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We investigated the Pockels effect in polycrystalline materials consisting of highly oriented polar fresnoite-type Sr2TiSi2O8 fabricated using perfectly surface-crystallized glass-ceramics (PSC-GCs). The chemical composition of the precursor glass was shown to significantly affect the crystallized texture, e.g., the crystal orientation and appearance of amorphous nanoparasites in the domains, resulting in variations in the Pockels constants. Single crystals exhibiting spontaneous polarization possessed large structural anisotropy, leading to a strong dependence of the nonlinear-optical properties on the direction of polarized light. This study suggests that variations in the Pockels constants (r13 and r33) and tuning of the r13/r33 ratio can be realized in PSC-GC materials.

The control of heat flow has become increasingly important in energy saving and harvesting. Among various thermal management materials, spinon thermal conductivity materials are promising for heat flow control at microscales because they exhibit high, anisotropic thermal conductivity resulting from spin chains. However, there has been only little development of the materials for controlling heat flow. Here, we present the line patterning of the spin chain structure on a SrCuO2 nanocrystalline film by laser scanning. When the polarization direction of laser light was orthogonal to the scanning direction, we found that the spin-chain structure anisotropically grew on the patterned line.

A transparent glass–ceramic containing borate crystallites in tellurite glass is reported. Controlled heat treatment resulted in surface crystallization of BaB4O7 in the BaO–B2O3–TeO2 glass system. The second-order optical nonlinearities observed in these surface crystallized glasses were attributed to the distortion present in the BaB4O7 unit cell as evidenced by x-ray powder diffraction studies. A d33 value of 0.08pm∕V was obtained from the Maker fringe analysis for a surface crystallized glass.

The technique for the writing of crystal lines in glass by Nd:YAG laser irradiation, selective atom heat processing, is applied to rare-earth (Sm3+, Dy3+) barium borate glasses, and nonlinear optical β-BaB2O4 (β-BBO) crystal lines with a width of ∼5 μm are fabricated. The crystal lines show uniform colors in polarized optical microscope and second-harmonic generation (SHG) measurements. The azimuthal dependence of SHG signals in the crystal lines is the same as that in commercially available β-BBO bulk single crystals. It is demonstrated that the crystal lines written by YAG laser irradiations are β-BBO single crystals with the c-axis orientation along the laser scanning direction.

Crystal lines with second-order optical nonlinearity have been successfully fabricated at the surface of 10Sm2O3.35Bi2O3.55B2O3 glass by continuous irradiation of Nd:YAG laser. The laser-induced crystalline phase was confirmed to be Bi0.7Sm0.3BO3 by x-ray diffraction measurements, and second-harmonic generation (SHG) from the phase was clearly observed. An array structure of crystal lines was fabricated by laser writing under automatic computer control, and Maker fringe patterns of SHG were observed, indicating that the direction of polarization in the structure with a crystal line array was parallel to the sample surface. In addition, we measured polarization optical microphotographs, and found uniform phase retardation for a whole length of crystal lines. It is strongly suggested from these results that crystal lines by laser irradiation are formed in single-domain crystalline phase (single crystal) with second-order nonlinearity.

We examined the crystallization and crystal growth of ZnO in the viscoelastic supercooled liquid (SCL) phase based on microscopic and spectroscopic observations. The resulting glass–ceramics exhibits complex ZnO dendrites that precede the formation of Zn-rich regions because of phase separation. Compressive stresses are concentrated at the interface of ZnO crystals and SCL phase, suggesting that the domains are subject to a resistance during ZnO crystal growth. Defects were detected in the crystallized ZnO and attributed to the suppression of smooth ordering of the Zn-rich regions in the SCL phase.

High thermal conductivity materials are in great demand for heat-flow control and heat dissipation in electronic devices. In this study, we have produced a glass-ceramics that contains spin-chain compound SrCuO2 and have found that the glass-ceramics yields high thermal conductivity of ∼5 W K−1 m−1 even at room temperature. The glass-ceramics is fabricated through crystallization of inhomogeneous melt-quenched oxides made from SrCO3, CuO, Li2CO3, Ga2O3, and Al2O3. Transmission electron microscopy and X-ray and electron diffraction reveal that SrCuO2 crystallites with a size of 100–200 nm are precipitated in the glass-ceramics. The highness of the thermal conductivity is attributable to two sources: one is elongation of phonon mean free path due to the crystallization of the inhomogeneous structure or structural ordering. The other is emergence of the heat carriers, spinons, in the SrCuO2. This highly thermal conductive glass-ceramics is expected to be utilized as base materials for heat-flow control devices.

