KNbO3 Crystals Research Articles

Mechanism of the electro-optic effect in the perovskite-type ferroelectric KNbO3 and LiNbO3

Based on the theoretical model published by Chen et al. [Chem. Phys. Lett. 397, 222 (2004)], we present an efficient scheme for studying the mechanism of the electro-optic (EO) effect in perovskite-type ferroelectrics employing the electronic density-functional theory and the generalized gradient approximation. The results show that the main contribution to the EO effect in KNbO3 and LiNbO3 crystals comes from the relative displacements of the central metal ion and oxygen in the NbO6 oxygen octahedron induced by a low frequency applied electric field. These displacements change the band wave functions of the electrons in the crystals through electron-phonon coupling, which then affects the electronic response of the lattice to the incident optical wave (i.e., induces a change of refractive index). Furthermore, the relationship between the microstructure of perovskite-type ferroelectrics and the clamped EO effect and the physical meaning of the parameter βk(ij) published by Chen et al. are explained.

Crystal Growth of KNbO<sub>3</sub> by Solution-Dropping Method

By the solution-dropping method imitating the growth of stalagmite in nature, piezoelectric KNbO3 crystal was prepared onto a substrate by dropping metastable K2NbO3F aqueous solution. The transformation from K2NbO3F to KNbO3 was accelerated by separately dropping H2O2 solution at the same time. Under the conditions of K2NbO3F concentration of 4.2×10-2 mol/l, H2O2 concentration of 9mol/l, dropping rate of 1.7ml/min, dropping time of 2 h, and substrate temperature of 20°C, KNbO3 crystals of 2 to 5μm and polycrystalline film composed of KNbO3 crystals of < 1μm were grown at aqueous temperatures of 20°C and 60°C, respectively.

Large and stable thickness coupling coefficients of [001]C-oriented KNbO3 and Li-modified (K,Na)NbO3 single crystals

Large thickness coupling factors kT⩽0.7 are measured in both [001]C-oriented (45° cut in the orthorhombic ac plane), monodomain and [001]C-poled, domain-engineered single crystals of KNbO3. kT is extremely stable over a large temperature range, remaining high (⩾0.65) even upon heating and cooling through the orthorhombic-tetragonal phase transition. This is likely related to another finding that spontaneously high kT values (⩽0.68) are observed in as-grown (unpoled), [001]C-cut, polydomain KNbO3 crystals. kT is also stable upon thinning to thicknesses below 60μm, as required for high-frequency applications. Finally, large kT values (⩽0.7) are reported in [001]C-poled, polydomain, Li-doped (K0.5Na0.5)NbO3.

Deviation from stoichiometry of KNbO3 crystal in fiber shape grown using pulling down technique

Potassium niobate crystals were grown from high temperature solutions using the floating zone pulling-down technique. The starting solution compositions of K and Nb were changed from K:Nb=50:50 to 58:42. The lattice parameter, lattice volume and phase transformation temperature depended on the starting solution compositions. The solid solution region near the KNbO3 crystals was estimated to be expanding to a K-rich composition.

Self-similarity of the electro-optical effects

Based on the electro-optical coefficients, the Miller-δ coefficients of KNbO3, LiNbO3, BaTiO3 and several semiconductor crystals were calculated. The results show that the coefficients are frequency independent and are the same for unclamped and clamped values, so it is feasible to take the Miller-δ coefficients to characterize the nonlinear optical properties. Furthermore, a novel mechanism of the electro-optical effects is presented which maintains that the electro-optical effect results from the self-similarity of the nonlinear system, manifested as the recopy of the linear properties and the Miller-δ coefficients are the proportion factors of self-copy.

Synthesis and Dielectric Characterization of Potassium Niobate Tantalate Ceramics

