One-dimensional steady-state photorefractive spatial solitons in centrosymmetric paraelectric potassium lithium tantalate niobate

We report the first observation of spatial one-dimensional photorefractive screening solitons in centrosymmetric media and compare the experimental results with recent theoretical predictions. We find good qualitative agreement with theory.     

Anomalous electron emission from lithium niobate and lithium tantalate single crystals

Anomalous electron emission from the surface of LiNbO₃ and LiTaO₃ single crystals excited by soft x radiation has been discovered and is investigated. The absence of anomalous emission from the face of these single crystals is established. The experimental results confirm the theoretical conclusions that the anomalous emission is caused by the presence of a maximum in the distribution of the potential in the near-surface layer. The dependence of the lifetime τ of the anomalous emission on the dielectric constant ɛ of the ferroelectric is calculated. Fiz. Tverd. Tela (St. Petersburg) 39, 679–682 (April 1997)     

Fast photorefractive response of vanadium-doped lithium niobate in the visible region

A series of vanadium-doped lithium niobate crystals was grown and their photorefractive properties were investigated with a 532 nm laser. At a total light intensity of 471 mW/cm(2), a short response time of only 0.57 s was achieved for 0.1 mol.% vanadium in LiNbO(3). The photorefractive process is dominated by the diffusion field instead of the photovoltaic field. The dominant charge carriers are electrons. The possible mechanism for the fast photorefractive response is discussed.     

Optical properties of lithium niobate and lithium tantalate crystals with impurities and defects

The photoinduced and Raman scattering in lithium niobate and lithium tantalate crystals with impurities and defects have been studied. An exciting laser beam propagated either along the ferroelectric Z axis or perpendicular to it. The conditions for exciting transverse and longitudinal polar optical modes in Raman spectra are established. The regularities of the excitation of Raman spectra in several polarization geometries (X(ZZ)Y, Z(XX, Y Y)Z, Z(XX, Y Y)Z, X(ZX)Y, X(ZX)X and X(ZX)X) have been investigated. Additional (extra) spectral lines are interpreted as a manifestation of a biphonon enhanced by the Fermi resonance and the result of violation of selection rules for pseudoscalar modes of the A ₂ type due to the reduction of the point symmetry group caused by the presence of impurities and defects in real crystals. The conditions for exciting coherent longitudinal and transverse modes in lithium niobate and lithium tantalate single crystals upon stimulated Raman scattering are analyzed. The temperature evolution of the spectra recorded in the X(ZZ)Y geometry near the ferroelectric phase transition point is explained based on the concept of effective soft mode and analysis of the isofrequency opalescence effect. Strong photoluminescence is found in copper-doped lithium niobate crystals.     

Field-induced light deflection in lithium niobate and lithium tantalate: the possible configurations

Field-induced light deflection by ferroelectric domain walls in lithium niobate (LN) and lithium tantalate (LT) is theoretically studied. The phenomenon can occur not only when both the incident wave vector k and the electric field E are parallel to the z-axis—as demonstrated by experiments made so far—but also when E and/or k are perpendicular to z. In particular, for E parallel to x and k parallel to y, the deflection phenomenon is predicted to have the same characteristics as in triclinic or monoclinic ferroelastics: the large deflection angle is related to the natural birefringence, whereas the deflection amplitude is proportional to the small tilt angle of the neutral lines, which is here induced by the electric field. In periodic domain structures, interference between deflected waves occurs, and the deflected intensity is expected to be largely enhanced when the Bragg condition is satisfied. This transverse configuration is thus specially attractive to characterize periodically-poled crystals.     

Raman spectra of photorefractive lithium niobate single crystals

We have conducted comparative studies of the Raman spectra of lithium niobate (LiNbO₃) crystals of different compositions for excitation in the visible and near IR regions. We have observed that the photorefractive effect is one of the factors leading to line broadening. For this reason, the linewidths may be greater upon Raman excitation in the visible region than for excitation in the near IR region. This may be explained by formation in the crystal, when illuminated by laser radiation in the visible region, of a three-dimensional sublattice of nanostructures and microstructures (with refractive index and other physical parameters different from the parameters of the host crystal) from which photorefractive light scattering occurs. Formation of nanostructures and microstructures makes an additional contribution (besides the contribution due to random and dynamic disorder in the arrangement of the structural units) to the broadening of the Raman lines in the visible region of the spectrum. Illumination of the crystal by radiation in the near IR region does not induce a sublattice of nanostructures and microstructures, due to a significantly smaller photorefractive effect.     