We report quantitative measurements on the decay of optical absorption at various elevated temperatures in ultraviolet (UV)-irradiated SiO2 glasses doped with Ge. From the Arrhenius plots of decay time constants, the activation energy of the Ge E′ center in the UV-irradiated glasses has been determined to be 2.4±0.3 eV, while the reported activation energy of Ge E′ decay in UV-poled glasses is 0.4±0.1 eV. The present experimental results strongly suggest that the decay process of the Ge E′ center induced in UV-irradiated glasses is quite different from that in UV-poled glasses.

In a 15.7 GeO2⋅84.3 SiO2 glass prepared by a vapor axial deposition method, optical absorptions at 4.92 and 5.08 eV were observed from 300 to 600 K. The results were then compared with electron spin resonance (ESR) measurement using the same samples. The absorption at 4.92 eV was found to increase with increasing temperature, while the optical absorption at 5.08 eV decreased with increasing temperature. These changes of optical absorption indicate that the concentration of neutral oxygen monovacancy is reduced at higher temperatures. The changes were quite reversible with the temperature. Concentration of Ge E′ center, estimated from ESR, increased with increasing temperature, which is in contrast with the decrease of neutral oxygen monovacancy. The present result strongly suggests the thermal equilibrium reaction between neutral oxygen monovacancy and Ge E′ center.

We report on a remarkably large second-harmonic generation (SHG) in a germanosilicate glass (15.7 GeO2–84.3 SiO2 in mol %), which was induced by poling while the sample was irradiated with the ArF laser at the wavelength of 193 nm. The SHG coefficient, d, thus induced was measured at various poling electric-field strengths using the Maker-fringe technique. The value of d increases with increasing poling field and then saturates around 1.5×105 V/cm. The largest d coefficient value obtained in this work, which is 3.4±0.3 pm/V, seems to exceed d22 of LiNbO3. Furthermore, polarization dependence of the induced SHG shows an anisotropy of the refractive index.

We have investigated formation of Ge-related defects in a GeO2–SiO2 glass fiber preformed by poling with ArF laser excitation. Electric field dependence of the induced-defect concentrations was measured by means of the optical absorption. Several color centers such as the germanium electron-trapped centers and the Ge E′ center were induced. Concentrations of the induced germanium electron-trapped centers and Ge E′ center increase with increasing the electric field. Conversion efficiency from the germanium electron-trapped centers to the Ge E′ center was found to be independent of the electric field. The present result strongly suggests that the poling with ArF laser excitation is effective in the present GeO2–SiO2 glass for the formation of germanium electron-trapped centers.

A production of polycrystalline phase with glassy precursor, i.e., glass-ceramics processing, has a great potential to fabricate the sophisticated materials. In this study, we have prepared the Bi2O3–Fe2O3–BaO–B2O3 system precursor, in which BiFeO3 phase is crystallizable, and examined the phase formation and physical properties in resulting glass-ceramics. We found both ferroelectricity and ferromagnetism in the glass-ceramics consisting of BiFeO3 phase, i.e., multiferroic glass-ceramics. This study has demonstrated an alternative method for synthesis of polycrystalline BiFeO3 material.

The propagation losses for fresnoite-phase crystallized glasses fabricated from 35BaO–15TiO2–50GeO2, 28BaO–18TiO2–54SiO2, and 35SrO–20TiO2–45SiO2 (STS45) glasses using a cut-back technique are reported. The values are comparable with those for LiNbO3 waveguide single crystals and transparent Y3Al5O12 ceramics. In addition, the propagation loss data indicate that the losses depend on the relationship between the polarization direction and crystal growth direction. The lowest propagation loss value (0.6 dB/cm) was measured for the STS45 crystallized glass; this value is the lowest value ever reported for an oriented crystallized glass.

We compare the photorefractive effect in proton‐exchanged (PE) and annealed proton‐exchanged (APE) LiNbO3 optical waveguides at the irradiation wavelength of 488 nm. We measure the intensity dependence of the saturated index change, the build‐up time constant, and the photorefractive sensitivity in PE and APE waveguides. In APE waveguides the photorefractive sensitivity is about three times that of PE waveguides up to an irradiation intensity of 100 W/cm2. The contribution of the dark conductivity to the photorefractive effect in PE and APE waveguides is found to be dominant in the intensity range used in our experiments.

