by their electronic absorption spectra and by TEM. The transparent, porous monoliths are excellent substrates for surface enhanced Raman spectroscopy (SERS). Small molecules such as pyrazine diffuse into the monoliths and are detected by using SERS. The sol-gel matrix stabilizes the gold particles (in comparison to colloids in liquid media) and SERS can be used to detect molecules that penetrate the matrix.

Gold-dispersed BaTiO₃ thin films, which will be used for third-order nonlinear optical devices, were prepared by sol-gel method with spin-coating using HAuCl₄ (DOT) 4H₂O, Ba(CH₃COO)₂ and Ti [O(CH₂)₃CH₃]₄ as starting materials. The thin films were heat-treated in air at temperatures ranging from 400 degree(s)C to 800 degree(s)C for 1 h. Thin films with different volume fractions of fine gold particles were successfully prepared. The peak position of the surface plasmon resonance of gold particles shifted toward the long wavelength as the heat-treatment temperature and the volume fraction of gold increased. The BaTiO₃ matrix was amorphous up to 400 degree(s)C, but became crystallized at temperatures above 500 degree(s)C. The fine gold particles became coarse between 600 degree(s)C and 800 degree(s)C. The mean particle size of gold was 20 nm at 800 degree(s)C. The nonlinear optical property of these thin films was measured by the degenerate four-wave mixing (DFWM) method using a frequency-doubled Nd:YAG laser with 20 ps pulse duration. Third-order nonlinear susceptibility (Chi) ⁽³) of gold-dispersed BaTiO₃ thin film with 10 vol% of gold was 2.5 X 10^-8 esu.

In this paper we discuss our approaches and results for the preparation of improved semiconductor quantum dot materials by the sol-gel method. By taking advantage of the low- temperature sol-gel route, the nanostructure and optical properties of CdS quantum dots were greatly improved for device applications. Pore-free sodium borosilicate (NBS) glasses and organically modified silicates (ORMOSILs) were used as matrices for the CdS nanocrystallites. Results from both degenerate four-wave mixing and pump-probe techniques indicated large third-order nonlinear responses of the order of 10^-6 esu from CdS-doped NBS glasses. By using potassium ion exchange, the first CdS-doped channel waveguides were fabricated in NBS glasses. Propagation of femtosecond laser pulses through the waveguide was investigated. Pulse breakup effect that may be the result of soliton formation was observed in these waveguides. Polydimethylsiloxane (PDMS) was uses as the organic component to modify the silica gel. CdS-doped ORMOSILs derived from the PDMS- TEOS system exhibits improved mechanical property and film-forming ability compared to purely inorganic gels. Both CdS-doped PDMS-TEOS ORMOSILs and NBS glasses were much more resistant to photodarkening than the glasses made by melting. The use of the bifunctional ligand 3-aminopropyltriethoxysilane (APTES) significantly reduced the average particle size and narrowed the size distribution of CdS quantum dots in silica gels and densified NBS glasses.

An original sol-gel technique is used to prepare sodium borosilicate glasses doped by II-VI semiconductor nanocrystallites. A complete study of CdS and CdSe crystallites is presented, as well as preliminary results on CdTe crystallites. The synthesis procedure involves the preparation of complex cadmium-based molecules in an aqueous solution to which other inorganic precursors are added in order to start the gelation of the vitreous matrix. In the cases of CdSe and CdTe, selenious and telluric acids, respectively, added to the starting solution induce the formation of crystallites by a redox reaction during thermal degradation of the gel. This gel is then melted and quenched to provide the glass. The effects of further thermal annealing, at temperatures around and above the T_g of the glass, are observed by optical absorption measurements. The latter are analyzed by using a theoretical calculation of excitonic energies and oscillator strengths, based on the envelope-function approximation. This model includes the confinement-induced complex mixing of valence-band states. Our results show that disorganized clusters with a rather narrow size-distribution controlled by initial conditions are present before the annealing. The latter quickly improves the crystalline quality. Meanwhile, the average size is slightly increased and the width of the size-distribution reduced down to an optimum, after a few hours (less than 10). For instance, CdS crystals of approximately 4 nm in diameter, with a half width at half maximum of the size distribution of approximately 0.23 nm are obtained after 6 hours of annealing.

Silica-titania optical waveguides doped with CdS and PbS were prepared by the sol-gel method. Thiourea was used as an internal source for sulfur. Cadmium and lead were introduced as acetates. Using tetraethoxysilane (TEOS) and Ti(OBu)₄ as precursors for silica and titania, films with typical thickness of 0.15 - 0.2 micrometers were obtained. In order to obtain thicker layers TEOS was substituted with methyltriethoxysilane (MTES). The effect of this precursor on solution and waveguide properties is discussed.

We have prepared CdS-doped SiO₂ glasses from silica gels containing several types of dithiocarbamatocadmium complexes. The CdS microcrystallites doped in SiO₂ glasses which were obtained from these complexes showed very slight discrepancy from stoichiometry. The maximum doping amounts were influenced by stability and solubility of the complexes in solution. In the optical absorption spectra and the photoluminescence spectra of glasses of Cd/Si equals 0.01 prepared using Cd[S₂CN(C₂H₅)₂]₂ and Cd[S₂CNCH₃(C₂H₄OH)]₂, the absorption edge and the emission peak observed for the glass using Cd[S₂CNCH₃(C₂H₄OH)]₂ were at longer wavelengths compared with Cd[S₂CN(C₂H₅)₂]₂. These differences are ascribed to the formation of larger CdS particles by agglomeration or precipitation because Cd[S₂CNCH₃(C₂H₄OH)]₂ is more labile in solution than Cd[S₂CNCH₃(C₂H₄OH)]₂. In the photoluminescence spectra, the emission peak with photoluminescence decay composed of several hundreds picosecond order and several nanosecond order was only observed at around the absorption edge. There was no change in the photoluminescence decay components by changing the kinds of dithiocarbamatocadmium complexes.

Ge nanocrystal-embedded SiO₂ glasses were successfully prepared by a sol-gel process. The glasses synthesized through hydrolysis of Si(OC₂H₅)₄ and GeCl₄ were heated in the presence of hydrogen at 400 to 700 degree(s)C, in which Ge⁴⁺ ions were reduced to precipitate nanosized Ge crystals. Glasses doped with Ge nanocrystals of a diameter of approximately 5 nm showed the optical absorption edge at approximately 2.5 eV and a strong room temperature photoluminescence with peaks at 2.35, 2.15 eV and weak at 1.85 eV. Large Ge crystals precipitated by heating above 800 degree(s)C showed no photoluminescence.

