Doped Lithium Niobate Crystals Research Articles

Optical fabrication of (2 + 1)-dimensional photorefractive photonic lattices by improved Billet split lens

A convenient way to optical fabricate (2 + 1)-dimensional photonic lattice microstructures was experimentally demonstrated in photorefractive crystal. An improved Billet split lens combined with a Fourier transform lens can generate large area interference of multiple plane waves. The (2 + 1)-dimensional square photonic lattice microstructures are optically induced in an iron doped lithium niobate crystal. Induced lattice microstructures are analyzed and validated using several different experimental methods. This method has good flexibility and scalability, stable equipment, and does not require complex adjustment systems. It can be used to optical fabricate more types of (2 + 1)-dimensional lattice microstructures by design and processing of improved Billet split lens reasonably.

Люминесцентные свойства номинально чистых и легированных кристаллов ниобата лития, полученных по разным технологиям

The dependence of photoluminescence of nominally pure and Zn (Mg)-doped lithium niobate crystals on composition, kind, concentration of doped element and doping technology, was observed. The photoluminescence did not depend on doped element kind and was decreased with increasing in stoichiometry. A decrease in the luminescence of NbNb–NbLi centers was observed in weakly Zn (Mg)-doped LiNbO3 crystals, the emission of NbNbO6 groups dominated in strongly ones. LiNbO3 : Zn (Mg) crystals obtained by homogeneous doping had the emission of both NbNb — NbLi centers and NbNbO6-groups.

Second-order Raman spectra of a LiNbO3:Tb crystal

Additional bands have been discovered for the first time in the Raman spectra of terbium doped lithium niobate crystals. The bands have been determined in the high-frequency region and have been attributed to the overtone states of the second order of polar fundamental modes of the crystal lattice. It has been established that Raman spectrum of terbium doped lithium niobate contains bands with frequencies exceeding the overtones of the highest-frequency fundamental polar modes of A1(Z) and E(X, Y) symmetry types. The observed features of the second-order Raman spectra have been interpreted as a manifestation of bound states of phonons – biphonons.

Tilt control of the charged domain walls created by local switching on the non-polar cut of MgO doped lithium niobate single crystals

Growth of the wedge-like domains with charged domain walls was studied under local switching by a biased tip on nonpolar cut of MgO doped lithium niobate crystal. The tilt of the domain wall from the polar direction was controlled by voltage and number of switching pulses. The tilt reduction with voltage was observed. The essential tilt increasing up to 4° caused by width growth was revealed for application of the several pulses with the same parameters. The obtained results were attributed to the step generation in the external field and kink motion in the field produced by the charged kinks.

Pyroelectric-controlled bending of a self-trapped optical beam in a photorefractive iron doped lithium niobate crystal

We present the experimental demonstration of a self-trapped optical beam bending in a photorefractive Fe-doped lithium niobate (LN:Fe) crystal controlled by the pyroelectric effect. Formation of self-trapped beams with typical [Formula: see text]50[Formula: see text][Formula: see text]m diameter and large bending of [Formula: see text]140[Formula: see text][Formula: see text]m are depicted in a 1[Formula: see text]cm length LN:Fe crystal for a laser beam at 632.8[Formula: see text]nm wavelength and 0.5[Formula: see text]mW power with a 30∘C crystal temperature change. The self-trapped beam bending is opposite to the crystal [Formula: see text]-axis. The underlying physics is elaborated and numerical simulations are performed. The long-living waveguiding channels with controlled curvilinear trajectories are promising for optical information routing.

Enhanced Ultraviolet Damage Resistance in Magnesium Doped Lithium Niobate Crystals through Zirconium Co-Doping.

MgO-doped LiNbO3 (LN:Mg) is famous for its high resistance to optical damage, but this phenomenon only occurs in visible and infrared regions, and its photorefraction is not decreased but enhanced in ultraviolet region. Here we investigated a series of ZrO2 co-doped LN:Mg (LN:Mg,Zr) regarding their ultraviolet photorefractive properties. The optical damage resistance experiment indicated that the resistance against ultraviolet damage of LN:Mg was significantly enhanced with increased ZrO2 doping concentration. Moreover, first-principles calculations manifested that the enhancement of ultraviolet damage resistance for LN:Mg,Zr was mainly determined by both the increased band gap and the reduced ultraviolet photorefractive center O2−/−. So, LN:Mg,Zr crystals would become an excellent candidate for ultraviolet nonlinear optical material.

Анализ влияния легирования на структурные дефекты в LiNbO3.

Changes in a structural state of homogeneous doped lithium niobate crystals with magnesium in the region of 5 mol. % were studied by X-ray diffraction methods. It was found that the changes in the structure of LiNbO3: Mg occured not only in the region of the impurity defect, but also in the region of the main motive of the structure. Bond lengths in the octahedra of the main motive and the Nb–Li distance along the polar axis of the crystal changed. The magnesium introduced into the vacant lithium octahedron distorted the shape of the octahedron more strongly than niobium.