Phase separation often makes glass structure inhomogeneous. Nevertheless, a uniform texture consisting of single-crystal domains has been found by crystallization in nonstoichiometric glass, i.e., perfect surface-crystallization (PSC). Although the PSC is probably caused by evolution of SiO2-rich amorphous nanoparasites on the domain, its mechanism has not been understood yet. In this study, we examined an early-stage of PSC in a strontium-titanosilicate glass by means of nanoscopic observations. It was demonstrated that the binodal-like nanometric phase-separation and crystal-growth of fresnoite-type phase occur simultaneously at the crystal interface. It was strongly suggested that the PSC process is driven by the phase separation.

Glass-ceramics (GC) consisting of nano-sized Nb-dielectrics were fabricated in BaO–Na2O–P2O5–Nb2O5 system, and their crystallization and physical features were studied. Precursor of 0.67(2BaO–0.5Na2O)–1P2O5–2.5Nb2O5 glass co-crystallized Ba2NaNb5O15 (BNN) and NaNbO3 phases, which are about 10 nm in size, and resulting GC possessed an optical transparency comparable to the precursor glass, and optical band-gap of the GC was close to that of BNN. It is suggested that the fabricated GC is not merely a GC with transparency but Nb-dielectric GC, in which light scattering due to the crystallized phases evolution is minimized, i.e., practically transparent GC material.

Li2Ge4O9 is known to be a ferroelectric phase with Curie temperature at Tc ∼ −93 °C. In this study, we found both the lowest-frequency phonon and heat-capacity anomalies around 100 °C in the Li2Ge4O9 phase, demonstrating their presence in the structural phase-transition. The phase transition is probably triggered by regularization of distorted GeO6 octahedra in the paraelectric phase, resulting in another phase consisting of the regular octahedral GeO6 unit, which was supported by in situ measurement of emission of 2E → 4A2 transition in the Mn4+-doped phase. It is deduced that the phase transition is more dynamic than ferroelectric phase-transition originating in ordering of the Li ions.

This letter elucidates the formation mechanism of interstitial Zn defects in wide band-gap semiconductive willemite-type Zn2GeO4 via nanocrystallization in a zincogermanate glass. The results of time-development structural observations suggest that Zn2GeO4 nanocrystals precipitate in the nanometric Zn-condensed region, which occurs prior to nanocrystallization. The Zn-condensed environment probably promotes the capture of Zn ions in the interstitial sites of the Zn2GeO4 structure during the structural ordering of the supercooled liquid phase. The Zn-condensation mechanism is also discussed.

We demonstrated the selective introduction of interstitial Zn defects (Zni•) into willemite-type semiconductive Zn2GeO4 by nanocrystallization of 15Li2O–15ZnO–70GeO2 glass. The resulting nanocrystallized glass consisting of Zn2GeO4 exhibited a long-lasting photoluminescence (LLP). It is suggested that the Zni• is produced by capturing a zinc ion through six-membered rings of the Zn2GeO4 phase during nanocrystallization and the excess Zni• acts as an electron trap for the LLP. This defect activation through nanocrystallization is a promising new approach to functionalization.

We consider the relationship of nucleation tendency with crystallizing-phase structures in silicate glasses using glassy sanbornite (1BaO–2SiO2) that shows homogeneous nucleation resulting crystallization of Ba5Si8O21 and β-BaSi2O5 phases with a layered structure of a network modifier and a network former. This trend is also confirmed in other silicate glasses with homogeneous nucleation tendency, which has been assessed by Zanotto and other researchers. It is suggested that the structural dimension of the crystallizing phase governs the nucleation tendency in silicate glasses.

In situ observation of inelastic light scattering in Raman and Boson regions was performed in a niobiogermanate glass with a high nucleation ability during heating in order to examine its nanocrystallization dynamics. The relation of structural heterogeneity to the precursive stage of crystallization is also discussed. It is suggested that evolution of nuclei occurs in a nanometric niobate-rich region due to density fluctuation in the supercooled liquid phase prior to nanocrystallization.

The transition between the supercooled liquid (SCL) and crystal phases was investigated by in situ observation of boson peak in a glassy fresnoite (Ba2TiSi2O8) with an extremely high nucleation ability. The glassy fresnoite revealed anomalous boson behavior, in that the boson peak's full width at half maximum diverged at T/Tg∼1.2. This behavior suggests that an elastically softer region exists at the interface of the fresnoite nucleus/crystallite in the SCL state.

