Nb2O6 Crystals Research Articles

Helically twisted nonlinear photonic crystals.

Nonlinear photonic crystals with a helical structure in the second-order nonlinear coefficient (χ(2)) are fabricated using infrared femtosecond laser poling in ferroelectric Sr0.61Ba0.39Nb2O6 crystals. The quasi-orbital angular momentum of the helical χ(2) structure can be imprinted on the interacting photons during nonlinear optical processes, allowing the topological charge of the generated photons at new frequencies to be controlled. Here we study the case of a double-helix nonlinear photonic structure for the generation of a second-harmonic vortex beam from a Gaussian pump beam without phase singularity. The conservation law for orbital angular momentum in the second-harmonic process is also verified, with the topological charge of the pump photons being fully compensated by the double-helix structure. The flexible control of light carrying orbital angular momentum (OAM) at new frequencies will find important applications in both classical and quantum photonics, such as nonlinear wavefront shaping and multidimensional entanglement of photons.

Nonlinear generation of an optical bottle beam in domain-engineered ferroelectric crystals.

Nonlinear wavefront shaping in periodically poled ferroelectric crystals has received great attention because it offers a convenient way to generate a structured light beam at new frequencies. In contrast to structurally uniform beams like Laguerre-Gaussian or Hermite-Gaussian modes, here we demonstrate the possibility to generate a spatially varied optical bottle beam via a frequency doubling process in a domain-engineered Sr0.61Ba0.39Nb2O6 (SBN) crystal. The nonlinear holography method was employed to design the modulation pattern of the second-order nonlinear coefficient χ(2), and the femtosecond laser poling was used to imprint the χ(2) pattern into the SBN crystal via ferroelectric domain inversion. The second harmonic bottle beam with zero intensity in its center that is surrounded in all three dimensions by light was observed with the incidence of a fundamental Gaussian beam. These results are useful for nonlinear generation and control of structured light at new frequencies, which has important applications in nonlinear photonics and quantum optics.

Strontium barium niobate crystals with various chemical compositions probed by Raman and Brillouin scattering

The Sr x Ba1 – x Nb2O6 crystals with x = 0.33, 0.5, 0.61, 0.75 were studied by Raman and Brillouin light scattering in a wide temperature range. The experimental temperature dependences of the central peak's integral intensity and the position and width of the longitudinal acoustic mode were plotted using derivative analysis. It is shown that the temperature at which the abnormal behavior begins (the so-called Burns temperature) is only slightly different for all the crystals under study, while the phase transition temperatures differ significantly. It allows asserting the identical microscopic nature of the Burns temperature in Sr x Ba1 – x Nb2O6 crystals.

Order-disorder nature and elastic anomaly of successive phase transition of (K0.5Na0.5)NbO3 proved by broadband Brillouin scattering

ABSTRACT The importance of the environment-friendly lead-free piezoelectric materials has attracted much attention. The (K0.5Na0.5)NbO3 (KNN) crystals with the perovskite structure undergo the cubic (C) to tetragonal (T) ferroelectric phase transition at T CT = 430 °C, and the tetragonal to monoclinic (M) structural phase transition at T TM = 190 °C. The elastic properties and relaxation process of these phase transitions of the KNN crystals have been studied by broadband Brillouin scattering. The softening of the LA and TA velocities was observed in the vicinity of T CT and T TM. The power law behavior of the velocity indicates three-dimensional fluctuations. The broadband central peak showed the critical slowing down in the vicinity of the T CT and T TM. The slowing down is attributed to the order-disorder nature of the off-center displacement of Nb in KNN.

