xNb2O6 Crystals Research Articles

Temperature dependence of the spontaneous polarization, acoustic and strain anomalies in strontium barium niobate crystals of different chemical compositions probed by the second harmonic generation technique

In SrxBa1−xNb2O6 crystals (x = 0.33, 0.5, 0.61, and 0.75), temperature dependences of spontaneous polarization, strain, elastic modules, and second harmonic generation (SHG) signal are compared. It is revealed that SHG describes the temperature dependences of dipole moments in polar nanoregions in paraelectric phase. In the vicinity of the phase transition in paraelectric phase, SHG reflects the temperature behavior of relatively large and long-lived polar asymmetric regions as indicated by the presence of the intermediate temperature range on the temperature dependence of this nonlinear response.

Floating-zone growth and characterization of CaxBa1−xNb2O6 crystals

Transparent and pale yellow calcium barium niobates (CaxBa1−xNb2O6) single crystals with larger Ca content ranging from 0.224 to 0.366 have been grown out by floating zone technique. The growth of CBN crystals in air is oriented along the 〈001〉 direction. The CBN crystals have been characterized by X-ray diffraction and X-ray single crystal diffraction. The composition uniformity of CBN-35 crystal rod has been checked by EPMA measurement. The deviations of Ca mole fraction between initial melt and the crystal are 0.1–1.4mol%, which is much lower than crystals prepared by the Czochralski method. All the as grown crystals had high transmittance above 80% with increasing wavelength from about 500nm. With increasing Ca content in crystal, symmetric bending mode v5 and symmetric stretching mode v2 broadened and moved to the low wave number. The relative intensity of Raman peak v5 compared to v2 increasing gradually indicates that the NbO6 octahedral distortion degree increased.

Ellipsometric study of near band gap optical properties of SrxBa1−xNb2O6 crystals

Complex refractive indices N=n+ik for a-cut SrxBa1−xNb2O6 single crystals with four nominal values of composition x=0.40; 0.50; 0.61; 0.75 were studied by spectroscopic ellipsometry in the photon energy range 2–5eV of crystal transparency and fundamental electronic absorption. The ellipsometric data were evaluated taking into account an uniaxial anisotropy and an unintentional surface roughness. We use a global composition dependent Sellmeier fit as a reference for monitoring the crystals composition. The optical model yields indirect and direct optical band gap energies Eg in the spectral range 3.77–4.03eV and 4.28–4.55eV, respectively, depending on the Sr/(Ba+Sr) content ratio x and light polarization. The congruently melting composition Sr0.61Ba0.39Nb2O6 shows distinct dispersion features in the imaginary part of the UV complex refractive index with respect to other compositions.

Ferroelectric microdomains and microdomain arrays recorded in strontium–barium niobate crystals in the field of atomic force microscope

Microdomains and various one-dimensional (1D)- and two-dimensional (2D)-microdomain arrays were formed under dc-voltages applied to the tip of an atomic force microscope (AFM) in ferroelectric SrxBa1−xNb2O6 crystals. Detailed studies of the characteristics of the AFM—recording and decay kinetics of the written arrays have shown that the crucial factors of the stability of a domain array are its dimensionality and discreteness (described by a distance Δ between the recorded point domains forming the array). The dependence of the stability on the discreteness of domain ensembles is analyzed. With decreasing Δ, the decay times of the domain ensembles increases. The stability of 2D arrays (domain squares, complex-shaped arrays composed of the domain ensembles of opposite polarity) by orders of magnitude exceeds that of 1D-arrays (domain chains and lines) provided all factors of recording being the same. As an illustration, the decay time of individual (spatially separated) domains and quasicontinuous domain lines are tens of minutes and about 20 h, respectively, whereas a quasicontionuous domain square persists within at least ten days. We assume the existence of cooperative interactions in microdomain ensembles, which reveal themselves even in arrays consisting of spatially separated point domain.

Thermal lens spectroscopy through phase transition in neodymium doped strontium barium niobate laser crystals

In this work the anisotropic thermal lens (TL) effect was studied in Nd3+ doped SrxBa1−xNb2O6 crystals with two different stoichiometries (x=0.47 and 0.61). TL measurements were carried out as a function of temperature from 30 up to 140°C so that the effect of phase transition was investigated. A strong enhancement in the TL effect during the ferro-to-paraelectric phase transition has been observed. Additionally, we have found an inversion of the TL signal with the crystal stoichiometry. This fact is explained in terms of a different thermal expansion along the c axis for both crystals, as measured by x-ray diffraction experiments. The obtained results open the possibility to tailor a crystal stoichiometry that minimizes the TL effects.