Photonic waveguide structures in photorefractive lithium niobate with pyroelectric mechanism of nonlinear response

Compensation for the nonlinear diffraction of narrow laser beams with wavelengths of 532 and 633nm and the formation of photonic waveguides and waveguide circuits due to the contribution of pyroelectric effect to the nonlinear response of lithium niobate crystal have been experimentally demonstrated. Complete compensation for the linear and nonlinear diffraction broadening of light beams is obtained upon uniform heating of an undoped sample from room temperature to 55?C.     

Electric-field-controlled two-dimensional Raman-Nath diffraction from paraelectric potassium lithium tantalate niobate

Electric-field-controlled two-dimensional Raman-Nath diffraction has been realized using a photorefractive diffraction grating. The grating was produced by two-wave coupling (at a wavelength of 632.8 nm) at small incidence angles using a potassium lithium tantalate niobate single crystal. Results for the Raman-Nath diffraction from the g₄₄ grating are presented, in which the externally applied field is perpendicular to both the grating vector and the wave vector of the incident beam. Two pairs of coherent beams were used to record the grating for two-dimensional Raman-Nath diffraction. The wave vector and the polarization of one pair lay in the (x, z) plane, and those of the other pair lay in the (y, z) plane. The influence of the applied electric field was studied, and the results show that the intensity of the Raman-Nath diffraction could be controlled by the direction and intensity of the applied field.     

Improved ultraviolet photorefractive properties of vanadium-doped lithium niobate crystals

A series of vanadium-doped lithium niobate (LN:V) crystals has been grown and their photorefractive properties were investigated. For 0.1 mol. % V-doped LiNbO(3), a fast photorefractive response of 160 ms was obtained with a 351 nm laser and a total light intensity of 583 mW/cm(2). The measurements of the x-ray photoelectron spectrum and electron paramagnetic resonance show that V(3+), V(4+), and V(5+) ions exist in these LN:V crystals. V(3+) and V(4+) ions correspond to the 420 and 475 nm absorption peaks, respectively. The fast photorefractive response and high sensitivity indicate that LN:V is a suitable candidate for UV photorefractive applications.     

Study of the subsurface photorefractive effect in lithium niobate

A new phenomenon of a strong irreversible variation of the refractive index (n–0.2) in a skin layer (thickness 300 Å) has been discovered. The effect arises under the strong absorption of high-power uv radiation (249 nm, 5 ns, 20 MW/cm²). The characteristics of the subsurface photorefractive effect (SPRE) were identified by using the data on measurements of the reflection coefficient of a crystal and of the diffraction efficiency of a recorded phase grating.     

Magnetic-field-induced changes in the photorefractive sensitivity of lithium niobate

The effect of a magnetic field on the average photorefractive sensitivity of an undoped LiNbO₃ crystal is studied by phase-mismatched second-harmonic generation. The experimental data obtained show the photorefractive sensitivity to reverse sign as the external magnetic field exceeds B ₁=?0.38±0.04 T. The magnetic field is oriented perpendicular to the crystal optical axis and to the plane of laser radiation polarization. The variation of the photorefractive sensitivity is associated with paramagnetic iron centers, whose photoionization probability depends on the direction of their magnetic moment relative to the optical axis.     

Complex study of bulk screening processes in single crystals of lithium niobate and lithium tantalate family

Comprehensive study of depolarization field bulk screening was carried out in lithium niobate and lithium tantalate single crystals with various stoichiometry. Three used complementary methods are based on: 1) dependence of coercive field on delay time; 2) decrease of optical contrast of domain wall trace; 3) relaxation of light diffraction intensity on domain walls. The following parameters of bulk screening process were obtained: relaxation time constants, type of relaxation law, maximal value of bias field and “true” value of coercive field. Advantages and disadvantages of used experimental methods were analyzed.     

Enhanced terahertz-wave parametric generation and oscillation in lithium niobate waveguides at terahertz frequencies

We observed parametric-generation efficiency of 1.61% from 1064 to 1071 nm and at 162 microm in a 0.5 mm thick, 45 mm long z-cut congruent lithium niobate waveguide with a pump energy of 2.2 mJ and a pump pulse width of 5.8 ns. We also measured an ultralow-threshold intensity of 70 MW/cm2 for a 1064 nm pumped parametric oscillator resonating at 1071 nm and emitting at 162 microm with a 1 mm thick, 45 mm long lithium niobate waveguide.     