Processing mediated enhancement of ferroelectric and electrocaloric properties in Ba(Ti0.8Zr0.2)O3–(Ba0.7Ca0.3)TiO3 lead-free piezoelectrics

High-performance Ba(Ti0.8Zr0.2)O3–(Ba0.7Ca0.3)TiO3 (BCZT) fiber crystals were grown using laser-heated pedestal growth (LHPG) for a systematic investigation of processing effects on structure and physical properties. By changing the growth rate (200−5 mm/h), fiber crystals with varying stoichiometry were realized: controlled composition, Ba0.872Ca0.128Ti0.925Zr0.075O3-δ, for the optimized crystal (5 mm/h), relative to the ceramic sample (SS) synthesized by the ceramic method. SS and optimized fibers exhibited larger energy storage densities of 700 and 868 mJ/cm3, respectively. Electrocaloric (EC) measurements by direct and indirect methods yielded ΔT of 1.3 K & 0.95 K and EC responsivity (ΔT/ΔE) of 0.3 and 0.43 K mm/kV for SS and 5 mm/h respectively, at 30 kV/cm, at least two-folds higher compared to 200 mm/h fiber. The EC responsivity of our samples is comparable to that of Pb(Mg,Nb)O3 crystals. These results demonstrate that BCZT has potential application as EC elements when the composition is controlled by fine-tuning of growth parameters.

Dye wastewater treatment driven by cyclically heating/ cooling the poled (K0.5Na0.5)NbO3 pyroelectric crystal catalyst

In this work, a novel pyroelectric catalysis technique is developed for decomposition of dye wastewater using the lead-free (K0.5Na0.5)NbO3 pyroelectric crystal catalyst under the environmental temperature variation. 91.2% of rhodamine B dye can be decomposed after (K0.5Na0.5)NbO3 pyroelectric catalyst undergoing 100 cold-hot 30–60 °C cycles. The catalytic activity can be obviously influenced by the poling electric field of catalyst materials. With the increase of the poling electric field from 0 to 4.5 kV/mm, the dye decomposition ratio correspondingly increases from 3.3% to 91.2%. The poling treatment is helpful to enhance the polarization of the (K0.5Na0.5)NbO3 crystal, which can also result in the increase of the pyroelectric coefficient, yielding to the enhancement of pyroelectric catalysis. The scavenger experiments are carried out, in which the main active species in the pyroelectric catalysis are found to be superoxide radicals, hydroxyl radicals and holes. The effects of various factors including dye concentration, catalyst dosage, pH and type of dyes on pyroelectric catalysis performance of (K0.5Na0.5)NbO3 are also investigated to obtain the optimum condition. The results of three consecutive recycling-utilization tests indicate that (K0.5Na0.5)NbO3 crystal possesses good chemical stability. With these advantages of high dye decomposition ratio and good recycling utilization, (K0.5Na0.5)NbO3 crystal is potential in developing the novel pyroelectric catalysis technique for decomposition of dye wastewater through utilizing the thermal energy from the environmental cold-hot temperature variation in future.

Anomalous domains formed under AFM-TIP voltages in Sr0.61Ba0.39Nb2O6 crystals and their suppression

Submicro- and nanosized domain patterns are demanded by various applications. The most attractive method for fabrication of structures of these scales is the domain writing by an AFM-tip voltage Utip. The use of this method is limited by the appearance of so-called anomalous domains, in which a small area under the tip is polarized oppositely to the poling field. We present the studies of anomalous domains in zero-field cooled (ZFC) and field cooled Sr0.61Ba0.39Nb2O6 crystals. A correlation between the spatial distribution of the domain shape and the bias Ub of local hysteresis loops was found in ZFC crystals, namely, in the points with a larger Ub the anomalous domains appeared at higher Utip. Based on this correlation, we managed to prevent the formation of anomalous domains by a strong preliminary poling of the crystal resulting in an essential increase in Ub all over the bulk. The dependences of the domain diameter D on Utip and the exposure time tp are described by the unique linear and power D ∼ tpk functions, respectively. These dependences are not affected by the appearance of an anomalous region growing with Utip and tp.

Electron-Beam Domain Patterning in Sr0.61Ba0.39Nb2O6 Crystals

The characteristics of electron-beam domain writing (EBDW) on the polar and nonpolar surfaces of the field-cooled (FC) and zero-field cooled (ZFC) Sr0.61Ba0.39Nb2O6 (SBN) crystals are presented in the range of accelerating voltage U from 10 to 25 kV. The exposure characteristics of the domain diameter d and length Ld (when writing on the polar and nonpolar surfaces, respectively) were measured. With increasing exposure time, d tends to a saturation value, whereas Ld grows linearly, the frontal velocity Vf being of 40 μm/s. At U = 25 kV the achieved d and Ld are of 7 and 40 µm, respectively. The observed peculiar features of EBDW—specifically the domain widening with exposure times and the effect of the polarization state of the crystal on the domain stability—are accounted for by the relaxor features inherent to this material. The effects of electron-beam (EB) irradiation on the local hysteresis loops is evidence of a domain fixation.

