In the past decade, sol-gel processing has been actively employed for the preparation of new ceramics and for the fabrication of ceramics into many complex geometries. Sol-gel processig is particularly suitable for optical materials. The application of sol-gel technology to the synthesis and fabrication of optical materials is termed “Sol-gel optics.” A review is presented on the basic principles of the sol-gel process and examples are given for some important materials in sol-gel optics. These include many configurations of silica glass, organic-impregnated glasses, organically modified ceramics (ORMOCERS), ferroelectric thin films, amorphous ferroelectrics, nonlinear optical nanocomposites and gradient index lenses. Some future developments in sol-gel optics are discussed.

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Current state and the future prospect of sol-gel process as the fabrication method of r-GRIN elements with large geometry were discussed. The method based on the partial leaching of index modifying cations such as Ti, Ge, Zr from alkoxy-derived silicate gels is advantageous in fabricating r-GRIN rods with large geometry and precise refractive index profile. The largest delta-n attained by this method is estimated to a 0.08-0.1. The method based on the interdiffusion of index-modifying cations in the liquid phase of a wet gel is suitable to fabricate materials with large geometry and large delta-n. The interdiffusion method is particularly suitable to fabricate r-GRIN rods with prescribed spectral variation.

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Growth techniques of crystals for optical applications are reviewed. LiNbO₃ and LiTaO₃ for electro-optical devices are grown by the Czochralski method. KDP, LiNbO₃, KTP, and BBO for nonlinear optical applications are grown from aqueous solutions, melts and high temperature flux solutions. Garnets, sapphire and crysoberyl are main host crystals for lasers, and PbMoO₄ and TeO₂ are common crystals for acousto-optical applications. They are usually grown by the Czochralski method. Calcite and rutile provide polarizing elements and are grown, respectively, from either high temperature or hydrothermal solutions and from melts. Quartz crystals for phase plates are grown by the hydrothermal method. BaTiO₃, SBN and BSO are photorefractive and photoconductive crystals. BaTiO₃ is grown from high temperature solutions and the rest by the Czochralski method. The rare earth iron garnets give optical isolator elements. The bulk crystals are grown by the floating zone method and thin crystals by the liquid phase epitaxy. Crystals for scintillators, CdWO₄, Bi₄Ge₃O₁₂, and Ce: Gd₂SiO₅, are grown mostly by the Czochralski method. Al₂O₃ for windows is grown in a thin plate form and MgO for similar purposes by the electric fusion technique.

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Electro-optic waveguide switches and modulators, using LiNbO₃ crystal, which are the key components in the future optical communication systems, are reviwed. The advantages of the Titanium diffused waveguides include, no degradation of electro-optic effect, low propagation loss, low fiber-coupling loss and ease of fabricaiton. The operating principles and general design considerations of directional couplers switches, cross waveguide switches and balanced-bridge type switches are briefly reviewed. The design and development considerations of a simple polarization independent switch with low drive voltage for OTDR applications, is described. Optical matrix switches for photonic switching systems and optical modulators for high speed optical communication systems are also briefly reviewed.

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The influence of the rare earth elements on the non-resonant type third-order optical nonlinearity was studied by third harmonic generation in germanate glasses. The nonlinearity basically increased by the increase in La₂O₃ content and decreased by the increase in field strength of the rare earth elements. The relationship between χ⁽³⁾ and refractive index of the glasses distinctly deviated from that for other germanate glasses. Large hyperpolarizability of the bonding pairs of oxygen and rare earth metal ions seems to contribute to the raise of χ⁽³⁾.

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The glass-forming region in the system TeO₂-BaO-TiO₂ and the crystalline phases obtained by heat treatment of glasses were examined to fabricate transparent TeO₂-based glasses containing ferroelectric BaTiO₃ crystals. The glasses containing up to 30mol% BaO and 20mol% TiO₂ were obtained. The glass transition temperatures, 341°-463°C, and crystallization temperatures, 463°-589°C, increased with increasing BaO and TiO₂ contents. By controlled crystallization, transparent 70TeO₂-15BaO-15TiO₂ glasses containing BaTiO₃ crystals with a diameter of about 5-10μm were successfully prepared. The small difference in the refractive indices, n, between the TeO₂-based glass, n=2.1, as matrix and BaTiO₃ crystals, n=2.39, leads to transparency, even though the particle size is much larger than the wavelength of visible light.