The Bi₂S₃ microcrystallite doped thin films and glass lumps have been successfully prepared by the sol-gel process, the size of the microcrystallite in glass heated for different times at 400 degree(s)C was decided by the method of HRTEM. The optical transmission valley shifted towards longer wavelengths with longer heat-treatment time at 500 degree(s)C in the Bi₂S₃ doped thin films, showing the experimental evidence of quantum size effects. The red-shift of emission peaks in luminescence spectrum excited with longer wavelength is attributed to the broad distribution of particle size in Bi₂S₃ doped glass. The nonlinear refractive index of the Bi₂S₃ quantum dots glass which is studied by the method of Z- scan gives n₂ equals 3.3089 X 10^-9 esu. The optical second generation (SHG) of the Bi₂S₃-doped glass also is observed using a Nd:YAG colliding mode locked laser.

The luminescence of inorganic ions, organic neutral molecules (or polyatomic anions or cations) incorporated in sol-gel glasses (formed during a programmed process by hydrolysis of alkoxides and subsequent polycondensation) usually gives higher (to far higher) yields than in low-viscosity water or alcohols. Photodegradation can be avoided in lasing perylimide dyes firmly fixed in the glass built up around them. Some quantum-chemical comments are made.

Gel silica glasses of type VI have been widely used in making laser or NLO dye doped devices by the post-doping method. Both mechanical and optical properties of the as-prepared optical materials can be improved by introducing a third phase into the essentially porous matrix through impregnation. In our study, we have used three different phases to fulfill this purpose: i.e., polymer, sol and ormosil. The finished materials are characterized in relation to their textural structures, bulk structures, phase interaction and susceptibility to environmental impurities such as water. Our results have shown that pores of the initially porous matrix are partially or fully eliminated depending on the type of phase impregnated. In both sol and ormosil impregnated samples, the primary substrate phase interacts with the secondary impregnated phase through the formation of Si-O-Si bonds via dehydroxylation of silanol groups from the two phases. The organic component from the ormosil may also interact with the primary inorganic-based substrate via molecular forces such as hydrogen-bonding. In the PMMA-impregnated materials, surface silanol groups from the gel matrix interact with ester carbonyls of PMMA through hydrogen-bonding. Bulk structures of the primary and impregnated phases are well retained. PMMA-modified samples are hydrophobic; ormosil modified samples are hydrophobic to some extent depending on the actual organic groups; but sol-modified samples are hydrophilic due to the exposed surface silanol groups. In general, organically modified optical composites are best used in the near UV and visible region and their application in the near IR is restricted to some extent due to the overtone and combination vibrational energy absorptions by both the organic and inorganic species present in the materials.

Poled hybrid siloxane-oxide coatings synthesized through hydrolysis and condensation of Si(OMe)₄ and TDSP precursors leads to SHG values of 10 pm/V. The glass transition temperatures of the hybrid coatings increase with the TMOS content and the thermal curing time. These modifications related to increases in the condensation and crosslinking degree of siloxane and silica species have a strong influence on the relaxation behavior of the NLO chromophores grafted in these hybrid networks. Hybrid organic inorganic nano composites made of metal oxo-polymers (zirconium and neodymium oxo species) linked to siloxane species [(CH₃)Si-O_3/2] exhibit Nd³⁺ luminescence properties. At low rare earth concentrations the emission lifetime is mainly governed by some energy transfer processes occurring between the Nd³⁺ ions. While for higher Nd³⁺ concentrations the emission lifetime is controlled by Nd-Nd interactions and by an increase of the quenching probability due to the presence of some residual hydroxy groups.

A number of different organic molecules have been used as optical probes of the sol-gel process. There is relatively little information, however, as to whether these molecules remain well isolated within the sol-gel structure or if they tend to form dimers or higher aggregates within the network. This issue is particularly important for doped sol-gel optical materials as dimer formation can exert a significant influence on the optical properties of dyes. The present paper uses the optical absorption characteristics of copper phthalocyanine tetrasulfonate (CuPcts) to determine how the state of the dye is affected by the chemical changes during the sol-gel process. The absorption spectra of CuPcts indicate that the dye molecules are dimerized in acid-catalyzed silica xerogels prepared from TMOS. The dimerization is largely controlled by the chemical environment inside the pores. By using appropriate reference solutions, we are able to identify the factors which cause dimerization and the stages of the sol-gel-xerogel transformation when the dimers form. These factors include the quantity of solvent remaining in the pores, the alcohol/water content of the solvent and its acidity. It is shown that by modifying the sol-gel processing conditions and the solvent chemistry within the pores, it is possible to reduce significantly dimer formation in silica xerogels.

Transparent gels, either purely inorganic or organically modified, are obtained through a sol- gel process circumventing the usual hydrolysis-condensation path. This non-aqueous route, which may also be described as a `non-hydrolytic' process, results in gels containing low amounts of hydroxyls vis-a-vis conventional gels. The mechanical properties of the gels range from rubbery to hard according to the functionality of the precursor. Erbium-doped methylsilsesquioxane shows luminescence at 664 nm under pumping with an argon line at 488 nm. Pure SiCl₄-derived gels show an additional emission, ascribed to the ⁴F_7/2 yields ⁴I_3/2 transition at 710 nm.

Tetraphenylporphinetetrasulfonic acid (TPPS), which is known as a photo-chemical holeburning (PHB) dye, was incorporated in silica gels obtained by the sol-gel process from tetraethoxysilane (TEOS). The form of TPPS must be free-base (H ₂P), which exists in basic condition, to be active in PHB. In this study to obtain transparent and dense silica gels doped with free-base TPPS, a two-step hydrolysis process using solutions containing NaOH or using ammonia were developed. In the first step we used acidic catalyst to get denser and transparent gels and in the second step basic catalyst was adopted to obtain free-base TPPS. Monolithic silica gels can be obtained by this two-step process from the hydrolysis of TEOS. By the spectroscopic analysis of TPPS incorporated in the gels, it is found that the free base form of TPPS is retained at the molar ratio of NaOH/TEOS above 10^-3. The spectra of monocation are observed at the molar ratio of NaOH/TEOS below 10^-3. And the dimerization of TPPS incorporated in the TEOS gels does not occur at higher NaOH concentration than that in the TMOS gels.

Optical absorption and emission behavior have been investigated in aluminosilicate sol-gel materials containing the sodium salt of tetra-4-sulfonatophenylporphyrinatocopper(II). Concentration-dependent nonlinear absorption was observed in the porphyrin-doped specimens. Up-converted optical emission from the second excited doublet band was studied, and a saturated behavior was observed as a function of incoming excitation energies. These phenomena have been attributed to excited state absorption from both doublet and quartet manifolds. Intersystem crossing and quartet-quartet absorption effects are believed to dominate the nonlinear processes, especially in the case of strong excitation. Rate equation analysis also suggests that the quartet manifold is extensively involved for intersystem relaxation and excited state absorption. Good agreement was obtained between experimental observations and the presented rate equations.

