Valentin Grachev

Macroscopic and microscopic defects and nonlinear optical properties of KH2PO4 crystals with embedded TiO2 nanoparticles

Results from the successful growth of high quality KH2PO4 (KDP) crystals with incorporated TiO2 anatase nanoparticles and the characterization of these crystals using several complementary methods are presented. The study allowed the nature and distribution of macroscopic and microscopic defects in the KDP:TiO2 crystals to be clarified. The relationship between these defects and the distribution of TiO2 nanoparticles, and the influence of incorporated nanoparticles on the nonlinear optical properties of composite crystals in comparison with pure crystals were also elucidated. Visual observations, transmission and scanning electron microscopy have shown that the anatase nanoparticles were captured mainly by the pyramidal growth sector and, to a considerably lesser extent, by the prismatic growth sector. Energy dispersive x-ray analysis was able to confirm that the growth layer stacks contain the TiO2 particles. Fourier transformation infrared spectra have clearly shown the presence of an absorption band at...

Site selective substitution Pt for Ti in KTiOPO4:Ga crystals revealed by electron paramagnetic resonance

Electron Paramagnetic Resonance at low temperatures has been used to characterize potassium titanyl phosphate (KTiOPO4) single crystals grown by different techniques. Irradiation with 20 MeV electrons performed at room temperature and liquid nitrogen temperature caused an appearance of electrons and holes. Platinum impurities act as electron traps in KTiOPO4 creating Pt3+ centers. Two different Pt3+ centers were observed, Pt(A) and Pt(D). The Pt(A) centers are dominant in undoped samples, whereas Pt(D)—in Ga-doped KTP crystals. Superhyperfine structure registered for Pt(D) centers was attributed to interactions of platinum electrons with 39K and two 31P nuclei in their surroundings. In both Pt(A) and Pt(D) centers, Pt3+ ions substitute for Ti4+ ions, but with a preference to one of two electrically distinct crystallographic positions. The site selective substitution can be controlled by the Ga-doping.

Second axial Fe3+ center in stoichiometric lithium tantalate

The axial Fe3+ center Fe1 with the crystal field parameter b20≈3130×10−4cm−1 is well studied in congruent lithium tantalate crystals. The second axial Fe3+ center Fe2 was discovered and investigated by the electron paramagnetic resonance in stoichiometric samples prepared by vapor transport equilibrium treatment. The crystal field parameter of the Fe2 center (b20≈2050×10−4cm−1) is significantly smaller than for Fe1. The electron nuclear double resonance measurements have shown that hyperfine interactions of the Fe3+ electrons with the surrounding Li nuclei for Fe2 are stronger than for Fe1. Therefore, the conclusion was made that in the case of Fe2 center the iron ion substitutes for Ta and has Li nuclei in the nearest neighborhood, whereas in the case of Fe1 center it substitutes for Li, has Ta nuclei as nearest neighbors and Li nuclei in the second shell only.

Paramagnetic defects in KH2PO4 crystals with high concentration of embedded TiO2 nanoparticles

Qualitative transformations of spectra of Electron Paramagnetic Resonance, EPR, were found in KH2PO4 crystals grown from liquor with 10−5–10−1 wt. % of anatase TiO2 nanoparticles in comparison with nominally pure KH2PO4. The nanoparticles have larger segregation coefficient for prismatic parts of the crystals than for pyramidal ones. Significant decrease in resonance absorption, complete disappearance of EPR lines of Fe3+ and Cr3+ centers, and appearance of four weak lines of equal intensities together with broad asymmetric lines with g-factors about 2.07–2.5 was observed in pyramidal parts grown with concentration of TiO2 nanoparticles larger than the threshold value 10−2 wt. %. The four lines were attributed to non-controlled impurity As substituted for P. In the presence of TiO2 nanoparticles, non-paramagnetic AsO43− clusters trap electrons becoming AsO44−. Disappearance of Fe3+ and Cr3+ centers was explained by their recharge to “EPR-silent” states and/or pairing at the surface of TiO2 nanoparticles.

Single, dimer and trimer chromium centers in lithium niobate

Abstract Chromium doped lithium niobate crystals of different stoichiometric composition were investigated by means of electron paramagnetic resonance. It was found that characteristics of observed spectra have the strong dependence on chromium concentration and crystal stoichiometry. The parameters of exchange interaction of dimer chromium center were determined.

