Ratnakar R. Neurgaonkar

Photorefractive properties of strontium-barium niobate

We have grown and optically characterized strontium‐barium niobate crystals, including both undoped and cerium‐doped crystals having two different Sr/Ba ratios (61/39 and 75/25). By measuring the coupling of two optical beams in the crystals, we have determined the following photorefractive properties: the effective density, sign, and spectral response of the dominant charge carrier, the grating formation rate, dark conductivity, and carrier diffusion length. We find that electrons are the dominant photorefractive charge carriers in all of our samples; the typical density of photorefractive charges is ∼1×1016 cm−3 in the undoped samples. The grating formation rate increases with intensity, with a slope of ∼0.3 cm2/(W s) over an intensity range of ∼1–15 W/cm2 in undoped samples. Cerium doping improves both the charge density (increased by a factor of ∼3) and the response rate per unit intensity (∼5 times faster).

Progress in photorefractive tungsten bronze crystals

We review the current status of the photorefractive tungsten bronze family crystals in terms of their growth problems and applications, with special emphasis on the current results for the Sr1−xBaxNb2O6 solid-solution system. Ferroelectric morphotropic phase-boundary materials are discussed as an appropriate goal for future development.

Piezoelectricity in tungsten bronze crystals

The range of piezoelectric properties available in tungsten bronze crystals reviewed includes both longitudinal and shear mode responses. Specific crystal compositions investigated include Sr1-xBaxNb2O6 (SBN), Ba2-xSrxK1-yNayNb5O15 (BSKNN), K3Li2Nb5O15 (KLN), (Ba, Sr)6Ti2Nb8O30 (BSTN), Sr2-x-CaxNaNb5O15 (SCNN), Sr2KNb5O15 (SKN) and Pb1-xBaxNb2O6 (PBN). SBN, SCNN and SKN exhibit large piezoelectric d33 coefficients, while BSKNN, PBN:60 and KLN have large d15 shear coefficients. In La3+ -modified SBN:60, d33 can exceed 1000 pC/N, although its relaxor character increases with increasing La3+ content. The role of dopants in the 12-, 9- and 6-fold coordinated crystallographic sites is described with respect to the relaxor character of SBN crystals. The high piezoelectric and dielectric constants in these single crystals suggest their use in high frequency ultrasonic tomography. Both piezoelectric and electrostrictive options would appear to be available in the bronzes and the very high perfection of the availa...

Tungsten bronze Sr1−xBaxNb2O6: a case history of versatility

Abstract We review the chronological development of ferroelectric tungsten bronze Sr1-x Ba x Nb2O6 (SBN) single crystals in terms of crystal growth and applications ranging from pyroelectric to surface acoustic wave (SAW) to nonlinear optics, with specific emphasis on the compositions SBN:60, SBN:75 and SBN:50. In addition to a number of bulk crystal applications, these crystals are also now being used as substrates for the growth of various lattice-matched thin films, including ferroelectric and superconducting oxide materials.

Development And Modification Of Photorefractive Properties In The Tungsten Bronze Family Crystals

The Sr1-xBaxNb2O6 (SBN) and Ba2-xSrxK1-yNayNb5O15 (BSKNN) tungsten bronze solid-solution systems are shown to be promising photo-refractive materials. Because of the versatility of the bronze structure, both the response time and spectral response can be controlled by altering the type of dopant and its crystallographic site preference. This paper reviews the current status of the tungsten bronze crystals SBN and BSKNN for photorefractive applications in terms of their growth, electro-optic character, and the role of cerium dopants. Ferroelectric morphotropic phase boundary (MPB) bronze materials are also discussed as potentially important for future development.

Digital holographic storage system incorporating optical fixing

Digital holograms have been written in stoichiometric Pr:LiNbO(3) in a two-color recording scheme, demonstrating what is to our knowledge the first all-optical nondestructive readout of digital data. Using writing light at 800nm and gating light at 476 nm, we stored and retrieved 256-kbit digital data pages with a raw bit-error rate BER of <10(-4) .

Czochralski single crystal growth of Sr.61Ba.39Nb2O6 for surface acoustic wave applications

Abstract Ferroelectric Sr.61Ba.39Nb2O6 (SBN) single crystals approximately 2 cm. in diameter and 5–6 cm. long have successfully been grown by the Czochralski technique. All the crystals were grown in the [001] direction in argon or oxygen, and it was found that the degree of difficulty of growth increased as the diameter of the crystals increased. Temperature dependence measurements on the poled Z-cut SBN crystals showed that this composition has temperature compensated orientations which make this material useful for surface acoustic wave devices.

A thermodynamic phenomenology for ferroelectric tungsten bronze Sr0.6Ba0.4Nb2O6 (SBN:60)

The tetragonal tungsten bronze ferroelectrics in the strontium barium niobate system have been extensively studied over many years. As for many of the bronzes, a crude interpretation of the experimental data has been attempted in the past using the simple Landau‐Ginsburg‐Devonshire expansion of the Gibbs free energy as a Taylor series in powers of the polarization, lumping all the temperature dependence into the lowest order term. In this paper new measurements are presented for the temperature dependence of dielectric polarization, permittivity, and the E‐field dependence of the permittivity. It is shown that for a realistic fitting of the data, the Taylor expansion must be taken to at least the eighth power term, and that the coefficients of terms up to the sixth power must be taken as functions of temperature. Since the phenomenology describes equilibrium behavior, it is the total static polarizability that is being explored in this treatment. The nature of this temperature dependence strongly suggests...

Ferroelectric and structural properties of the tungsten bronze system K2Ln3+Nb5O15'Ln = La to Lu

The K2Ln3+Nb5O15 system has been investigated for a range of rare-earth ions (La3+ to Ho3+, Y3+). The temperature-dependent dielectric properties indicate that all but the La3+-containing materials have a tungsten bronze orthorhombic structure. Two phase transitions occur above room temperature, one paraelectric:ferroelectric and the other ferroelectric:ferroelastic. These transitions shift toward higher temperatures as the size of the rare-earth ion decreases. The lattice constant determination for this system indicates that the larger ions (La3+ to Gd3+) predominantly prefer the 12-fold coordinated site, Tb3+ to Ho3+ are distributed over both the 12- and 9-fold coordinated sites, and the smaller rare-earths (Er3+ to Lu3+) do not yield the tungsten bronze phase. Because of their low dielectric constants, these bronzes appear to be promising for some optical and pyroelectric detector applications.

Electrical fixing of photorefractive holograms in Sr0.75Ba0.25Nb2O6

Photorefractive holograms stored in Sr0.75Ba0.25Nb2O6 crystals are electrically fixed at room temperature. The fixed holograms can be read out directly or after a positive-voltage pulse is applied that can dramatically enhance the diffraction efficiency. Single gratings as well as images are recorded and fixed.