The lifetime of the near-infrared emission of oxazine-1 and 1,1',3,3,3',3'- hexamethylindotrycarbocyanine iodide (HITC) into gel-glasses prepared from TMOS or TEOS are studied by variable frequency phase and modulation data. Analysis of the lifetime by a fitting procedure to an assumed model revealed two lifetimes with a low value of the reduced chi-squared for the studied doped gel-glasses. The longer lifetime is attributed to the adsorbed species, while the shorter to species which are affected by the remaining solvent in the pores. The contributions of these species to the total fluroescence are also resolved.

The sol-gel synthesis of organic-inorganic composite materials with push pull azobenzene molecules covalently bonded to the silica matrix is used for fabricating low cost optical channel waveguides for integrated optics applications and optical information storage. The refractive index change is obtained through the orientation of the nonlinear active molecules induced by a trans-cis-trans photoisomerization process. The index change and anisotropy are created by illuminating the thin film at a wavelength located inside the absorption band of the chromophores. At short irradiation times, the induced birefringence and dichroism are the consequence of the usual reorientation phenomenon due to the trans-cis-trans isomerization. At long pumping times, a photobleaching is observed corresponding to an irreversible transformation in the silica matrix. This property is used to write channel waveguides and gratings.

Organic dye molecules have been widely used in solution as amplifying media in tunable lasers. The possibility of trapping these molecules in a solid state matrix may lead to a new field of application outside the laboratory. Many types of matrices can be used. We present results with new hybrid organic/inorganic xerogels prepared by hydrolysis-condensation of the methyl- or vinyl-triethoxysilane precursor under acid-catalyzed hydrolysis and basic-catalyzed condensation. Organic methyl or vinyl groups attached to the silica network provide a favorable environment to dye fluorescence. We have studied the effect induced by using different basic catalysts: N-(2-aminoethyl)-3-aminopropyltrimethoxysilane and 3- aminopropyltriethoxysilane are found to lead to the best matrix characteristics. Perylenes and pyrromethenes were chosen because of their thermal- and photo-stability. After gelation, 10 mm thick samples, polished to a surface roughness of about 4 nm, are placed in a plano- concave cavity as gain media and pumped by a frequency doubled nanosecond Q:switched Nd:YAG laser. Efficiency is the greatest with a pyrromethene 597 doped sample: more than 6 mJ output energy for a 10 mJ pump energy (2 J/cm² fluence) leading to a slope efficiency of 63%. The lifetime of the laser emission when the same point of the sample is sequentially used is also an important characteristic. Best results are obtained with perylene red dye: for a pump energy of 0.7 mJ at 1 Hz repetition rate, the output energy is still 50% of the initial value (0.1 mJ) after 80,000 shots. For a pyrromethene 597 doped MTEOS sample, it appears that the measured lifetimes are independent of the operating frequency in the 0.2 - 20 Hz range, indicating a reasonable heat conductivity for this type of matrix. We have also studied tunability by adding a grating or prism to the cavity: a typical tunability of over 50 nm or more is obtained with many of these dyes. Finally we report laser activity obtained with rhodamine B doped xerogels in which the dye molecules were covalently bonded to the silica network in order to stabilize the organic molecule structure.

A silylated Coumarin 4 (derCoum) laser dye has been incorporated over a large range of concentrations in sol-gel silica composites. Optically transparent films of derCoum and Coumarin 4 (Coum) doped silica were obtained; and their absorption and fluorescence spectra and fluorescence efficiency were measured. Dye extraction was investigated as a function of sol-gel processing conditions. Dye extraction results indicated that prehydrolysis of the derCoum and full hydrolysis of TMOS resulted in films from which the dye could not be extracted, suggesting that all the dye is bonded within the sol-gel matrix. The silylated dye films showed higher fluorescence efficiency at all concentrations with respect to the normal dye film.

Various compositions of inorganic-organic composites (ormocers and nanocomposites) have been synthesized and used as matrices for NLO push-pull molecules. It has been shown that dielectric properties and electrical conductivity during the sol-gel transition (curing parallel to corona poling) are the most important factors for obtaining high (Chi) ⁽²) efficiencies. With low conductivity systems, (Chi) ⁽²) values up to 9.7 pm/v have been obtained.

A photochromic dye, spirooxazine (SO), was incorporated in organic-inorganic composite (OIC) materials by the advantages of sol-gel processing. It has been found that the photochromic response is high enough and the fading rate is similar to the dye-in-ethanol solution while the photofatigue resistance is strongly dependent on the matrix composition and the starting compounds. In the present work, we present results on the effect of matrix composition and starting compounds as well as additives on the photofatigue resistance of SO- OIC photochromic coatings. Sol-gel coatings synthesized from methyltrimethoxysilane (MTMS) and glycidyloxypropyltrimethoxysilane (GPTMS) as starting compounds, and using 1H,1H,2H,2H-perfluoroalkyltrimethoxysilane (FAS) and 1-methylimidazole (MI) as additives provide the SO dye with a favorable matrix environment in terms of photofatigue, so that the overall photochromic performance of the dye can be optimized. The photofatigue resistance reaches the same level as, while both the photochromic response and fading rate are much better than the SO-PMMA coatings.

A new hybrid organic-inorganic material has been developed which has excellent properties as an optical material and host for molecular dopants. The undoped material has very low optical loss at 1.06, 1.3, and 1.55 micrometers as well as in the visible region. In addition, a 20 mW HeNe laser beam directed through the sample is barely visible. The structure of this material has been studied using ²⁹Si MAS-NMR, high-resolution TEM, and NIR spectroscopy. These results are presented and discussed in terms of the chemical composition, processing conditions and low optical loss of the material. A wide variety of molecular dopants has been successfully incorporated into this host. The linear optical properties of several perylene laser dyes and metal dithiolene (Chi) ³ molecules are presented. Results for lasing experiments, damage threshold, and (Chi) ³ measurements are being reported separately.

In this paper, the optical spectra and optical properties of vanadium oxygen phthalocyanine having propyl substitutent [PPcVO] in different matrices and forms at different concentration have been studied. The optical recording performances of multilayer films is reported.