Structural analysis of the dominant axial Fe3+ center in LiNbO3 crystal by electron nuclear double resonance

Measurements of the angular dependencies of Electron Nuclear Double Resonance (ENDOR) were used for the analysis of the substitution site and local environment of the dominant axial Fe3+ center in LiNbO3 crystals. All components of hyperfine and quadrupole tensors for several shells of Li and Nb nuclei were determined. Since the found isotropic part of the hyperfine interaction of the iron electrons with the Li nuclei was small, it was justified to approximate the anisotropic part by the dipole-dipole interaction of electron and nuclear spins. From the comparison of the calculated and observed ENDOR spectra of the Li nuclei, it was derived that the Fe3+ ion occupies a Li site but is shifted from the regular Li position by 0.009 nm toward the octahedral structural vacancy. There is no evidence for the presence of any charge compensators in the nearest Li and Nb shells, which indicates a distant mechanism of excess charge compensation. ENDOR of Nb nuclei shows that there is a large isotropic hyperfine inter...

Determination of g-tensors of low-symmetry Nd3+ centers in LiNbO3 by rectification of angular dependence of electron paramagnetic resonance spectra

Two procedures for facilitation of line tracing and deciphering of complicated spectra of electron paramagnetic resonance (EPR) were developed: a correction of microwave frequencies for every orientation of external magnetic field on the base of known values of g-tensor components for a reference paramagnetic center and followed rectification of measured angular dependences using plots of effective deviation of g2-factors of observed lines from effective g2-factors of the reference center versus angles or squared cosines of angles describing magnetic field orientations. Their application to EPR spectra of nearly stoichiometric lithium niobate crystals doped with neodymium allowed identifying two axial and six different low-symmetry Nd3+ centers, to determine all components of their g-tensors, and to propose common divacancy models for a whole family of Nd3+ centers.

Gallium hole traps in irradiated KTiOPO4:Ga crystals

Nominally pure and gallium doped single crystals of potassium titanyl phosphate (KTiOPO4) have been studied by Electron Paramagnetic Resonance at low temperatures before and after irradiation. Irradiation with 20 MeV electrons performed at room temperature and liquid nitrogen temperature caused an appearance of electrons and holes. Gallium impurities act as hole traps in KTiOPO4 creating Ga4+ centers. Two different Ga4+ centers were observed, Ga1 and Ga2. The Ga1 centers are dominant in Ga-doped samples. For the Ga1 center, a superhyperfine structure with one nucleus with nuclear spin ½ was registered and attributed to the interaction of gallium electrons with a phosphorus nucleus or proton in its surrounding. In both Ga1 and Ga2 centers, Ga4+ ions substitute for Ti4+ ions, but with a preference to one of two electrically distinct crystallographic positions (site selective substitution). The Ga doping eliminates one of the shortcomings of KTP crystals—ionic conductivity of bulk crystals. However, this doe...

Magnetic Resonance Study of Non-Equivalent Centers Created by 4f-Ions in Congruent and Stoichiometric Lithium Niobate

Lithium Niobate doped with 4f-ions is of great interest for both fundamental science and advanced applications including high efficiency lasers with frequency conversion, elements for an all-optical telecommunication network and quantum cryptography. Our study has shown that 4f-ions create an unexpected variety of completely different non-equivalent centers in both stoichiometric and lithium deficient congruent crystals. Dominant Nd1 and Yb1 centers have C3 point symmetry (axial center), whereas all Er and most other Nd and Yb centers have the lowest C1 symmetry. Distant defects create small distortions of the crystal field at the impurity site, which cause line broadening, but do not change the C3 symmetry of observed EPR spectra. Defects in the near neighborhood can lower center symmetry from C3 to C1. We concluded that Nd1 has distant charge compensation, whereas the charge excess in low-symmetry Nd(Li) centers is compensated by near lithium or niobium vacancies. Since no axial centers were found for Er, models with cation vacancies can not describe our experimental data. The dominant axial Yb1 center has no defects in its surrounding. One axial and one low-symmetry Yb centers are self compensating Yb(Li)-Yb(Nb) pairs. Six other centers are different complexes of Yb and intrinsic defects. Obtained data can be used for defect engineering for tailoring properties of photonic materials.

Point Defects and Physical Properties of Ferroelectrics: Lithium Niobate

The strong dependence of the properties of ferroelectric lithium niobate crystals on the concentration of intrinsic (non‐stoichiometric) and extrinsic (impurity) defects is analyzed. Spectra of optical absorption and magnetic resonance spectroscopy are compared for crystals with different compositions. The results show clearly that the possibility to vary the concentration of intrinsic defects offers extraordinary informative opportunities for the investigation of the fundamental physics of the material.