The effects of photorefraction on electro-optic field sensors

We have measured the electro-optic response of LiNbO3 and SrxBa1−xNb2O6 crystals to be used for electric-field sensors. Our results indicate that optically induced refractive index variations (photorefractivity) in the crystals affect the temporal stability and sensitivity of the sensors. Spatial distributions of the refractive indices produced from the photorefractivity create incoherence in the polarization of the probing laser passing through the crystal (optical probe). In LiNbO3 crystals, this spatial incoherence was negligible and sensor responsivities close to the theoretical maximum were attained. However, in SrxBa1−xNb2O6 crystals, strong spatial variations of the principal refractive indices resulted in an extremely incoherent polarization of the optical probe and reduced the electro-optic responsivity dramatically. The photorefractive-induced sensitivity loss is modeled using a distribution function (rather than a constant value) to describe the phase of the optical probe. These results emphasize the importance of an optically stable material, with little or no photorefractivity, when designing an electro-optic field sensor.

Structure of the OH− stretching vibrational band in SrxBa1−xNb2O6

The stretching mode of the hydroxyl ions in SrxBa1−xNb2O6 crystals with x=0.38 to 0.79 has been measured by the Fourier transform infrared spectroscopy technique. The main absorption band at about 3493 cm−1 is accompanied by a shoulder on the low energy side. The shape of the shoulder depends strongly on the Sr/Ba ratio of the crystal which allows a rough estimation of the composition. The main absorption band including the shoulder has been decomposed using Lorentzian and Gaussian curves and the parameters of three components have been determined. The components are attributed to OH− transitions in different cation environments strongly depending on the crystal composition. The temperature dependence of the OH− absorption band is approximately linear in the 20–200 °C range and does not show any peculiarities at the phase transition temperature.

Radio frequency magnetron sputtering deposition of hetero-epitaxial strontium barium niobate thin films (SrxBa1−xNb2O6)

Excellent electro-optic properties of SrxBa1−xNb2O6 crystals (SBN:x) motivate the attempts to control the deposition of high ordered SBN thin films with the aim of optical waveguiding and processing integration. We have examined the ability of sputtering techniques to deposit stoichiometric SBN thin films. Composition analysis has enabled us to probe the mechanisms which control the target-film composition transfer and to define an experimental strategy for stoichiometry control. Epitaxial (001) SBN thin films have been obtained on MgO (100) substrates, which exhibit two in-plane orientations (±31°) mirror symmetric to the MgO cell axis.

Optically induced domain waveguides in SrxBa1−xNb2O6 crystals

Optically induced refractive index patterns, which can be used for waveguides, were formed in SrxBa1−xNb2O6 crystals using an erasable fixing mechanism, based on domains formation by the screening effect. The sign and strength of the refractive index change of the fixed waveguides was controllable by an applied electric field. Fixed patterns are shown to allow the storage of many spatially multiplexed holograms.

Controllable narrow band and broadband second-harmonic generation by tailored quasiphase matching with domain gratings

We demonstrate second-harmonic generation for various discrete input wavelengths by a controllable quasiphase matching method. This is done by forming ferroelectric domain gratings in SrxBa1−xNb2O6 crystals. The method allows flexible preparation of tailored quasiphase matching for prespecified wavelengths over prespecified bandwidths. Conversion efficiencies of 5% were achieved.

Image and hologram fixing method with SrXBa_1−XNb_2O_6 crystals

We demonstrate a new fixing method with Sr(x)Ba(1-x)Nb(2)O(6) crystals. This is done by inducing ferroelectric domains in the crystal in real time, by a screening mechanism. Images or their holograms can be recorded in small separate locations in the crystal. This crystal pixelization permits the recording of many pictures and their individual readout and erasure.

Broadband second-harmonic generation in SrxBa1−xNb2O6 by spread spectrum phase matching with controllable domain gratings

We have demonstrated, for the first time to our knowledge, second-harmonic generation for a broad input wavelength range of 750–1064 nm in SrxBa1−xNb2O6 crystals, with a controllable spread spectrum of quasiphase matching. Second-harmonic conversion efficiencies of up to ∼1% were observed. This was done without any temperature or angular tuning of the crystal. The phase matching was obtained by inducing alternating ferroelectric domains in the crystal in real time, using a novel fixing process which is based on screening. The broadband capability of the conversion is allowed by a spread in the range of the domain widths.