A transparent colored thin gel layer (100 - 300 nm thick) was formed on a glass surface by the dipping-withdrawing coating (1 - 3 mm s^-1) method. The colored coating layer was dried at ambient atmosphere, and then heated at 100 degree(s)C for 15 min. Cationic dyes, such as triphenylmethane, phenoxazinium, phenazinium, and phenothiazinium dyes, can be soluble uniformly under the silica sol processing conditions where hydrolysis and condensation of tetraethoxysilane or methyltriethoxysilane in the presence of water and acid occur. Superfine particles of some organic pigments also are dispersed uniformly in such sol in high concentration, and they are coated on the glass by similar method. The effects of nickel complexes and matrix gels on the photofading of the colored thin gel film using organic cationic dyes by the irradiation of filtered visible light (> 390 nm) using 500 W-Xe lamp have been studied. Anthraquinone, phenazinium, and phenoxazinium dyes with terminal triethoxysilyl group to connect them directly with silica matrix of the gels were prepared, as reactive dyes, and reactivity of the reactive dyes under conditions of sol-gel processing has been investigated.

New hybrid organic-inorganic materials synthesized at room temperature and doped with laser dyes have been tested for laser action. The new ormosil material has excellent optical properties and is an effective host for many organic molecules. Two members of the perylene family of laser dyes, which are characterized by increased degrees of photostability, have been successfully incorporated into this host. The spectroscopic properties of the perylene dyes are not significantly altered by incorporation into ormosil. Dye doped samples with polished faces have been studied by longitudinal excitation in a compact cavity by the second harmonic of a Nd:YAG laser emitting 27 mJ in 6 ns pulses at 532 nm. Aspects of laser performance including slope efficiency, photostability, and output wavelength are reported. Experiments were conducted to establish the normalized photostability, which is the accumulated pump energy absorbed by the system per mole of dye molecules in the gain region before the output pulse energy falls to one-half of its initial value. These measurements allow a comparison between different dyes and hosts.

Recent studies on metal dithiolene complexes in solution and PMMA have shown this range of materials to exhibit high third-order nonlinear optical coefficients, (Chi) ³, and excellent figures of merit. Phthalocyanine dyes have been shown to exhibit optical limiting due to the nonlinear effect of reverse saturable absorption. We report here on the third-order nonlinear optical characterization of several metal dithiolene and phthalocyanine compounds doped in sol-gel derived materials. These hosts provide a physically and chemically stable environment for the nonlinear compounds, and offer the potential of high levels of doping. The dithiolene and phthalocyanine compounds were incorporated into partially densified sol-gel silica glass (xerogels) by the post-doping technique, and into the hybrid organic-inorganic materials at the sol-stage. To obtain optical quality surfaces on the porous xerogel, a unique polishing technique has been developed. Picosecond and nanosecond pulsed laser studies of nonlinear refraction and absorption are presented, together with laser damage and micro-hardness measurements. For the dithiolenes, concentrations of the order of 10¹⁸ molecules/cm³ for both the hybrid material and the xerogel were studied using the degenerate four wave mixing (DFWM) technique. (Chi) ³ values up to 1.4 X 10^-19 m²/V² (equivalent to 1.0 X 10^-11 esu) were observed in the sol-gel host.

Higher energy laser action in partially densified sol-gel glass, sol-gel glass composites and polymethylmethacrylate (PMMA) doped with laser dyes is reported. Post-doping, in which the active dye is diffused into the densified porous sol-gel matrix, has been developed into an effective method for the impregnation of organic species. The sol-gel glass composite is formed by refractive index matching the porous sol-gel glass by impregnation with polymethylmethacrylate. This is achieved by in-situ polymerization of methylmethacrylate. This reduced optical attenuation from 0.7 dB/cm to 0.4 dB/cm. By formation of the composite the chemical environment of some dyes is improved. Laser performance is described for the dyes rhodamine 590, pyrromethene 567, and the perylene derivatives perylene orange (KF 241) and perylene red (KF 856), in the three solid state hosts when pumped with the second harmonic of a Nd:YAG laser. As a comparison, their performance is also described in solution. For laser performance, the best combinations of dye and host in terms of both efficiency and photostability were found to be rhodamine 590 in sol-gel glass and perylene orange in sol-gel glass/PMMA composite. Long pulse lasing has also been demonstrated with rhodamine 590 in sol-gel glass and sol-gel glass/PMMA composite and pyrromethene 567 in sol-gel glass, using a flashlamp pumped coumarin 504 dye laser as a pump source, emitting 3 microsecond(s) pulses at 504 nm. We also give the first report on de-oxygenated pyrromethene 567 doped sol-gel glass/PMMA composites. De-oxygenation improved the photostability of this dye in the composite by a factor of four.

The photochromic gel thin films using 1'-butyl-3',3'-dimethyl-6-nitro-spiro[2H-1- benzopyran-2,2'-indoline] (1) and 1'-butyl-spiro[2H-indole-2,3'- [3H]naphtho[2,1-b][1,4]oxazine] (2) dispersed in sol in the sol-gel processing were prepared and photochromic behaviors of these films were investigated. A good transparent coating layer on glass surface was formed in the range of ca. 6 - 10 wt% of 1 or 2 to alkoxysilane, and was colored by UV irradiation. The absorption band formed by UV-irradiation disappeared by thermal decay and also by Vis irradiation for 1. The thermal fading of the colored form to the spiro form 1 or 2 is dependent on a matrix of the gels, the colored forms in the film starting from methyltriethoxysilane (MTES), which is expected to include larger pores than in the film starting from tetraethoxysilane (TEOS) or silane oligomer, show faster thermal fading which roughly follows the first order kinetics. The colored form in the gel is stabilized, compared with that in solution or bulk gel, and it is suggested that there are some kinds of colored species in thin gel films containing spiropyran 1, which may be some aggregates, whereas only a colored species from spironaphthooxazine 2 is suggested. Photochromic behavior of 2 in sol was also examined.

The photochromic organic molecule 2,3-diphenyl indenone oxide (DPIO) has been incorporated into transparent silica gel-PMMA nanocomposites. Under ultraviolet illumination, the colorless DPIO molecule transforms into 1,3-diphenyl-2-benzo-pyrillium-4- oxide (DPBO), which is deep red. Upon exposure to visible light, DPBO transforms reversibly into DPIO. Some DPBO, however, irreversibly decomposes into 3,4-diphenyl isocoumarin (DPIC) and other by-products. In this preliminary study, the rate of photochromic fatigue for a variety of samples of DPIO-doped PMMA, silica gel-PMMA nanocomposites, and ORMOSILs is presented and discussed. The prospects for DPIO-based photochromic materials exhibiting long-term stability are discussed.