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Au microcrystal-doped glass films with high optical nonlinearity were prepared by the sol-gel method with dip-coating using NaAuCl₄⋅2H₂O and tetraethyl orthosilicate (TEOS) as starting materials. The glass film with an Au/Si atomic ratio of 0.01 was successfully prepared. The nonlinear susceptibility χ⁽³⁾ of the film was 7.7×10^-9esu. Microcrystals with a radius of about 7Å and of about 30Å coexisted in the glass film. The nonlinear susceptibility of Au microcrystals itself in the film, χm⁽³⁾, was 1.0×10^-7esu and was two times larger than that observed in Au-doped glasses prepared by the melting method. Subsequent heat treatment of the films at 1000°C changed the absorption spectra and decreased χ⁽³⁾ by a factor of two. The atomic ratio of Au/Si was able to be increased to 0.04.

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The third order nonlinear optical susceptibilities χ⁽³⁾ of PbO-SiO₂ and PbO-B₂O₃ glasses have been measured by the third harmonic generation (THG) method using silica glass as reference. It is found that the third order nonlinear susceptibilities χ⁽³⁾ of each series of glasses increase with increasing lead content, corresponding to the increase in the polarizability per unit volume and the decrease in the energy gap of the glasses. The largest χ⁽³⁾ values obtained are 31.1×10^-14esu for a 66PbO⋅34SiO₂ glass and 31.2×10^-14esu for a 70PbO⋅30B₂O₃ glass, about 11 times larger than that of pure silica glass.

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Third-order nonlinear optical susceptibility, χ⁽³⁾, of the sol-gel derived α-Fe₂O₃ thin film has been evaluated by the third-harmonic generation (THG) method. χ⁽³⁾ of α-Al₂O₃ single crystal (1120) with the same corundum structure as α-Fe₂O₃ has also been determined. The χ⁽³⁾ of the α-Fe₂O₃ film was 5.8×10^-11esu, which is comparable with those of organic materials with high nonlinearity. The large χ⁽³⁾ value of the α-Fe₂O₃ film may be ascribed to the high refractive index of 3.00 and to the three-photon resonance as well. The χ⁽³⁾ of α-Al₂O₃ single crystal was 2.9×10^-14esu, which is three orders of magnitude lower than that of α-Fe₂O₃ film and almost the same as that of fused silica. The small χ⁽³⁾ value of α-Al₂O₃ single crystal was attributed to the small refractive index and wide optical band gap.

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CdS or CdSe-doped polymer films (acetyl cellulose and polyvinyl alcohol) were prepared by a mutual counter diffusion method. CdS or Ga-doped alumina films were produced by a sol-gel process. Each CdS- or CdSe-doped film exhibited photoluminescence by excitation with an Ar⁺ laser. In particular, the CdS-doped alumina film did not emit photoluminescence derived from a deep trap level, but only one derived from the absorption edge. Moreover, photoluminescence of the Ge-doped alumina film was first observed.

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Quantum dot materials based on PbS-doped sodium borosilicate glasses were prepared by the sol-gel process using silicon methoxide (TMOS), boron ethoxide, sodium and lead acetates as starting materials. The chemical state of Pb species in the glasses fully densified at 590°C was identified to be PbS by X-ray photoelectron spectroscopy and electron diffraction. The amount of PbS crystallites in the glasses was 4-13wt% of mother glasses. The size of the crystallites was 5-15nm in diameter. The absorption edge of the glasses was near 2500nm, which was slightly shifted to higher energy from 3200nm for the bulk PbS crystal.