Generation of regular optical patterns and photonic structures by a single Gaussian beam in a photorefractive LiNbO3:Fe crystal

We report 2D micrometric scale regular optical patterns formation induced by a laser beam propagating through a Fe doped lithium niobate (LN:Fe) crystal. A single mode CW laser radiation at 632.8 nm wavelength and beam powers from 0.5 to 10 mW and LN:Fe samples with 0.03 wt% Fe concentration and thicknesses of 4 mm and 10 mm were used in the experiments. The observed phenomenon of optical patterns formation in photorefractive crystal is explained with the light-induced complex refractive index variation with a central minimum (negative lens) and symmetric maxima (positive lenses) on each side due to photovoltaic effect in the crystal. Light-induced complex lenses produce beam defocusing from the beam center and focusing on the periphery. Mutual interference of the beams leads to a regular optical pattern formation. Regular optical pattern, in its turn, leads to the formation of quasi-crystalline photonic structure in LN:Fe crystal. Experimental results are supported by numerical simulations.

Two-Color Quantum Correlation between Down-Conversion Beams at 1.5 and 3.3 μm from a Singly Resonant Optical Parametric Oscillator

We demonstrated a two-color quantum correlation between the down-conversion beams with a telecommunication wavelength at 1.5 μm and mid-infrared wavelength at 3.3 μm generated by a singly resonant optical parametric oscillator (SRO) operated above the pump threshold with a magnesium-oxide doped periodically-poled lithium niobate crystal in the cavity. A maximum of 1.8 dB noise reduction of the intensity difference of the twin beams was measured at the analysis frequency of 5 MHz. Based on a theoretical model for the quantum correlation between the twin beams given by a semi-classical approach, the influence of the analysis frequency and pump parameter on the quantum correlation between the twin beams was discussed theoretically and experimentally. The quantum correlation between the twin beams was degraded at the analysis frequencies above 5 MHz due to the limitation of the bandwidth of SRO cavity and was degraded at the analysis frequencies below 5 MHz due to the excess intensity noise of the pump. The two-color quantum correlated twin beams at 1.5 and 3.3 μm have potential applications in high-precision measurements beyond the shot noise level.

Polaron approach for nondestructive readout of holograms in photorefractive LiNbO3:Fe crystal in an external magnetic field

A novel method of magnetic field assisted nondestructive optical probing of photonic lattice structures in photorefractive crystals is demonstrated experimentally. High contrast annular symmetry micrometric scale refractive lattice structures were recorded by nondiffracting Bessel beam technique with the use of 532 nm, 17 mW power laser beam in Fe doped lithium niobate crystal. The stability of recorded lattices against erasure during readout by probe Gaussian beam at the recording wavelength has been studied. Time evolutions of the diffraction efficiency of the lattices for readout beam power of 17 mW in zero magnetic field and in external magnetic field of 0.85 T were measured. The measurements showed up to 3.4 times increase of erasure constant for magnetic field assisted readout of stored lattices. The physical model based on the approach of small polarons in external magnetic field to explain the experimental results is discussed.

High Homogeneity of Magnesium Doped LiNbO3 Crystals Grown by Bridgman Method

A series of LiNbO3 crystals doped with various MgO concentrations (0, 3%, and 5 mol%) was simultaneously grown in one furnace by the modified vertical Bridgman method. The wet chemistry method was used to prepare the polycrystalline powders, and the growth conditions were optimized. The full width at half maximum of high-resolution X-ray rocking curves for (001) reflection of 5 mol% Mg doped lithium niobate (LN) crystal was about 8″, which meant it possessed high crystalline quality. The OH− absorption spectra shifted to 3534.7 cm−1, and the UV absorption edge violet shift indicated that 5 mol% MgO successfully doped in LN and exceeded the threshold. The extraordinary refractive index gradient of 5 mol% Mg doped LN crystal was as small as 2.5 × 10−5/cm, which exhibited high optical homogeneity.

Impact of a Dopant Impurity Electronic Structure on Physical Properties, Defect Structure, and Features of Lithium Niobate Doping Technology

The macro- and microstructure of doped LiNbO3 crystals is studied, their transmission spectra are investigated, and the effective coefficients characterizing the dopant impurity distribution are determined. We analyze the literature data on phase diagrams of the ternary system Li2O–Nb2O5–dopant oxide and the electron configurations of dopant elements, and the results suggest that this can be used as a basis for designing technological conditions for growing doped lithium niobate crystals and predicting their quality. With p‑elements (boron), structurally and compositionally uniform LiNbO3 crystals can be grown, with the doping impurity not being incorporated in the crystal. s- and d-metals (magnesium and zinc) have a similar effect on the melt and properties of resulting LiNbO3 crystals, which have periodic domain structures and similar types of point defects. Due to their electronic structure, f-metals (cerium) bring the melt to such a structure that this enables the formation of a periodic domain structure in LiNbO3 crystals during their growth.

Coordinate dependence of the photo-response in metal – lithium niobate – metal sandwich structure

The light induced current in doped lithium niobate crystals with two electrodes of different metals (sandwich system) includes the photovoltaic and the thermoinduced effects. The anomalous coordinate dependences of the light response are revealed and investigated experimentally.