Nb₂O₅ prepared by a sol-gel process in the form of coatings are new materials which have outstanding properties: (1) The coatings present electrochromic properties and exhibit a blue coloration under Li⁺ insertion with 100% reversible variation of the optical transmission in the visible and near infrared range between 80% and 20% and have a high chemical stability (tested up to 2000 cycles). (2) They are semiconductors and present a photoelectric effect when illuminating in the UV region ((lambda) < 360 nm). These films are therefore very promising for use in electrochromic devices and as electrodes for photoelectrochemical purposes and the development of nanocrystalline solar cells.

The PbO-TiO₂ amorphous monoliths with TiO₂ content from 60 to 80 mol% were prepared by the sol-gel method. The amorphous monoliths obtained were somewhat porous, but had a high refractive index of more than 2.0. The third-order optical nonlinear susceptibility, (Chi) ⁽³), of them evaluated from third-harmonic generation (THG) was 5.7 X 10^-13 esu for the transparent monolith containing 60 mol% TiO₂, and increased with increasing TiO₂ content. The largest (Chi) ⁽³) was 1.4 X 10^-12 esu, for the monolith with 80 mol% TiO₂. Ti-K edge x-ray absorption (XANES and EXAFS) analyses were carried out for gels and heat-treated monoliths. XANES spectra of Ti were very similar to that of crystalline PbTiO₃ in which Ti is coordinated with six oxygen atoms. Similarly, EXAFS analysis on the basis of Fourier transformation and double-shell fitting method revealed sixfold Ti atoms both in the heated products and even in gels.

Transparent and uniform thin films of crystalline KNbO₃ were synthesized on different substrates such as SrTiO₃, ZrO₂, MgO and Pt by the dip-coating method using a double alkoxides solution. Both KNbO₃ and Fe-doped KNbO₃ films were epitaxially crystallized at 750 degree(s)C for 3 hours in air on SrTiO₃ single crystal substrates. Excess potassium in the sol-gel solution (K/Nb equals 1.1) was important to obtain single phase films. The structure of the films has been studied with x-ray and electron diffraction, and transmission electron microscopy. Different molar ratios of 2-ethylhexanoic acid (2-EHA) to alkoxides were applied to control the hydrolysis rate of the sol-gel solution. Dense films were produced after using the modified alkoxides solution as stock solution.

Lead zirconate titanate thin films on silicon wafers and titanium foils were fabricated by the sol-gel technique. Polycrystalline and amorphous PZT thin films were obtained at the heat- treatment temperature of 650 degree(s)C and 400 degree(s)C, respectively. The microstructures of these films ere determined by x-ray and electron diffraction and HRTEM techniques. the optical refractive index of both polycrystalline and amorphous PZT films were measured by ellipsometer. The optical refractive index of polycrystalline films increases with increasing film thickness; however, the optical refractive index of amorphous films is independent of the film's thickness. the electrical properties of the PZT thin films were measured. for both polycrystalline and amorphous PZT thin films a ferroelectric hysteresis loop and pyroelectric current were observed. The dielectric permittivity of amorphous PZT is much lower than that of polycrystalline PZT. For the optical applications, the advantages of ferroelectriclike amorphous PZT materials are low processing temperature, ease of deposition on a variety of substrates (including glass and plastic), and transparency without grain boundaries. In this paper, the preparation, optical, and electrical characterizations, as well a microstructures of these thin films, are reported.

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 film's thickness was about 0.2 - 0.5 micrometers .

We have successfully incorporated Sm²⁺ ions into alumino-silicate glasses using a sol-gel method. The gels synthesized through hydrolysis of Si(OC₂H₅)₄, Al(OC₄H₉)₃, and SmCl₃(DOT)6H₂O were heated in air, followed by heating in the presence of hydrogen, in which samarium ions were reduced from the trivalent to divalent state. Glasses incorporated with Sm²⁺ showed an intense emission with peaks at 683, 700, and 725 nm due to ⁵D₀ yields ⁷F_0,1,2 transitions, respectively, of the Sm²⁺ ions. Decay time was estimated to be 0.6 msec.

(beta) -BaB₂O₄ ((beta) -BBO) thin films were successfully synthesized by the sol-gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of barium metal with boron triethoxide or 2,4,6-triethoxycyclotriboroxane in a mixture of ethanol and 2-ethoxyethanol. As-precipitated powder from the precursor solution by hydrolysis crystallized to (gamma) phase, which was transformed to (beta) phase. The transformation temperatures of powders from (gamma) to (beta) phase of the ethoxide system and the boroxane system were 600 and 680 degree(s)C, respectively. The calcination of precursor films in a H₂O-O₂ gas mixture was found to be effective for the decrease of crystallization temperature of (beta) -BBO films on Pt sheets. The precursor films prepared from the ethoxide system and the boroxane system crystallized to (Beta) -BBO on Pt(111)/glass substrates at 500 and 550 degree(s)C, respectively. The BBO films on Pt(111)/glass substrates showed c-axis preferred orientation. The (Beta) -BBO films on Pt(111)/glass substrates showed the second harmonic generation (SHG) of the intense 532 nm light on irradiation with 1064 nm Nd:YAG laser. The efficiency of SHG for the BBO films was also estimated.

Whole cell parasitic protozoa have been entrapped within sol-gel porous silica matrices. Stationary phase promastigote cells of Leishmania donovani infantum are mixed with a silica sol before gelation occurs. They remain trapped within the growing oxide network and their cellular organization appears to be well preserved. Moreover protozoa retain their antigenic properties in the porous gel. They are still able to detect parasite specific antibodies in serum samples from infected patients via an enzyme linked immunosorbent assay (ELISA). Antigen- antibody associations occurring in the gel are optically detected via the reactions of a peroxidase conjugate with ortho-phenylenediamine leading to the formation of a yellow coloration. A clear-cut difference in optical density is measured between positive and negative sera. Such an entrapment of antigenic species into porous sol-gel matrices avoids the main problems due to non specific binding and could be advantageously used in diagnostic kits.

Thin films composed of alternating layers of amorphous barium titanate (BaTiO₃) and amorphous silica (SiO₂) have been fabricated by the sol-gel technique. Thin films of pure amorphous BaTiO₃ were also fabricated by the sol-gel technique for comparison. A special dip-coating and heating apparatus was built in order to coat and fire films of up to 20 layers. The cycles were adjustable and time-controlled. Upon firing to 400 degree(s)C, the films remained amorphous. The thickness and refractive index of each film, varying with concentration of solution, were measured by ellipsometry. The films were characterized by x- ray diffraction, electron diffraction and scanning electron microscopy (SEM). UV-visible absorption spectra of the films also confirmed the multilayered structure.