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In this work, MNA (2-methyl-4-nitroaniline) and substituted carbazoles were incorporated into silica matrix to form composite thin films on the glass substrate by a sol-gel processing for NLO applications. In the composite thin films, the thermal stability was dependent on the type of functional groups incorporated into the organic compounds. Carbazole-2 (5-nitro-9-hydroxylethyl-carbazole) which has a terminal -OH group showed enhanced thermal stability, whereas carbazole-1 (5-nitro-9-ethyl-carbazole) and MNA, which do not have a reactive functional group, showed rapid degradation upon thermal treatment. This strongly suggests that the thermal stability of incorporated organic compounds can be enhanced by a reactive terminal group in organic compound without severely damaging the NLO property.

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The optical properties of Er³⁺ ions doped in aluminate glasses were investigated.
(1) Among three intensity parameters obtained from absorption spectra by Judd-Ofelt theory, Ω₂ and Ω₄ showed a large value compared with those in other oxide glasses.
(2) The multiphonon decay rates of the ⁴S_3/2 and ⁴F_9/2 levels were obtained from the radiative transition probabilities and concentration dependence of nonraiative decay rate. The values were nearly equal to those in a germanate glass. The energy gap dependence of the multiphonon decay rate was consistent with the phonon energy of this host, 800cm^-1 obtained from the phonon sideband measurement.
(3) The upconversion fluorescence at 550nm was observed with excitation at 650nm corresponding to the ⁴F_9/2←⁴I_15/2 absorption, for which the Ω₄ value dominate the transition probability.
(4) The profile of the excitation spectra of upconversion fluorescence by a tunable dye laser showed a deviation from that of the ground state absorption. This result indicates that the excited state absorption from the ⁴I_13/2 level contributes the upconversion in the present glasses.

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The energy transfer processes among Nd³⁺, Yb³⁺, and Er³⁺ ions in glasses were investigated by fluorescence spectra and decay times. The results were interpreted on the basis of a kinetic model which describes the interactions between rare earth ions in terms of microparameters characterizing the probabilities of energy migration. The efficiencies of Nd³⁺→Yb³⁺ energy transfer were found to increase in the order of phosphate<fluorohosphate<germanate<silicate<aluminate. All the observed emission intensities originating from the ⁴I_13/2 level of Er³⁺ were mainly determined by the efficiencies of energy transfer from Yb³⁺ to Er³⁺ in both Yb³⁺+Er³⁺ and Nd³⁺+Yb³⁺+Er³⁺ codoped glasses, and strongly dependent on the concentration ratio of Er³⁺/Yb³⁺ for triply doped glasses.

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The processes of excitation energy transfer among Nd³⁺, Yb³⁺ and Er³⁺ ions in multicomponent aluminate glasses without any classical glass formers were invetiated experimentally, using a conventional spectroscoy and a kinetic model which describes the interactions between active ions. The efficiency of Nd³⁺→Yb³⁺ energy transfer is found to be so high as to nearly reach 100%. In Yb³⁺ and Er³⁺ codoped system, a mutual migration of energy occurs. The energy transfer from Yb³⁺ to Er³⁺ is considerably less efficient, owing to the influence of efficient back-energy transfer.

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Silicon carbide deposits were grown on graphite substrates in a horizontal water-cooled quartz tube by thermal decomposition of high purity methyltrichlorosilane (MTS) carried by hydrogen. Morphological and infrared property analyses of films deposited by different process parameters were performed by using scanning electron microscopy (SEM) and IR spectrometry. The morphology of deposits, such as smooth-featureless, nodular, dendritic, faceted, and particles, etc., obtained by the CVD process was dependent on the experimental variables. The dependence of the kinetics of deposition on substrate temperature, reaction pressure and H₂/MTS ratio was investigated.

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New aluminate glasses were obtained with the conventional melt-quench technique in the system SrO-ZnO-Al₂O₃. The glass forming region was determined. The glasses were colorless and had good chemical durability in laboratory conditions. When irradiated with UV-light, a broad absorption was induced with a peak at 470nm (2.64eV) and an ESR signal (77K and 300K) was induced with an apparent g-value of 2.006. Both the changes in optical density and the intensity of the ESR signal increased with the ZnO content. From the characteristic of ESR signal, coloring of glass was concluded to a due to the formation of hole trapped on oxygen and electron center resting on zinc ions.

