Dewei Gong

Double-loop hysteresis in tetragonal KTa0.58Nb0.42O3 correlated to recoverable reorientations of the asymmetric polar domains

KTa1−xNbxO3 single crystals are typically regarded as normal ferroelectrics. Such crystals are thought to undergo a paraelectric-ferroelectric phase transition from the cubic to the tetragonal phase where generally a single hysteresis loop is expected. Nevertheless, a double-loop hysteresis is clearly observed in the tetragonal phase of KTa0.58Nb0.42O3. Our investigations reveal further that the double loop exhibits good ergodicity during changes in temperature under various applied electric fields. The origin of this unusual double-loop hysteresis in tetragonal KTa0.58Nb0.42O3 is revealed with the recoverable reorientation of the asymmetric polar domains.

Spectroscopic and upconversion properties of erbium-doped potassium lithium tantalate niobate crystals under 800 nm femtosecond laser excitation

The single crystals of tungsten bronze-type potassium lithium tantalate niobate doped with three concentrations erbium with good quality are grown using top-seed solution growth method. The unpolarized ultraviolet-visible-near infrared optical absorption spectra of these single crystals are measured at room temperature. The green and red upconversion emission properties are analyzed with respect to steady-state spectra and decay behaviors under 800 nm femtosecond laser excitation. The spectroscopic properties of Er3+ ions are studied on the basis of the Judd–Ofelt theory. 12 Er3+ ions absorption bands are observed ranging from 350 to 1650 nm, the lower limit is also the ultraviolet absorption edge of these crystals. The fluorescence branching ratio and radiative lifetime are calculated to support the upconversion mechanism. The relationship of the Judd–Ofelt parameters Ω4 Ω4>Ω6. Experimental results indicate that upconversion emission is ...

Enhanced photorefractive properties of paraelectric potassium–lithium–tantalate–niobate by manganese doping

Paraelectric potassium–lithium–tantalate–niobate, or KLTN, crystals have been grown with two different Mn concentrations: 0.25 and 0.5 mol%. Detailed investigations have been made on the photorefractive (PR) properties of the as-grown crystals by the two-wave mixing technique. High diffraction efficiencies of 90% and 62.5% and PR sensitivities of 1.0 × 10−9E0 cm2 J−1 and 3.0 × 10−9E0 cm2 J−1 were achieved for 1.5 mm thick Mn0.25 : KLTN and Mn0.5 : KLTN, respectively. The measured two-wave mixing gains for 0.25 mol% and 0.5 mol% reached large values of 27 cm−1 at the external field of E0 = 3.4 kV cm−1 and of 25 cm−1 at E0 = 3.9 kV cm−1, respectively. The normalized PR response time under 1 W cm−2 illumination is less than 1 s at 532 nm. It is shown that Mn can effectively enhance the PR properties of KLTN which is a very promising ideal material for electroholographic applications.

The dependence of orientational optical nonlinearity in dye-doped liquid-crystal films on the polarization direction of the recording beams

We study, both experimentally and theoretically, the dependence of the absorbance and the two-beam coupling diffraction efficiency of dye-doped liquid-crystal films on the angle between the director vector of the liquid-crystal molecules and the polarization direction of the recording beams, which is shifted continuously from parallel to antiparallel corresponding to the director vector of the liquid-crystal molecules. The sample used in our experiment is the nematic liquid-crystal 5CB-doped with Disperse Red 1 (DR1). Also, there is no electronic field applied to it. The diffraction efficiency of the probe beam increases with the increase of the recording light absorption. Coupled-wave equations are developed to describe the laser-induced refractive index change, grating formation, and coherent wave-mixing effect. The analytical solutions of the recorded holographic grating and the diffraction efficiency of the probe beam are obtained for the case of taking losses into account. The results show that the diffraction efficiency can be improved by choosing suitable material parameters and recording parameters

High dielectric tunability of KTa0.60Nb0.40O3 single crystal

The dielectric nonlinear behavior of KTa0.60Nb0.40O3 single crystal grown by the top-seeded solution growth method was studied. High dielectric tunability (77%) and low loss (2.3%) was obtained at 1 kHz and room temperature, indicating that the KTa0.60Nb0.40O3 crystal is an attractive candidate for tunable device applications. The dielectric nonlinear behavior was further studied in terms with the Landau-Ginzburg-Devonshire thermodynamic theory completed with the Langevin term. It is found that the “extrinsic” polarization plays a remarkable role in the dielectric nonlinear behavior and it gives an amount of 42.1% contribution to the dielectric tunability with E = 1 kV/mm.

Electric field control of a Bragg diffraction optical beam splitter based on a cubic K 0.99 Li 0.01 Ta 0.63 Nb 0.37 O 3 single crystal

We have realized an electric field controlled Bragg diffraction optical beam splitter based on a photorefractive Bragg diffraction grating. In our experiments, the splitter was produced by wave coupling (532.0 nm) with a potassium lithium tantalate niobate single crystal. In the process of splitting, the incident beam could be split into multioutput beams by the splitter. The influence of an externally applied electric field was studied, and the results show that the intensity of the Bragg diffraction could be controlled by the electric field. The polarization properties of the splitter are discussed.

Optical properties of the a-axis lithium niobate single-crystal fiber with applied electric field

The optical properties of the a-axis lithium niobate (LiNbO3) single-crystal fiber are theoretically studied in this paper. We solve the electromagnetic field equations of the anisotropic a-axis single-crystal fiber and numerically analyze the mode characteristics of the a-axis LiNbO3 single-crystal fiber which only conducts the second-order elementary mode. We discuss the effect of the applied electric field on the refractive index anisotropy and the mode characteristics of this type of fiber which only conducts the second order elementary mode.

Charge compensation mechanism in La-doped potassium sodium tantalate niobate ceramics

(K0.95Na0.05)1?3xLaxTa0.60Nb0.40O3 ceramics (KNTN-La-x; x?=?0, 0.003, 0.007, and 0.01) were fabricated by the conventional solid-state method. The charge compensation mechanism was investigated by analyzing the current?voltage characteristics of the KNTN ceramics with various La doping contents. It was found that La doping induced semiconductivity in KNTN ceramics. This indicates the presence of the electronic compensation mechanism in La-doped KNTN ceramics, in addition to the well-studied ionic compensation mechanism. Further investigation revealed that the charge compensation mechanism could be altered by annealing, and that La was converted to become ionically compensated.

Electric field control of a two-dimensional higher order diffraction optical beam splitter based on a cubic K0.99Li0.01Ta0.63Nb0.37O3 single crystal

An electric field controlled two-dimensional higher order diffraction optical beam splitter has been realized based on a photorefractive higher order diffraction grating. In experiments, the splitter was produced by wave coupling (632.8 nm, 532.0 nm) at a small incident angle with a potassium lithium tantalate niobate single crystal. In the process of splitting, the incident beam of different wavelengths (632.8 nm, 532.0 nm) could be split into multi-output beams by the splitter. The influence of an externally applied electric field was studied and results show that the intensity of higher order diffraction could be controlled by the electric field. The polarization properties of higher order diffraction were discussed. An electric field controlled five–three optical beam splitter was investigated theoretically.

Mode Characteristics of the Strongly Anisotropic

The mode theory of the strongly anisotropic a axis step-index single-crystal fiber is studied in this paper. The electromagnetic field equations of the a axis step-index single-crystal fiber are solved under the strongly anisotropic condition, and the transverse electric field components are obtained. The dispersion equations far from cutoff, near cutoff, and in the guided mode region are discussed, respectively. Numerical results of the mth-order elementary modes are presented for the strongly anisotropic a axis step-index single-crystal fiber.