In this paper an overview is presented of the state-of-the-art of optical sensors which employ sol-gel-derived coatings. The technique is particularly suited to the side-coating of optical fibers or waveguides in evanescent-wave sensors because precise control of sensitivity- determining parameters, such as the coating thickness and length, is achievable. Sensors based on entrapped organic and inorganic dyes, enzymes and other biomolecules have been reported. The main features of the process are illustrated by examples of chemical sensors and biosensors from the literature. In particular, the development of an oxygen sensor based on the quenching of fluorescence from a sol-gel-entrappd ruthenium complex is described. This sensor may be operated in intensity or decay-time mode. The latter offers many advantages over intensity-based sensing and may also be used to provide useful diagnostic information concerning the distribution/accessibility of the sensor fluorophore. Issues which require further investigation before this technology can proceed to the stage of industrial development are also highlighted.

Liquid crystal microdroplets trapped into silica gel glasses by sol-gel processes may be used for display applications. Gel-glass dispersed liquid crystals (GDLC) are switched between opaque and transparent states by applying external ac-voltage signals; no polarizers are required. Contrast ratio is enhanced in the projection mode as scattering from liquid crystal microdroplets is increased. Color emissive projection displays may be obtained by placing fluorescent RGB masks on the back plane; a UV lamp over the front plane excites the chromophors located under switched pixels, whereas unswitched pixels scatter the incoming UV light and the residual fluorescence. This setup takes advantage of the photostability of sol- gel glasses as compared to similar systems such as polymer-dispersed liquid crystals (PDLCs).

By the end of 1994, flat-panel displays for lap-top computers and televisions will exceed $4.0 billion in total sales. Currently, most such displays are based entirely upon an advanced liquid crystal technology requiring several precisely aligned LC layers. A new technology for flat- panel displays, based upon the solid-state luminescence of sol-gel derived microspheres has been proposed. Silica gel-derived microspheres can be prepared at near ambient temperatures, doped with either optically-active organic dye molecules or lanthanide ions. Microspheres doped to give red, blue, and green luminescence can be arranged in a pixel pattern to form the basis of a display screen. A single monochrome LC layer is used to modulate the excitation light, or pump source, that activates the fluorescence of each pixel.

A r-GRIN rod of PbO-K₂O-B₂O₃-SiO₂ system having an effective diameter of about 13 mm and a smooth and refractive index change of 0.095 was prepared by a sol-gel process. The introduction of acetic acid in a bath for the formation of compositional gradient in a gel was effective to hinder the migration of lead ions during fixing of the formed profile. The performance of the r-GRIN rod was confirmed to be satisfactory for the practical application by the test on a compact camera.

Micro Fresnel lenses were prepared from photosensitive organic-inorganic nanocomposites of the Ormocer-type by irradiation of films of 10 micrometers in thickness with an interference pattern of two laser beams, mixing a planar reference wavefront and a spherical object wavefront. To monitor the polymerization behavior of the organic groups during irradiation under real time conditions, a characterization method was developed to study changes in optical thickness nd by measuring changes of index of refraction and shrinkage during polymerization. Three-dimensional Fresnel structures were obtained by removing the unpolymerized areas by a solvent. Variation of index of refraction can also be obtained by diffusion of monomers in regions of higher light intensity. A model, similar to the Colburn- Haines model, is presented.

Solid state tunable lasers were prepared by the sol-gel method either in bulk form or as thin wave guiding films deposited on glass or quartz. The active materials were perylimide dyes providing laser emission between 550 - 630 nm. The lasers obtained were extremely photostable surviving more than 1 million pulses of the second harmonic Nd:YAG laser, providing 50 mJ/pulse. Slope efficiencies of more than 20% can be obtained when longitudinal pumping is used at appropriate geometries. New composite materials based on polycondensation of alkoxysilanes with organically modified silanes are proposed as the laser host for leaking wave guides and titanium oxide modified films for wave guide lasers.

The realization of low-loss optical waveguides by the sol-gel method and the subsequent production of strip waveguides by direct laser densification are described. These waveguides have been characterized by both the optical and structural point of view, and the results are discussed.

Advances in integrated optics devices require the development of materials and technologies that can transmit, guide, receive, multiplex, demultiplex, modulate, and demodulate optical signals. These requirements are crucial for the realization of advanced integrated optic devices that fully employ the potentially wide bandwidth (approximately THz) of optical signal processing and computing. The authors are developing an innovative new technology for the photolithographic fabrication of sol-gel derived integrated optic devices (sol-gel IODs). The fabrication procedure is based on direct photolithographic writing of IODs onto a photoactive sol-gel glass matrix. Sol-gel glasses are made by a two-step process: first a gel film is chemically formed and dried to a porous state, and second the porous film is densified into solid glass at high temperature. An organometallic photosensitizer is doped into the porous matrix after the first step. Waveguide patterns are then formed by straightforward photolithographic techniques, and the unexposed sensitizer is removed. The final densification step locks in the waveguide patterns, creating a durable, impermeable integrated optic device. Depending on the choice of sensitizer, these waveguide patterns can be passive (simply having a higher index of refraction than the surrounding host glass) or active (possessing optical properties that can be influenced by the application of electric fields). The uncomplicated nature of this process makes this a very promising approach for the fabrication of commercially viable integrated optic devices.

The fine-patterning process is very important for the application of sol-gel derived thin films to optical and electronic devices. In the present study, a new technique for the patterning of oxide thin films has been developed. Gel films obtained from metal-alkoxides chemically modified with (beta) -diketones have optical absorption bands in the UV-range characteristic of the (pi) -(pi) * transition in chelate bonds. The irradiation of UV-light corresponding to this transition dissociated the chelate bonds and simultaneously decreased the solubility of the gel films in acidic solutions or alcohols. These findings are indicative of the occurrence of some structural changes in the gel films. The change in solubility of organically modified gel films by the UV-irradiation has been applied to the fine-patterning process; the gel films were irradiated with UV-light and leached in appropriate solutions, followed by the heat-treatment. The present process enables us to make fine-patterns of ZrO₂ and TiO₂ films on a variety of substrates.

A series of sol-gel derived, La-doped PbTiO₃ films, with compositions Pb_1-x/100 La_x/100 Ti_1-x/400 O₃ (PLTX) with La content, x equals 0, 5, 10, 15, 20, 25, 28, 30, and 32 were prepared on Corning 7059 substrates. These films were fired at 400 - 600 degree(s)C for 30 mins. The phase assembly and phase development were dependent on the precursor chemistries (La content) and processing (firing temperature). The grain size of the crystallized PLT films decreased with higher La content, and PLT films with higher La contents appeared denser. The optical properties of the films were investigated using UV - VIS transmission spectroscopy. Depending on composition and processing conditions, PZT films (2500 A thick) with refractive indices of 2.00 - 2.56, sufficient for waveguiding on Corning 7059 glasses with refractive index of 1.53, and band gaps of 3.11 - 3.55 eV can be obtained. Exhibiting high refractive indices, these amorphous and crystalline PLT films can be utilized for both passive and active optical applications.