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Transparent glass-ceramics containing ZnAl₂O₄: Cr³⁺ have been prepared by heat treatment of the glass with a composition of 20ZnO⋅20Al₂O₃⋅50SiO₂⋅6TiO₂⋅4ZrO₂⋅0.1Cr₂O₃ (molar ratio). X-ray diffraction analyses indicated that ZnAl₂O₄ microcrystals with a crystallite size of 5 to 30nm precipitated in the specimens heat-treated at 850° to 1150°C. Cr³⁺ ions were incorporated into the ZnAl₂O₄ phase in the transparent glass-ceramics as demonstrated by the lattice constants determined from the X-ray diffraction. The emission due to the ²E→⁴A₂ transition was observed at room temperature for all the heat-treated specimens under the excitation of light at wavelength 507nm from a Xe lamp, representing that Cr³⁺ ions replaced the Al³⁺ ions in the ZnAl₂O₄ microcrystals. The emission spectra were similar to that of single crystalline Cr³⁺-doped spinel.

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Copper-ruby glasses, which exhibit an absorption maximum at about 570nm, were prepared by simultaneous sputtering of copper and silica glass. The glasses turned red by heat treatment for less than 30min at 650°-800°C. The optimum heat treatment temperature was 700°C. Changes of optical absorption with heat treatment time were measured. X-ray diffraction analysis identified precipitated particles to ba Cu particles. The nonlinear optical property of these glasses was evaluated by the degenerate four-wave mixing method. The |χ⁽³⁾| value of this glass was 4.0×10^-9esu at the wavelength of 532nm

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The effects of the growth parameters of the Czochralski method on bubble formation were investigated. Pure Al₂O₃ single crystals were bubble free under both atmospheres of N₂+1.5%O₂ and N₂+20%H₂. Although Ti doped Al₂O₃ single crystals grown under an atmosphere of N₂+20% H₂ were bubble free, the crystals grown under an atmosphere of N₂+1.5%O₂ contained many bubble. The effective segregation coefficient of Ti ions was estimated to be greatly reduced under non-reducing atmospheres, possibly due to the valence state change of Ti from 3+ to 4+. The convection of the melt was remarkably enhanced under the reducing atmosphere and an influence of the atmosphere to the melt properties was suggested. The condition for the bubble formation is concluded to be similar to that of constitutional supercooling.

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Calcite single crystals could be hydrotherally grown in newly discovered NH₄NO₃ solution even at 150°C. The solubility of calcite in NH₄NO₃ solution was 4.0g/l at 100°C and 30MPa, and increased with increasing temperature and pressure. The dislocation densities of grown calcite crystals on as-cleaved and etched seeds were 4×10⁵ and 2×10⁴cm^-2, respectively. The growth rate on etched seeds was dependent upon both growth temperature and concentration of solution. The maximum growth rate was 45μm/day in 0.2M NH₄NO₃ solution at 200°C and 15MPa. Calcite crystals grown on etched seeds had the same order of dislocation density as that of high grade natural crystals, and were free from OH^- and NO₃^- ions. This moderate growth condition allows us to employ a Teflon liner for avoiding the contamination from the pressure vessel to grow the optical grade of calcite single crystals.

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MgTa₂O₆ single crystals, which adopt trirutile structure, were successfully grown by a floating zone method, and their optical properties were invetigated. The as-grown crystals were black and opaque, but subsequent annealing in air from 1200° to 800°C rendered the crystals pale-yellow and transparent. The refractive indices for ordinary and extraordinary rays were 2.07 and 2.18 under white light, respectively, and the birefringence was 0.11. The interference figure observed with a polarizing microscope was typical one for an optically uniaxial crystal, which indicates that the crystal was optically uniform. Transmittance in the wavelength range of 500 to 2000nm was about 75%, which approximately corresponded to the theoretical value calculated from the refractive index. The absorption edge of MgTa₂O₆ was at 290nm.