Photorefractive properties of paraelectric potassium lithium tantalate niobate crystal doped with iron

We report the successful growth of paraelectric potassium lithium tantalate niobate (KLTN) single crystal doped with iron. Detailed investigations have been made on the photorefractive properties of the as-grown crystal. The key parameters such as space-charge field, grating response time, photorefractive sensitivity and sign of the dominant charge carrier were obtained by two-wave mixing technique. 1.7 mm thick sample exhibits a high diffraction efficiency of 78% at the external field of 3.3 kV/cm and a sensitivity of 1.49 × 10⁻¹⁰E₀ cm²/J. The two-wave mixing gain coefficient increases linearly with external field, and reaches a large value of 19.4 cm⁻¹ at 4 kV/cm. Based on experimental results, iron is an effective dopant to KLTN which shows high diffraction efficiency and two-wave mixing gain coefficient.     

Electric field controlled higher-order diffraction images of paraelectric potassium lithium tantalate niobate

We report some electric field controlled photorefractive higher-order diffraction phenomena of a paraelectric phase potassium lithium tantalate niobate crystal doped with iron. In experiments, a p-polarized semiconductor laser (532 nm) was used to record grating at a small incident angle. Higher-order diffraction images were observed when the signal beam was focused behind and in front of the crystal. Then the higher-order diffraction images were reconstructed by a p-polarized He–Ne laser (632.8 nm) in the direction perpendicular to the surface. The higher-order diffraction images could be controlled by the external electric field. A theory about the higher-order diffraction images of the K and 2K grating is developed. The results show that the even order diffraction images of the K grating and the odd order diffraction of the 2K grating overlap each other. The odd order diffraction images of the K grating are diffracted in unattached direction. The electric field controlled higher-order diffraction image provides a useful method for optical information processing.     

Frequency mixing of photorefractive and ferroelectric gratings in lithium niobate crystals

Holographically recorded photorefractive gratings in periodically poled lithium niobate crystals (PPLNs) are investigated. The principal spatial frequency K of the grating is strongly suppressed. Sideband gratings with grating vectors K +/- G appear. From the measurements the domain grating vector G and the duty cycle of the domain structure can be obtained. These findings allow for fast nondestructive quality inspection of PPLN and are of importance for any optical application combining holography and PPLN.     

Influence of surroundings on photorefractive effect in lithium niobate crystals

In the paper results of the investigation of the influence of electric properties of the environment surrounding LiNbO₃ crystals on photorefractive effect induced in these crystals by Gaussian Ar⁺ laser beam with various intensities are presented. We show spatial and temporal dependences of changes of the refractive index obtained experimentally in LiNbO₃: Fe and LiNbO₃: Fe:Mn samples surrounded by media with different electric conductivities and different permittivities (water, air, water solution of CaCl₂). The space and time dependences of the refractive index changes induced by the Ar⁺ laser beam are observed by means of the Mach–Zehnder interferometer using light from HeNe laser. The experimentally obtained results are in a good agreement with those following from numerical calculations using the manifold mirroring method. The agreement between calculated and experimental results indicates that the polarization charge at the photorefractive crystal/surrounding medium boundary significantly influences the photorefractive process in the crystal. The experimentally observed slow spontaneous decrease of the refractive index change in a sample placed into a slightly conducting medium (air) after switching off the beam also indicates that the polarization charge in the sample's surroundings affects the photorefraction.     

Photocarrier transport in iron-doped potassium lithium tantalate niobate studied by time-of-flight measurement

The photocarrier mobility of Fe 0.03 wt%-doped potassium lithium tantalate niobate (K_0.95Li_0.05Ta_0.61Nb_0.39O₃) was investigated by time-of-flight (TOF) measurement. The longitudinal photocarrier response due to pulsed excitation leads to values of the drift mobility of μ_h = 1.45 × 10⁻² cm²/V s for holes, μ_e = 0.325 × 10⁻² cm²/V s for electrons, and a value for the range of holes (μτ)_h = 4.38 × 10⁻⁵ cm²/V at room temperature and at low field 3 KV/cm. The response time of holes and electrons (or the relaxation time) is determined to be 3.02 × 10⁻³ s and 3.74 × 10⁻³ s, respectively. The mobility of holes strongly depends on the field strength, and is observed to decrease with increasing bias field.     

Laser generation of acoustic waves at a periodic domain structure in lithium niobate

Generation of coherent acoustic oscillations due to the interaction of laser pulses with the periodic domain structure formed in a lithium niobate single crystal is observed. It is found that the excitation of acoustic waves is most efficient when the generated wavelength is equal to the period of the domain structure. The proposed mechanism of the optical generation of acoustic oscillations consists of the photogeneration of free carriers, which compensate the polarization fields within the domains, and the occurrence of alternating elastic stresses caused by the piezoelectric effect.