The preparation and dielectric properties of potassium niobate tantalate (KTN) have been investigated with the aim of exploring the material's potential for ferroelectric tunable applications. The samples were prepared both by conventional sintering in air and by uniaxial hot pressing. A relative average density greater than 92% was obtained with both methods. An inhomogeneous Nb/Ta distribution was found in the samples prepared by both methods, but the inhomogeneity extent was lower in the hot‐pressed samples. While both sintering processes resulted in ceramics of lower transition temperatures in comparison with the reported results on single crystals, a relative up shift of the temperature (Tmax) at which the dielectric constant is maximum was found for hot‐pressed samples compared with that of samples sintered in air. All the samples exhibited strong frequency dispersion in dielectric properties. The effect of DC bias at room temperature was measured and modeled using the Landau–Devonshire model. It was found that the nonlinear coefficient β of KTN, which is important for tunable applications, is comparable with the value reported on KNbO3 and SrTiO3 single crystals. A dielectric tunability of 16% and 42% at room temperature was demonstrated under 20 kV/cm for the sintered and hot‐pressed ceramics, respectively. The loss tangent, low at megahertz frequencies, was augmented to 9%–17% at low gigahertz frequencies. This is believed to be related to the frequency dispersion observed in the ceramics because of charged defects.

Diode laser measurements of the Ar 3p54s1 excited states in an inductively coupled RF plasma

Diode laser absorption spectroscopy has been used to study the four lowest energy excited states of atomic argon (1s2–1s5) arising from the 3p54s1 electronic configuration and produced in an inductively coupled RF discharge. Three of the states (1s2, 1s4 and 1s5) were detected using wavelength modulation spectroscopy with diode laser radiation in the wavelength range 425–430 nm, obtained by frequency doubling the output of a near infrared diode laser in a KNbO3 crystal. The 1s3 state was probed using direct absorption spectroscopy with the fundamental output of a diode laser operating at 795 nm. The 1s5 state dominates the total population of this manifold with a proportion of around 48%, which is essentially independent of plasma power and pressure. Translational temperatures varied with power but were essentially pressure independent. For example, at 20 mTorr total pressure and 400 W applied power, the temperature was found to be 384 ±15 K. The variations in concentrations of all four 1sn states with plasma power and pressure are reported and suggests that a statistical equilibrium between them has been established. The results are rationalized by utilizing a simple kinetic model, to determine the dominant production (electron impact excitation from the ground state) and loss (intra-manifold quenching and radiative decay) processes. Fast redistribution of population in the states, mainly via electron impact excitation to higher lying electronic states and subsequent radiative decay back down, in combination with modified radiative rates for the two radiative states as a consequence of radiation imprisonment, can partially account for the observed relative trends.

Stress-Free Domain Quadruplets in Ferroics

The coexistence of ferroelectric ferroelastic domains in some materials plays an important role in practical area of domain engineering. Here the problem is discussed theoretically from the point of view of elastic aspects. Domain quadruplets are considered, i.e. systems of four ferroelastic domains coinciding along one intersection line. Conditions at which the elastic energy of quadruplets is zero are specified; they would allow for the existence of permissible quadruplets. The problem has been solved for the species mm − P ϵds-mm2 representing KNbO3 crystals in which regular quadruplets have been discovered. It is concluded that such quadruplets are not permissible but can be formed when specific conditions for spontaneous strain are fulfilled. Theoretical results can be used to specify these conditions.

Large scale computer modelling of point defects in ABO 3 perovskites

We present results for basic intrinsic defects: F-type electron centers, free and bound electron and hole polarons in ABO3 perovskites. Both one-site (atomic) and two-site (molecular) hole polarons are expected to coexist, characterized by close absorption energies. Shell Model (SM) and intermediate neglect of differential overlap (INDO) calculations of the F center diffusion indicate that the relevant activation energy is quite low, ca. 0.8 eV. Further INDO calculations support the existence of self-trapped electron polarons in PbTiO3, BaTiO3, KNbO3, and KTaO3 crystals. The relevant lattice relaxation energies are typically 0.2 eV, whereas the optical absorption energies are around 0.8 eV. We also used the INDO method for modelling the atomic and electronic structure of KNbxTa1–xO3 (KTN) perovskite solid solutions. The Nb impurities in KTaO3 reveal off-center displacements starting with the smallest calculated concentrations. The calculated magnitude of the displacement is close to the EXAFS observation. Using the calculated energy gain for several concentrations of Nb in KTN, we construct the Ginzburg-Landau-type functional for the excess energy. The coefficients of this functional are concentration-dependent. This dependence allows us to define the type of concentration-induced phase transition in KTaO3 alloyed by Nb. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of non-linear Potassium crystals in the Terahertz frequency domain

Systematic measurements of the dielectric properties of KDP, KNbO3, KTP and KTA non-linear crystals in the Terahertz (THz) spectral range are presented. The index of refraction and the absorption coefficients are measured between 0.1 and 1.5 THz for different crystallographic orientations. The data are deduced from an experimental set-up based on standard Terahertz time domain spectroscopy system at room temperature. These data, key parameters for the optimization of non-linear THz generation by optical rectification as well as electro-optic detection, are analysed in term of non-linear capabilities. We finally review different methods making possible to generate THz wave in these crystals and compare their characteristics.