Determination of Electro-Optical Coefficients of Lithium Niobate Crystals

The electro-optical coefficients r22, rе of doped lithium niobate crystals were determined by the interference-polarization method, depending on the zinc concentration in the range 0.018-0.88 wt. %. The dependence of the electro-optical coefficient on the zinc concentration is nonlinear. The values r22, rе are determined.

Photoelectric Fields in Nominally Undoped and Doped Lithium Niobate Crystals

Photoelectric Fields in Nominally Undoped and Doped Lithium Niobate Crystals

Photo-induced effects in the doped ferroelectric

Light induced currents in thin lithium niobate crystal were described. New effects were observed in doped lithium niobate crystals with two electrodes of different metals (sandwich system). The light induced response includes the photovoltaic effect and the thermoinduced effect. Spectral and coordinate dependences of the light response are investigated.

Group and phase birefringence dispersion of pure and doped lithium niobate crystals obtained by analysis of interference pattern observed behind a plane polariscope

A sample of lithium niobate crystal with known thickness is set on a vertical rotary stage, placed in between two crossed plane polarizers and illuminated by a collimated beam of white light. The sample is rotated to an appropriate position, and interference fringes observed behind an analyzer are recorded by a fiber-optic spectrometer. The recorded channeled spectrum is then analyzed, and from measured positions of interference minima (dark fringes) and known crystal’s thickness, the group birefringence of the crystal sample is obtained as a function of wavelength. The fringe order versus position of the fringe within the spectrum is fitted by appropriate dispersion function, and as the result phase birefringence as a function of wavelength is found. The measurement is performed for a sample declared as undoped, a sample doped with 0.025 wt. % of Fe, and a sample doped with 0.025 wt. % of Fe and 0.075 wt. % of Mn in the wavelength ranges (470 to 780) nm and (900 to 1700) nm. A good agreement between group birefringence dispersion obtained from positions of interference minima and known sample’s thickness and that calculated from phase birefringence dispersion given by Sellmeier dispersion model (used for undoped sample) and Cauchy model (used for doped samples) is found.

Electro-optic properties of singly and doubly doped lithium niobate crystal by rare earth elements for optoelectronic and laser applications

Congruent lithium niobate (LN) doped with rare-earth elements is a promising host for integrated optic combining the properties straightforwardly associated to the dopant with the good electro-optic (EO) properties. By direct technique based on interferometric optical arrangement at a wavelength of 633 nm and at room temperature, we have experimentally determined the figure of merit F = n3reff, (n is the refractive index and reff, the effective EO coefficient) and finally calculated the EO coefficients reff of the third-column of the unclamped EO tensor of three series of singly doped LN with Yb3+, Ho3+, Tm3+ and two series of doubly doped Er3+–Yb3+. It is found that in all the studied opto-geometric configurations, the unclamped figure of merit and consequently the corresponding EO coefficients are relatively constant in the considered dopant concentration range. As the figure of merit, F qualifies crystals for EO modulation and laser applications, all reported results confirm that the LN singly or doubly doped with rare-earth elements are very promising versatile candidates for several multifunctional nonlinear devices in optoelectronic and laser applications.

Влияние электронного строения примеси на физические свойства, дефектную структуру и особенности технологии легирования кристаллов ниобата лития

Macro- and microstructure were researched for doped LiNbO3 crystals. The transmission spectra and the effective distribution coefficients of the dopant were studied. The analysis of literature data on ternary phase diagrams of systems Li2O-Nb2O5-dopant oxide and electronic configurations of dopants has revealed a possibility to predict technological growth conditions and quality of doped lithium niobate crystals. At this p-elements (boron) provide growth of structurally and compositionally uniform LiNbO3 crystals without incorporation of a dopant into the structure. Metals, s- and d-elements (magnesium and zinc) influence the melts and the crystal properties similar. At this non-periodical domain structures and similar types of point defects appear. Metals of f-elements (cerium) order the melt structure in such a way that provide forming of growth regular domain structure in LiNbO3 crystals due to their electron configurations. Keywords: crystal, lithium niobate, doping, optical microscopy.

Defect Complexes and Optical Properties of Doubly Doped Lithium Niobate Crystals

The effect of defect complexes related to the presence of protons on the optical properties of LiNbO3:Y(0.24):Mg(0.63) and LiNbO3:Gd(0.25):Mg(0.75) (wt %) crystals has been studied using IR absorption spectroscopy, photoinduced light scattering, and laser conoscopy. The results indicate that the absorption bands at frequencies of 3483 and 3486 cm–1 correspond to vibrations of a V Li - –OH− defect complex. To a Mg Li + - Mg Nb 3- –OH−defect complex in the LiNbO3:Gd(0.25):Mg(0.75) crystal correspond bands at frequencies of 3526 and 3535 cm–1. In the LiNbO3:Y(0.24):Mg(0.63) crystal, we have detected an anomalous opening of the speckle structure of the photoinduced light scattering indicatrix.