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Diffusion of aluminum, scandium and gallium ions in single crystals has been studied. Stoichiometric crystals have been grown by the Czochralski method using a double crucible and automatic powder supply system. Diffusant concentration profiles were measured by secondary ion mass spectrometry. The diffusivities of all ions depended on the Li/Nb ratio of single crystals. The diffusion coefficients were larger in the samples with lithium deficient composition than in that with stoichiometric composition. This behavior was explained by a cation vacancy model in the lithium niobate lattice.

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PLZT ((Pb, La) (Zr, Ti) O₃) ceramics was synthesized by the coprecipitation method using nitrates and ammonia. Dehydration was promoted by replacement with butanol and ethanol to prevent the aggregation of primary particles. Precursor gel consisted of well dispersed particles less than 10nm was obtained by replacement with butanol. A powder treated in butanol and calcined at 500°C for 2h was sintered to full density at 1150°C for 6h. Transparency of sintered body was related to its porosity. The sintered body obtained at 1200°C for 20h showed the transmittance of 20%. Mechanochemical effects on the synthesis and pulverization of PLZT ceramics were also examined. Grinding of the precursor gel powder promoted mechanochemical reaction. Grinding of calcined powders caused the decrease in the crystallinity. Sinterability was reduced by grinding, but annealing of calcined and ground powder recovered the original sinterability.

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The wetting of (Pb, La) (Zr, Ti)O₃ ceramics with various electrode metals were studied by measuring the contact angle in a vacuum atmosphere. Two kinds of PLZT were used: x=0.07, y=0.35, and x=0.12, y=0.60 in (Pb_1-x, La_x) (Zr_1-y, Ti_y)O₃. The metals studied were Ag, Al, In, Sn, Bi and Cu. The metals forming thermodynamically unstable oxides showed low contact angles: 98° with Ag and less than 90° with Cu. The contact angles were dependent on the composition of PLZT. The other metals indicated the obtuse contact angles as large as 140° since the metals forming the stable oxides can not move by being protected with the oxide. The wettability of a metal on the oxide ceramics can be predicted by the redox reactions of metal and a certain element in the ceramics. In and Sn were bonded with the PLZT but not with BaTiO₃.

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Self diffusion coefficients of oxygen in PLZT ceramics were evaluated to estimate the vacancy structure by the oxygen-18 isotope exchange reaction and the isotope analysis by the secondary ion mass spectroscopy (SIMS). In the piezoelectric composition of PLZT (10/50/50), the oxygen vacancy concentration was estimated as about one ppm/mol by means of oxygen diffusion measurements and it did not change by doping impurities except for manganese. It was considered that point defects existed at all sites of the perovskite structure of PLZT. On the other hand, the oxygen vacancy concentration increased to 100ppm/mol for manganese doped samples, and the reason was considered that manganese ions were reduced during sintering and that the oxygen vacancies were created to compensate the charge imbalance. In the transparent PLZT (9/65/35), the oxygen vacancy was ten times as much as that in PLZT (10/50/50). The increase in oxygen vacancy was ascribed to volatilization of lead or variation of solubility of lanthanum in B site. All samples showed extraordinary oxygen diffusion at high temperatures near the surface, which was ascribed to the volatilization of PbO.

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An experimental study on the utilization of an electro-conductive surface layer on a dielectric Si₃N₄ component formed by carbon ion (C⁺) implantation as a crack gage and a strain gage was conducted. Surface cracks were generated by the Vickers indentation. The electrical resistance of the electro-conductive layer increased with the generation of the surface cracks. The resistance was also influenced by stresses which appeared in the layer. Variation of electrical resistance caused by cracking and stresses was so small that the resisitance must have been measured with high accuracy.

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Peroxo polytungstic acids are promising inorganic resist materials which become insoluble in water by UV irradiation. After UV irradiation, part of oxygen molecules, decomposition product of the peroxo groups, was stably caged in the amorphous network. This mechanism is discussed in connection with the densification of thin films during UV irradiation.

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