Polarity-controlled ZnO thin films grown on different buffer layers were investigated as nonlinear optical materials for second harmonic generation. The effective nonlinear optical coefficient (deff) of ZnO grown on Cr-compound buffer layers showed a higher value than that of ZnO grown on MgO buffer layers. The correlations among the grain size and surface roughness with the values of deff were found to be strong. The deff of the ZnO film increased with decrease in the grain size and surface roughness. This relationship could be explained by the increase in reaction interface and the decrease in scattering probability at the surface as decrease in grain size and surface roughness, respectively.

Fabrication of transparent glass-ceramics containing ZnO nanocrystallites has been reported. The obtained material shows UV-excited photoluminescence consisting of both broad emission in the visible region and the free exciton emission at 3.28 eV. Since the observed emission depends on the precipitated state of ZnO in the glass matrix, the glass-ceramics obtained by this way will give an alternative selection of semiconductor material with unique optical and electronic functions.

We have found that an addition of SnO in a bismuth-borate glass, CaO–B2O3–Bi2O3–Al2O3–TiO2, decreases the optical absorption coefficient in the visible region, in which selective crystallization of TiO2 was observed after heat treatment. Since selective crystallization of TiO2 was also attained in the SnO-containing glass, the transparency of TiO2 crystallized glass can be improved independently of selective crystallization of TiO2. We have also demonstrated that the rutile-nanocrystallized glass with SnO addition shows a higher photocatalytic activity than the glass without SnO, indicating that this crystallized glass has a large potential for application as transparent photocatalytic materials.

Nanocrystallization of rutile and anatase was observed in a CaO–B2O3–Bi2O3–Al2O3–TiO2 crystallized glass. The color of the present crystallized glass was changed by the heat-treatment temperature, and x-ray diffraction (XRD) patterns of the crystallized glass show that the apparent color change is correlated with the formation of titania nanocrystallites. The particle size of crystallites observed in the transmission electron microscope image is comparable to that measured by XRD pattern. In addition, a large change of refractive index between the rutile particles and glass matrix, 0.8 or larger, was obtained. The present titania crystallized glass will be not only a photocatalytic material but also a promising candidate for random lasing devices.

A technique for the writing of crystal line in glass has been developed, in which a continuous-wave Nd:YAG laser (wavelength: λ=1064nm) is irradiated to the glasses containing transition metal (TM) ions such as Fe2+, Ni2+, and V4+. Laser energies are absorbed through d-d transitions of TM ions and dissipated to the lattice surrounding TM ions by nonradiative relaxation process, giving the increase in temperature in the laser irradiated region and inducing crystallization. This technique has been demonstrated for the writing of crystal lines consisting of nonlinear optical fresnoite-type Ba2TiGe2O8 and Ba2TiSi2O8 crystals in NiO-, Fe2O3-, and V2O5-doped (0.3–1mol%) BaO–TiO2–GeO2 and BaO–TiO2–SiO2 glasses. It is confirmed that crystals in the crystal lines are highly oriented along the laser scanning direction. The technique developed in this study is proposed to be called “transition metal atom heat processing.”

The optical properties of Ba2TiSi2O8 nanocrystals in transparent crystallized glasses were investigated. The transparent nanocrystallized glass with Ba2TiSi2O8 crystals having a particle size of ∼200nm was fabricated by crystallization (at 760 °C for 1 h) of a glass with the stoichiometric composition of Ba2TiSi2O8, i.e., 40BaO∙20TiO2∙40SiO2 glass. Both visible second-harmonic generation (green light, 532 nm) and luminescence (blue light, ∼470nm) could be observed in the transparent Ba2TiSi2O8 nanocrystallized glass, demonstrating the optical multifunctional nanocrystallized material. It was suggested that the blue luminescence from the transparent Ba2TiSi2O8 nanocrystallized glass originated from the oxygen-related defects of the SiO4 unit in the Ba2TiSi2O8 nanocrystals.

We report a remarkably reduced photorefractive sensitivity in Ti:LiNbO3 channel waveguides using substrates with reduced optical absorption near the absorption edge. In addition, the intensity dependence of the saturated index change was measured, which indicates that, based on the Glass model, the saturated value of photoinduced index change is independent of irradiation intensity in the high intensity limit.

A novel method of measuring the photorefractive sensitivity of Ti‐diffused channel waveguides in lithium niobate is proposed and demonstrated. It is suited for quantifying the dependence of the photorefractive sensitivity on the irradiation beam wavelength. From measurements at various irradiation intensities and wavelengths 0.633, 0.81, and 1.06 μm, we give an estimate of the crosstalk of a directional coupler as a function of irradiation intensity and wavelength.