Formation kinetics of Sr0.25Ba0.75Nb2O6 and Li2B4O7 crystals from 0.25SrO-0.75BaO-Nb2O5-Li2O-2B2O3 glass.

We have investigated the transition kinetics of Sr0.25Ba0.75Nb2O6 (SBN) and Li2B4O7 (LBO) crystals from 0.25SrO–0.75BaO–Nb2O5–Li2O–2B2O3 (SBNLBO) glass under isothermal and non-isothermal processes. With increasing temperature, there are two consecutive steps of crystallization of SBN and LBO from the glass. The Johnson–Mehl–Avrami function indicates that the crystallization mechanism of SBN belongs to an increasing nucleation rate with diffusion-controlled growth. The crystallite size of SBN ranges from 40 to 140 nm but it is confined to within 30–45 nm for LBO during the whole crystallization process. The relationship between the nano size and strain of SBN based on the Williamson–Hall method, and the change of activation energies of SBN and LBO crystallization analyzed by using the isoconversional model are discussed. A comparison of phonon modes between as-quenched glass and fully transformed crystals clearly shows that the low dimensional vibration modes in the structurally disordered glass change to highly dimensional network units with the formation of crystals.

Optical phonon behavior and magnetism of columbite Zn0.8Co0.2Nb2O6**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0403704), the National Natural Science Foundation of China (Grant Nos. 11304113, 11474127, and 11574112), and the Fundamental Research Funds for the Central Universities of China.

Columbite Zn0.8Co0.2Nb2O6 crystals were grown by optical floating zone methods. The x-ray diffraction (XRD) was used to check the structure information of the grown Zn0.8Co0.2Nb2O6 crystal. The room temperature and temperature-dependent Raman spectra were tested to investigate the optical phonon behaviors of columbite Zn0.8Co0.2Nb2O6, which exhibited a temperature stable property. The magnetics properties of Zn0.8Co0.2Nb2O6, measured by a physical property measurement system (PPMS), were also presented in this work.

Effect of reduction on the optical properties of Sr0.61Ba0.39Nb2O6 single crystals grown by optical floating zone technique

The effect of thermal reduction during growth of strontium barium niobate Sr0.61Ba0.39Nb2O6 (SBN61) crystal on its optical and electrical properties has been investigated. Single crystals of SBN61 were grown by the Optical Floating Zone technique in air ambience. Growth challenges such as formation of core and crystal cracking were eradicated by optimizing the synthesis and growth process parameters. The evidence of reduction was observed in the grown crystals that resulted in a characteristic gray colour. Further, the crystal became colourless on annealing in oxygen ambiance at ∼1200 °C for 20 h. The as-grown crystal is therefore identified as reduced SBN crystals due to formation of oxygen vacancies and the accompanied colour centers and charge compensated defects e.g. Nb4+ polaron in the as-grown crystals. X-ray powder diffraction analysis confirmed the formation of the desired phase and no structural change in the reduced crystal. Also, the absorption spectra revealed a broad absorption band centred around 500 nm in the as-grown reduced crystals confirming the thermal reduction of the crystal. Reduced crystals depicted a slight decrease in the optical band gap energy due to defect induced band tailing. An increase in the pyroelectric coefficient (p3) was observed for reduced SBN61 crystal in comparison to the crystal annealed in oxygen. The impedance measurement revealed higher electrical conductivity with smaller activation energy for the reduced crystal in comparison to the annealed SBN61 crystal. It shows that the thermal reduction induced defects influence the electrical conduction processes, causing the materials to be more conducting, which is desirable for a relaxor ferroelectric materials to be used as a thermoelectric generator application.