Enhanced Piezoelectric Properties of Potassium Niobate Single Crystals by Domain Engineering

The engineered domain configuration was induced into potassium niobate (KNbO3) crystals, and the piezoelectric properties were investigated as a function of domain size. First, single-domain treatment was investigated, and a combination of chemical etching and 2-step electric poling treatment was effective. Using the single-domain crystals, all piezoelectric related constants were determined. On the basis of these piezoelectric constants, the d31 and d32 surfaces were calculated. The piezoelectric properties of [001]c oriented KNbO3 single-domain crystals were measured and were completely consistent with the calculated properties. Finally, the engineered domain configurations were induced into KNbO3 crystals by the control of the temperature and the electric-field along the [001]c direction. The piezoelectric properties of these KNbO3 crystals with the engineered domain configurations showed much higher values than those of the single-domain crystal. Moreover, the piezoelectric properties increased with decreasing the domain sizes of the engineered domain configuration.

Effect of Poling Field on Piezoelectric Properties of KNbO3 Crystal Grown by Vertical Bridgman Method

Single crystals of KNbO3 were grown by the vertical Bridgman method. The crystals were cut into 6×3×0.5 mm3 plates, and dielectric constants and resonance characteristics were observed as a function of the electric field strength used in the poling treatment. For the [001]pc-poled crystal, which stands for [001] direction in pseudocubic crystal lattice, the dielectric constant increased once with increasing amount of poling field and then decreased again with higher fields. For [101]pc-poled crystal, both a reduction in dielectric constant and a higher phase angle shift in the resonant region were observed when the electric field exceeded 85 V/mm. The series of observations can be consistently interpreted using the domain structures estimated from the anisotropic dielectric constant. The vibrators having their longitudinal directions along the apc and bpc axes showed quite different piezoelectric responses, revealing that the polarization vector lies only in the [101]pc and [101]pc directions.

Noncritical phase matching of Nb:KTP crystal for blue light generation

The relationships between the temperatures and the principal refractive indices of 7.5 mol% Nb:KTP crystal is achieved from the measured refractive indices. The cutoff wavelengths of noncritical phase matching at different temperatures are calculated. Compared with the KNbO3 crystal, the Nb:KTP crystal shows the better performance for generating the blue light.

Study of diode-laser pumped self-Q-switched intracavity-doubled laser of <inline-formula><math display="inline" overflow="scroll"><msup><mrow><mi>Cr</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup><msup><mrow><mi>Nd</mi></mrow><mrow><mn>3</mn><mo>+</mo></mrow></msup><mo>:</mo><mi>YAG</mi></math></inline-formula> plus <inline-formula><math display="inline" overflow="scroll"><msub><mi>KNbO</mi><mrow><mn>3</mn></mrow></msub></math></inline-formula> material

A diode-laser pumped blue intracavity frequency-doubled self-Q-switched microchip laser of a chromium Cr4+ Neodymium Nd3+ codoped yttrium aluminum garnet crystal (Cr4+Nd3+:YAG) combined with a potassium niobate (KNbO3) crystal is developed. By coating the cavity mirrors with the films that suppress the 1064-nm operation and enhance the 946-nm laser, the 4F3/24I9/2 transition of the Nd3+ ion is facilitated to achieve the 946-nm laser oscillation. The 946-nm laser of the Cr4+Nd3+:YAG is self-Q-switched due to the saturable absorption of the Cr4+ ion. The full width at half maximum (FWHM) of the laser pulse at a 964-nm wavelength is about 5.07 ns. A self-Q-switched 473-nm laser pulse is sequentially obtained through intracavity frequency doubling of the 946-nm laser by the KNbO3 crystal, whose FWHM is about 4.30 ns. The 473-nm intracavity doubled laser has a good fundamental transverse TEM00 mode, because the self-Q-switched 946-nm laser has a good TEM00 mode that results from the absorption bleaching established by both the 808-nm pump laser and 946-nm oscillating laser. The constant FWHM results from both the response time of the self-Q-switching and the establishing time of the oscillating laser being much faster than the accumulated time of the pump laser energy. The constant peak power of the 946-nm self-Q-switched laser mainly depends on the modulation ability of the self-Q-switching.

