D.K. Tagantsev

Raman spectroscopy and the origin of electrooptical Kerr phenomenon in niobium alkali-silicate glasses

It is shown that magnitudes of the Kerr coefficient and permittivity of 19Na2O–11K2O–2B2O3–2CdO–(66−x)SiO2–xNb2O5 glasses, where 0<x<37, correlate with the content of the Na2O·Nb2O5 groupings, with that content being estimated by Raman scattering data. This allows assigning to these groupings, or to other structural entities containing such groupings (for example, crystal motifs), the responsibility for high-polarizability and Kerr sensitivity of the glasses.

The origin of electrooptical sensitivity of glassy materials: crystal motifs in glasses

Abstract The results of studying electrooptical Kerr sensitivity in heavy metal silicate and phosphate glasses and glass-ceramics are presented. A niobium–lithium-silicate glass demonstrating a record Kerr coefficient (266×10−16 m/V2) has been formed. Formation of the transparent glass-ceramics containing electrooptical sodium niobate microcrystals has been studied, and glass-ceramics demonstrating Kerr coefficients higher than 6000×10−16 m/V2 have been elaborated. On the base of the effective medium approximation, it is shown that the Kerr coefficient of these glass-ceramics depends on the volume fraction of sodium niobate microcrystals, vc as a linear function of vc(1−vc)−2 A conception of the origin of electrooptical sensitivity of glasses is proposed. This conception is based on the hypothesis that in glasses there exist regions with exactly crystalline ordering within 2–3 coordination spheres, with these regions having no phase boundaries. These regions are named the crystal motifs (CM). Due to the highly effective mechanism of nuclear polarizability of the electrooptical crystals, the motifs with the symmetry of such crystals are responsible for high permittivity and Kerr sensitivity of the glasses, and they play a role of pre-nuclei while electrooptical glass-ceramics are forming under glass heat treatment. It has been found that synthesized barium-titanate-silicate and niobium–lithium-phosphate glasses demonstrate extremely low Kerr coefficients, and they do not form transparent glass-ceramics with any electrooptical precipitates. This contradicts literature data and is explained by the difference in the conditions of glass synthesis, which are supposed to be responsible for the formation of proper CMs.

New glasses for graded-index optics: influence of non-linear diffusion in the formation of optical microstructures

Abstract We have designed novel glasses for the formation of graded-index diffractive optical structures. In these glasses silver ion exchange and rapid diffusion produce a large refractive-index variation (0.085). We model the non-linearity of the diffusion process by applying the Boltzmann–Matano technique to the optical profiles and use index variation rather than concentration in the second Fick equation. The simulation of graded-index diffraction gratings formed under thermal and electrically assisted diffusion shows that, other conditions being equal, more efficient gratings can be formed when the diffusion coefficient increases with the dopant concentration. According to the simulation, the application of an electric field of about 0.5 V/μm enables one to obtain a phase shift equal to 1.5 π rad if a grating with a period of 4 μm is formed in the novel glass material. For comparison, a phase shift of only 0.9 π is predicted when a similar grating is formed in the commercial BK-7 glass.

Formation of thick glass-ceramics films by ion-exchange

Abstract The formation of thick sodium-niobate glass-ceramic films on the substrate of lithium-silica-niobate glass was induced by the sodium-lithium ion exchange performed in the glass plates at the temperature exceeding the glass transition temperature. Used conditions of the ion-exchange processing allowed producing glass-ceramic films with the thickness up to 0.5 mm, and the choice of the processing temperature and duration allowed varying optical properties and thickness of the ceramic films.

Characterization of ion diffusion process in glasses with simple mode-index measurements

It is shown by numerical computations that self-similar index profiles imply self-similar mode spectra in planar waveguides. This leads to a simple method to estimate the diAusion activation energy and to evaluate the validity of Fick’s second law in diAusion modeling, under given processing condition, from measurements of mode spectra. The feasibility of the approach is verified experimentally with Na2O‐SiO2‐Al2O3‐B2O3 glass developed for ion exchange processing. ” 2000 Elsevier Science B.V. All rights reserved.

Prototype of hybrid diffractive/graded-index splitter for fiber optics

The first example of hybrid diffractive/graded-index optical in- tegration is presented. The demonstrated 1:3 beamsplitter for fiber optics consists of two GRIN lenses and one diffractive element, which are fab- ricated by ion exchange and stacked together to form a solid unit. The unit is coupled with the input optical fiber and with three output ones.

Glass crystallization by ion exchange: formation of NaNbO3 in thick films at the glass surface

Abstract Experimental results of studying a lithium–sodium high-temperature ion exchange in a lithium–niobium–silicate glass are presented. It has been discovered that the ion exchange brings forth glass crystallization. The crystals precipitating in the diffused layers and identified by X-ray diffraction (XRD) analysis are orthorhombic (and/or quasicubic) NaNbO3. Depending on the temperature–time conditions of the ion-exchange processing, surface glass-ceramic films of different thicknesses have been formed. The phenomenon and the conditions that produce it are discussed in terms of the theory of phase equilibria in liquid solutions. This work completes the topic (developed by the authors during the last few years) concerning the possibility to control any phase transformations in glasses (including liquid–liquid phase separation and crystallization) by the ion-exchange technique.

Decrystallization of crystallized glasses by ion exchange

Abstract Experimental results of studying the ion exchange process in crystallized glass are presented. For the first time, the disappearance of the crystalline phase within the layers undergoing the ion exchange process has been discovered. The samples of crystallized lithium silicate glass underwent ion exchange processing in a NaNO3 salt melt at the temperature where the viscosity of the initial (non-crystallized) glass is equal to about 108 Pa s. After 1.5-, 3- and 5-h processing, vitreous layers about 120, 190 and 270 μm thick correspondingly have been observed at the sample surface. The effect is discussed from the viewpoints of the thermodynamic theory of phase decomposition of glasses and the theory of phase equilibria in heterogeneous liquid solutions. A phenomenological description of the ion exchange diffusion in the vitreous material embedded with microcrystals is proposed.

Ion-exchange processing of glasses under non-isothermal conditions

Abstract The general solution of the diffusion equation with a diffusion coefficient depending on time is found. The solution is applied in describing ion exchange diffusion in glasses at arbitrarily varying temperatures in processing. It is shown that the processing yields identical concentration profiles if the integral, ∫t0D(ξ)dξ, taken over the processing time, t, is the same, where D(ξ) is the time-dependent interdiffusion coefficient, which is determined from the real temperature schedule during the process. The range of applicability of the formulated criterion is discussed. Applying that criterion to the technology of graded-index (GRIN) optics the results of the simulation of isothermal ion exchange can be used for processing at non-isothermal conditions.

Vitreous Materials for Electrooptics

A complex approach to the development of electrooptic glasses is proposed. The approach is based on the use of the traditional methods of structural-physical investigations in combination with the Rayleigh and Mandelstam-Brillouin scattering spectroscopic techniques. The structural features of niobate glasses and their properties are considered. It is shown that niobium selectively enters into the fluctuation microinhomogeneities in the alkali borosilicate vitreous matrix. The conditions for the precipitation of sodium niobate microcrystals are determined, and their influence on the increase in the Kerr constant for glass-ceramic materials is analyzed.