Tunable photonic crystal wavelength sampler with response in terahertz frequency range

Optical features of novel one dimensional photonic crystals with response in the terahertz frequency range are studied theoretically by outlining the transfer matrix method. Dependence of the ideal crystal reflectance to the size of the alternating SiO2 and InSb layers are studied systematically at the first step. Extracting the optimum values in which large photonic band gaps (PBGs) appear in the reflectance spectrum, two more defective structures are proposed by benefiting the electro-optical behaviors of Li:NbO3 crystal. Taking into account of the reflection alterations, tunability of the crystals’ response by the wave incident angle, external bias voltage applied to the defect layer(s) and the wave polarization is further studied. Results show that PBGs and resonant modes inside them are strongly dependent to all these controlling factors besides the dependence to the structural parameters. But however, to some extent different modes have different behaviors. The proposed crystals may satisfy the growing demands for development of the high speed photonic devices in the terahertz frequency range and have great potentials for applications such as tunable wavelength samplers/filters.

X-ray source on the basis of the piroelectric crystal Sr0.61Ba0.39Nb2O6

The properties of X-ray source based on Sr0.61Ba0.39Nb2O6 crystals (SBN-61) having a large pyroelectric coefficient γ = 85 nC/(cm2K) was studied. When the crystal was heated to 60 ° C, X-rays from a tungsten target with energy of 8.4 keV were detected. The maximum energy of the electron beam was 52 keV. The visualization of the electron beam with a grid electrode and a fluorescent screen showed that the electron beam was inhomogeneous in the polar plane of the crystal and varied during heating.The radiation intensity was unstable in time. The work of the X-ray source was limited to electrical breakdowns between the polar faces of the crystal. When the crystal was cooled, there was no X-ray emission, which could be due to the depolarization of the crystal by the total electric field or as a result of electrical breakdowns. The crystals SBN-61 and LiNbO3 are compared.

Strong anisotropy of electric field effects on uniaxial relaxor ferroelectric Sr0.75Ba0.25Nb2O6 crystals proved by acoustic emission

[001] oriented Sr0.75Ba0.25Nb2O6 uniaxial relaxor ferroelectric crystals have been studied by acoustic emission in the temperature range of 20÷200 °C and under an external electric field up to 1 kV/cm. Under the application of an electric field the temperature of a dielectric maximum exhibits a nontrivial behavior: it remains constant at first, secondly steep decreases down to some threshold field, and thirdly starts to increase as a field enhances, whereas the same temperature of a dielectric maximum under a bias electric field to [100] oriented Sr0.75Ba0.25Nb2O6 crystals exhibits a smoothed minimum before the start to increase as a field enhances (E. Dul'kin et al., J. Appl. Phys. 110, 044106 (2011)). Such a difference of electric field effects in c- and a-cut crystals is discussed from the viewpoint of random-bond-random-field model of relaxor ferroelectrics. By the comparison between experimental and theoretical data, a dipole moment of the PNR was estimated to be 0.1 (C cm).

A comprehensive structural analysis of relaxor ferroelectric Cr- and Ni-doped Sr0.61Ba0.39Nb2O6 crystals

A structural features, real compositions, and point defects of nominally pure and Cr- and Ni-doped Sr0.61Ba0.39Nb2O6 (SBN) crystals, which structure contains four chemically-different and ten crystallochemically-different atoms, have been determined using neutron, X-ray single crystal, X-ray powder, and X-ray synchrotron diffraction techniques, and EXAFS/XANES spectroscopy. The location of transition metal ions has been identified based on the analysis of electron (nuclear) density and confirmed by the EXAFS/XANES together with the definition of formal charges of Ni atoms in Ni-doped SBN crystals. The scope and application possibilities of diffraction methods and EXAFS/XANES spectroscopy for the determination of structural features and defect formation in functional bulk nominally-pure and Cr- and Ni-doped Sr0.61Ba0.39Nb2O6 single crystals is established.

Pairing High Piezoelectric Coefficients, d33, with High Curie Temperature (TC) in Lead-Free (K,Na)NbO3.