We show that including highly polarizable dopants in the pores of a xerogel matrix leads to interesting optical properties and applications, such as an all-optical molecular memory device. By submitting the sample to ultrashort linearly polarized optical pulses, we demonstrate that the dopants tend to align along the direction of the electric field of the optical wave like in classical optical Kerr effect, and that the new alignment of the molecules is partially maintained. We study the influence of the matrix-dopant interactions on this phenomenon. For example, for rhodamine-like molecules, we establish that the alignment ability as well as the relaxation times depend strongly on the structure of the gel network and on the coupling intensity between the host matrix and the guest molecule.

We report a novel photolithographic technique that facilitates the photogeneration of refractive index gratings in sol-gel derived silica fibers, based on the ability to produce changes in the index of refraction of a silica glass of the order of 0.1 to 0.001. A porous sol-gel fiber impregnated with a photosensitive organometallic compound exhibits optical changes when exposed to light. Changes in transmission and refractive index can be induced and are permanent after consolidation of the porous glass matrix. Gratings have been produced in doped sol-gel fibers by exposing the fiber to UV light through a contact mask. The photoinduced grating is permanent after heat treatment of the fiber to 1200 degree(s)C. The uncomplicated nature of this process makes this a very promising approach for the fabrication of commercial fiber optic refractive index gratings for fiber lasers, fiber optic sensors, filters, and multiplexing schemes.

Solution chemistry techniques are of interest for the deposition of thin film planar waveguides due to the ease of fabrication, the compositional range available, and the routine synthesis of guides with losses < 0.5 dB ((lambda) equals 632.8 nm). In this work, the wavelength dependence and attenuation of sol-gel waveguides were measured as a function of composition and time in storage, or aging. The compositions investigated in this work were (in mole %) 65SiO₂:35TiO₂, 50SiO₂:50TiO₂ and 41.5SiO₂:41.5TiO₂:17Al₂O₃ waveguides. Initial losses of the waveguides were < 0.5 dB/cm ((lambda) equals 632.8 nm). The initial wavelength dependence of the guides was also measured. In waveguides where the losses are intrinsically or Rayleigh scatter limited, a dependence of (lambda) ⁴ is expected. However, the wavelength dependence measured for the sol-gel guides varied with composition. After periods in storage, waveguide attenuation was remeasured, and the guides were found to have deteriorated, with higher losses. The rate of deterioration varied with composition. Possible causes of the deterioration of the guides are presented and discussed.

In pursuit of the ideal solid state dye laser, two possible host matrices were investigated. Porous silica with average pore diameters of 50 and 75 angstrom were doped with both rhodamine 6G and rhodamine 6G in an ORMOSIL composition, and their fluorescence and lasing properties were characterized. Measurements included absorption/scattering loss of undoped hosts, and fluorescence spectra, lasing efficiencies, and photostability of the dye- doped materials. Excitation was done with a doubled Nd:YAG at 532 nm. The center of the broadband laser emission (FWHM of 5 nm) was 560 nm. The optimum material in the form of a 12 mm diameter X 6.3 mm thick disk gave a lifetime to 50% power for a single 1 mm diameter irradiation site of 25,000 shots with an initial efficiency of 39%. The same material in the form of an 8 mm diameter X 15 mm long rod gave a lifetime of 95,000 shots and initial efficiency of 31% under the same conditions.

Characterization of erbium-doped silicate glasses produced by sol-gel method with a drying control chemical additive has been carried out. Both of the absorption and fluorescence spectra of these samples were similar to those of erbium-doped optical fibers prepared by modified chemical vapor deposition. The homogeneous monolithic erbium-doped silicate glasses with concentrations ranging from 500 to 10,000 ppm were constantly fabricated. Due to N,N- dimethyl formamide (DMF) in 2-step hydrolysis procedure, the processing time can be reduced significantly. The index of refraction and specific density of the samples were approximately 1.65 and 2.2 g/cm³, respectively.

LiAl₅O₈:Fe³⁺ film has been prepared by using the sol-gel method, and the optical properties of the film have been examined. X-ray diffraction analysis indicates that (110)-oriented LiAl₅O₈ film is formed when the gel film is heat-treated at 773 K. The x-ray diffraction patterns of the LiAl₅O₈ films heat-treated above 1173 K manifest diffraction lines assigned to reflections from the (110), (220) and (440) planes. In contrast, only the diffraction lines corresponding to the (220) and (440) planes are observed in the LiAl₅O₈ film heat-treated below 1073 K. These results suggest that the LiAl₅O₈ films heat-treated below 1073 K and above 1173 K possess disordered and ordered structures, respectively. The emission spectra of the LiAl₅O₈ films obtained by the heat treatment above 1173 K manifest a zero-phonon line at around 660 nm with vibronic sidebands in the long wavelength region. These emission bands are ascribed to the transition from ⁴T₁(⁴G) to ⁶A₁(⁶S) of Fe³⁺ in tetrahedral site in the ordered phase. The LiAl₅O₈:Fe³⁺ films obtained by the heat treatment below 1173 K, which contain microcrystals of less than 10 nm, are optically transparent.

Infrared absorption spectra were recorded for silica and silica-titania sol-gel films spin-coated on silicon substrates. Information of two types is derived from those spectra concerning the porosity of the films, which was independently calculated from ellipsometry results: (1) the IR spectra allow the determination of the `infrared thickness' of the porous films, i.e., the thickness that they will exhibit after full densification (independent of the film porosity), which is always less than the thickness obtained by mechanical profilometry (or ellipsometry); also, the volume percent porosity can be estimated from those two thickness values; (2) as the films are densified at increasing temperatures, the IR thickness remains constant, but the profilometer (and ellipsometer) thickness decreases, whereas the frequency of the dominant Si- O stretching mode near 1070 cm^-1 passes through a minimum, at approximately equals 500 degree(s)C for silica films and at approximately equals 450 degree(s)C for silica-titania (20 mol% TiO₂), suggesting a maximum porosity for those heat treatment temperatures; this was confirmed by a minimum in the refractive index, in the case of pure silica films only. The significance of these findings is discussed.