Total Refraction of P-Polarized Light at the Boundary of 90°-Domains in the Ferroelectric Crystal

A 90°-domain structure was fabricated using electrical poling in ferroelectric birefringent KNbO3 crystal. The refraction and reflection characteristics of the light at the boundary of the 90°-domain structure were investigated, and the novel behavior of total refraction (no reflection) has been found for the P-polarized light throughout the entire range of incident angle (0≤θ≤90°). Strong refraction of the light was observed after it exited from the crystal output surface at the incidence parallel to the domain boundary. These behaviors were explained analytically.

Detailed study of an efficient blue laser source by second-harmonic generation in a semimonolithic cavity for the cooling of strontium atoms.

We have constructed a blue laser source consisting of an amplified, grating tuned diode laser that is frequency doubled by a KNbO3 crystal in a compact standing wave cavity and produces as much as 200 mW of internal second-harmonic power. We have analyzed the unusual characteristics of this standing wave cavity to clarify the advantages and disadvantages of this configuration as an alternative to a ring cavity for second-harmonic generation. We emphasize its efficiency and stability and the fact that it has an inherent walk-off compensation, similar to twin crystal configurations. We demonstrate its utility for laser cooling and trapping of earth alkalis by stabilizing the laser to the 461-nm transition of strontium, using a heat pipe, and then forming a magneto-optic trap of strontium from a Zeeman-slowed atomic beam.

An EPMA Study on KNbO3 and NaNbO3 Single Crystals – Potential Reference Materials for Quantitative Microanalysis

Single crystals of KNbO3 and NaNbO3 were selected from the limited number of suitable alkali compounds that are available and evaluated as possible reference materials for the electron-probe microanalysis (EPMA) of alkaline niobates with a composition described by the general formula K1−XNaXNbO3. The EPMA study verified that KNbO3 and NaNbO3 single crystals are stable under the electron beam and compositionally homogeneous. A quantitative microanalysis confirmed the composition of pure KNbO3, while the NaNbO3 crystal contained 0.3 mass fraction % of Ca. A significant improvement in the accuracy of the quantitative EPMA of polycrystalline potassium–sodium niobates was achieved using these single crystals as standards. The crystals can also be useful as reference materials for the analysis of sodium and potassium in other materials.

Characterization of KNbO3 Crystal by Traveling Solvent Floating Zone (TSFZ) Method

The effect of solvent and feed rod composition on optical and electrical properties of KNbO3 crystals grown by the traveling solvent floating zone (TSFZ) method was studied. As-grown crystals were crack-free, blue, semiconductive and had a multidomain structure. Optical transmission decreased with increasing potassium content in the solvent and feed rod, while the absorption edge remained unchanged at 380 nm. Two discontinuities of electrical conductivity over the range from room temperature to 500°C were observed. These transition temperatures decreased with increasing potassium content, whereas the activation energy over the range from room temperature to 100°C remained unchanged at 0.2 eV.

Diode-pumped quasi-three-level 456-nm Nd:GdVO4 laser

A diode-pumped quasi-three-level Nd:GdVO4 laser emitting at a wavelength of 912 nm corresponding to the 4F3/2 — 4I9/2 transition of the neodymium ion and at the second-harmonic linewidth of 456 nm after intracavity frequency doubling in a KNbO3 crystal is studied. The maximum output power at the fundamental frequency was 2.96 W and the conversion efficiency amounted to 48 %. The second-harmonic output power achieved 220 mW for the conversion efficiency of 15 %.

High-efficiency blue-light generation by frequency doubling of picosecond pulses in a thick KNbO_3 crystal

We report an experimental demonstration of highly efficient single-pass second-harmonic generation from 859 nm to 429.5 nm with picosecond pulses in a thick KNbO3 crystal. Both the conversion-efficiency and quantum-noise properties of the generated blue pulses are measured at various pump intensities under a strong focusing condition. We find that the variation of the conversion efficiency of the picosecond second-harmonic generation is an oscillatory function of the input pump intensity (with a maximum efficiency of 56.5%) and is sensitive to the position of the input beam focus in the crystal. The quantum noise on the blue beam can be reduced below the shot-noise limit by 20% at low input power.