The largest piezoelectric properties, d33 = 416 pC/N and 490 pC/N, in KxNa1-xNbO3 ceramics have been reported for compositions close to polymorphic phase transition (PPT); however, they also have Curie temperatures, TC, of around 217-304 °C, considerably lower than those of undoped KNN ceramics (420 °C). High d33 along with high TC remains the ideal choice for applications but, unfortunately, not attained up to now. Here, we show that using KNN single crystals as seeds for template grain growth (TGG) of KNN ceramics enables dramatic improvements in the electromechanical properties while maintaining a high TC. The (001)-oriented (K0.5Na0.5)0.98Li0.02NbO3 ceramics engineered by TGG using (K0.5Na0.5)NbO3 crystals as templates exhibit a high d33 of 280 pC/N while maintaining the high TC of 430 °C. Enhanced piezoelectricity is attributed to long-range ordered ferroelectric domain patterns consisting of 90° and 180° domains, similar to single crystals. It is the first time that pairing high d33 and high TC in KNN, keeping a high PPT temperature, is achieved. This study is an unequivocal proof that it is possible to maximize d33, keeping a high TC in KNN without resorting to heavily doped compositions. This work opens the door to high-performance, rare-earth free, compositionally simple lead-free and low-cost electromechanical compounds, which can largely expand lead-free piezoelectrics applications.

Enhanced photorefractive performance of bulk cu-doped KNSBN crystals through surface electrostatic modification

The remarkable photorefractivity enhancement was observed and investigated in copper-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 (Cu:KNSBN) crystals due to charge accumulation on the surface, stemming from electrostrictive effect. This electrostatic modification to the surface was studied with conventional two beam coupling experiment and over 17 high diffraction orders were observed. To estimate the amount of charge accumulation, the surface charge density was measured through direct current measurement using a close loop circuit. It was believed that a thin phase grating was responsible for the diffraction pattern, which was confirmed by the related reading experiment. Corresponding calculation and analysis were given to highlight the strong refractive index modulation of the gratings.

Correlation between piezoresponse nonlinearity and hysteresis in ferroelectric crystals at the nanoscale

The nonlinear response of a ferroic to external fields has been studied for decades, garnering interest for both understanding fundamental physics, as well as technological applications such as memory devices. Yet, the behavior of ferroelectrics at mesoscopic regimes remains poorly understood, and the scale limits of theories developed for macroscopic regimes are not well tested experimentally. Here, we test the link between piezo-nonlinearity and local piezoelectric strain hysteresis, via AC-field dependent measurements in conjunction with hysteresis measurements with varying voltage windows on (K,Na)NbO3 crystals with band-excitation piezoelectric force microscopy. The correlation coefficient between nonlinearity amplitude and the amplitude during hysteresis loop acquisition shows a clear decrease with increasing AC bias. Further, correlation of polynomial fitting terms from the nonlinear measurements with the hysteresis loop area reveals that the largest correlations are reserved for the quadratic terms, which is expected for irreversible domain wall motion contributions that impact both piezoelectric behavior as well as minor loop formation. This study suggests applicability at local length scales of fundamental principles of Rayleigh behavior, with associated implications for future nanoscale ferroic devices.

High-order diffraction and nanolayer electrostatic modification in Cu-doped (K_05Na_05)_02(Sr_075Ba_025)_09Nb_2O_6 crystals

Electrostrictive and photoinduced charge accumulation in the nanoscale interfaces of air and copper-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 crystals and electrostatic modification to the nanolayer interfaces were studied with conventional two beam coupling. This interface modification was confirmed by (1) emerging 2 dimensional diffraction patterns with over 10 high orders and 6 rows; (2) remarkable electroinduced power change of specular reflection on the very first surface; and (3) the energy coupling of the first reflected beam with another coherent laser beam in the subwavelength scale. Distinct responses to perturbation show coexistence of a surface grating and bulk ones in the sample. Theoretical consideration on excitation of surface plasmon polaritons through photorefractive phase grating mediation was given to elucidate all the related observations.

Investigation of the domain structure and hierarchy in potassium–sodium niobate lead-free piezoelectric single crystals

We reported self-organized and hierarchized domain structures on various length scales ranging from micrometer to nanometer scale in K0.5Na0.5NbO3 crystals.