Time-resolved, fluorescence line narrowing (FLN) spectra and fluorescence lifetimes are used to characterize local structure in Eu³⁺-doped silica gels. Co-doping with Al³⁺ is shown to have a significant effect on the local structure of the Eu³⁺ ion in the silica matrix. A pulsed dye laser was used to selectively excite subsets of ions contributing to the inhomogeneously broadened ⁷F_O$ARLR⁵D_O transition. Resultant line-narrowed fluorescence spectra and lifetimes are interpreted in terms of site symmetry, energy transfer, Eu³⁺ ion clustering, and residual hydroxyl groups.

The geometry and electronic spectra of small clusters comprised of hydrated Cr³⁺ interacting or binding with siloxane structures have been calculated with a semi-empirical molecular orbital method. The atomic parameters for silicon, oxygen, and chromium were first optimized using crystallographic and gas phase geometries. The electronic spectrum of the chromium hexa-aqua species was computed and matched the experimental spectrum within a few 100 cm^-1 of the UV and visible absorption maximums. The theoretical models reproduced the experimental peak shifting of the ⁴A_2g yields ⁴T_2g and ⁴A_2g yields ⁴T_1g transitions and elucidate the changes in the environment that the chromium ion might experience within the sol-gel porous network.

Sol-gel derived SiO₂ glass objects are of interest because of their high purity. However, cracking of wet gels during drying, or of dried gels during hydrocarbon removal and sintering, are the main obstacles preventing the commercialization of this process. These difficulties can be overcome using gels with high strength and large pores. In this study, gels were prepared using Si(OC₂H₅)₄, C₂H₅OH, H₂O and acidic catalysts. The strength, shear modulus, microstructure, drying and sintering behavior of these gels were investigated as functions of catalyst type and aging time. Experiments indicate that the modulus of rupture must be high enough to overcome drying stresses. In addition, the shear modulus must be high enough so that the gels are sufficiently stiff to prevent bending during drying. Repetitive and reliable drying of relatively large gels (D equals 8 - 10 cm, L equals 40 cm) was achieved within a week. However, to obtain crack-free sintered glasses the xerogels must have larger pores (pore radius > 40 angstrom). This is required for easy removal of organic groups at 200 - 500 degree(s)C and of chemically bonded hydroxyl or silanol groups at 800 - 1200 degree(s)C.

In this work the sol-gel process was used to obtain porous silica gels through acid catalysis of tetramethyl orthosilicate, which was produced from silicon tetrachloride. The influence of the type and concentration of acid catalyst on macroscopic appearance, gelation kinetics, and ultrastructure of the gels was analyzed. The acids employed in this work were nitric, hydrochloric, hydrofluoric, and acetic. The porous gels were infiltrated with a monomer solution of MMA and polymerized. The structural characterization of gels and silica/PMMA composite was carried out in terms of density, specific surface area, pore volume and average pore size. Physical properties such as microhardness, optical transmission and refractive index were evaluated. The structural and physical properties were associated to the different catalysts used. A wide range of gel structures was obtained by changing the volume fraction, average size, and connectivity of pores.

Understanding the stability of porous Type VI silica gel matrices during diffusion of water is important because this new material is being used as a host for optically active composites. Aqueous solutions are being used during processing as a carrier for optically active phases for: filters, graded index lenses, laser dyes, and scintillators. A pore wall model has been developed to calculate the wall thickness of Type VI porous silica-gel matrices. The Austin Method (AM1) Molecular Orbital calculations have been performed to model silica structures with dimensions close to those expected for the pore walls. A base line value for the mean pore wall thickness was determined from comparisons between the mode and experimental results. This value separates environmentally stable matrices from unstable gels during water diffusion (or percolation). A ring opening hydrolysis mechanism is proposed to explain the role of water in fracture of the porous sol-gel silica matrices.

The sol-gel process allows the production of alumina with high purity and homogeneity, controlled nanostructure and in several shapes. These aspects are important to provide some characteristics to alumina that enable the application of this material in advanced devices, such as in opto-electronic devices. In this work, aluminum isopropoxide was synthesized and used in the preparation of alumina-gels. The effect of some of the variables of the mixture step such as hydrolysis temperature, pH and water/alkoxide ratio, in peptization and gelation, were evaluated. The obtained gels were dried at 105 degree(s)C and analyzed by x-rays diffraction, infrared spectroscopy, nitrogen adsorption and thermogravimetric analysis. It was noted the influence of temperature of alkoxide hydrolysis in the behavior of hydrolyzed mixture. So, acid addition enhances the gelation time of mixtures hydrolyzed at 30 degree(s)C and reduces the gelation time of mixtures hydrolyzed at 90 degree(s)C. The structural analysis showed the presence of an amorphous phase in xerogels derived from mixtures hydrolyzed at 30 degree(s)C and the presence of pseudoboehmite in xerogels derived from mixtures hydrolyzed at 90 degree(s)C. The different behavior of mixtures hydrolyzed at low and high temperatures was suggested to be caused by different mechanisms of surface charge formation in the structurally different hydroxides. It also could be noted that pseudoboehmite gels have less structural water and volatiles than amorphous gels. Pseudoboehmite gels produced in low pHs should be formed by a randomly and less compact arrangement of particles with plate shape.

Successful fabrication of nonlinear optical devices such as nonlinear waveguides and switches requires high nonlinearity of semiconductor quantum dot nanocomposites and their ability to form waveguides. Nano-sized CdS quantum dots with narrow size distribution can be fabricated via the sol-gel process by using a bifunctional ligand which anchors the Cd²⁺ ions homogeneously to the matrix. In this investigation, 3- aminopropyltriethoxysilane, H₂N(CH₂)₃Si(OCCH₅)₃, has been used to synthesize CdS quantum dots with small diameters and narrow size distributions in sodium borosilicate gels and fully dense glasses. This matrix composition allows waveguide fabrication by ion exchange of sodium with potassium and has the advantage of low- temperature densification to form optical quality glasses. In 8 nominal wt% CdS-doped 5Na₂O-30B₂O₃-65SiO₂ glass, the dots have an average size of 2.8 nm, a maximum in the distribution in the range of 2.0 - 2.5 nm, and a standard deviation of 0.9.

Cadmium tin oxide powder has been synthesized by the sol-gel route using the polyacrylamide gel. Details of the preparation and preliminary characterization of the product are reported.

Rhodamine 6G-doped hybrid gels were prepared by a sol-gel route. The inorganic component tetraethyl orthosilicate (TEOS) and the organic component glycerol propoxy triacrylate (GPTA) were simultaneously polymerized to create interpenetrating silica-GPTA networks. Crack-free, transparent dried gels in the shape of rods and fibers were obtained. Lasing properties of the hybrid rod-shaped monoliths were determined as a function of rhodamine concentration. In addition, gels in the shape of fibers were evaluated as fiber lasers for detecting ammonia vapor.