Optical alignment of Cu<Superscript>2+</Superscript> axial centers in KTaO<Subscript>3</Subscript>: Spectral dependence of the effect

The Cu _Ta ²⁺ -V_O axial centers in crystalline KTaO₃ were found to undergo alignment under the action of polarized light. The sign of the effect is shown to change depending on the wavelength of the aligning light. A parallel study of the spectral response of photoconductivity of the same samples led to the conclusion that the alignment of the copper centers is driven not by reorientation but rather by an anisotropic recharging of the centers, which involves both the conduction and valence bands of the crystal. This interpretation was supported by a study of the kinetics of thermal destruction of the copper center alignment.     

A Fe[Nb]-Li center in stoichiometric LiNbO<Subscript>3</Subscript> crystals: Mechanism of formation

A new iron center in stoichiometric lithium niobate crystals has been studied by the EPR method. The angular dependences of the EPR spectrum of the center have been used to derive the parameters of its spin Hamiltonian. The data amassed on the variation in the concentrations of two iron centers in lithium niobate crystals annealed in a Li₂CO₃ powder have provided an insight into the mechanism of formation of the new center, as well as corroborated its model proposed by us earlier. According to this model, the center represents a complex of two defects aligned with the polar axis in the crystal: the iron ion at the niobium site and an interstitial lithium ion filling the nearest structural vacancy (Fe³⁺[Nb]-Li⁺[V]). The structure of other Fe³⁺ centers revealed earlier in LiNbO₃ crystals, in which the iron ion occupies the niobium site, has been discussed.     

Structural-phase transformations at annealing of highly reflective mirrors based on aluminum and silver

Multilayered prototypes of Al and Ag total reflection mirrors with a protective coating of amorphous ZrO₂ oxide were analyzed using transmission electron (cross-sectional method), optical, and scanning (with electron probe microanalysis) microscopes. The prototypes’ reflectivity was measured by the recording of reflection spectra in the wavelength range 200–1100 nm. The structure of the mirrors was studied by X-ray diffraction reflection. High temperature annealing (8 h) did not change the stoichiometric composition of the protective coating. The reflectivity of the Al mirrors in the wavelength range 700–1100 nm and the Ag mirrors in the wavelength range of 500–1100 nm remained unchanged in spite of recrystallization of the polycrystalline metal films and the incipient process of amorphous ZrO₂ crystallization. The mass of the crystalline oxide after annealing was several percent of the total amorphous oxide mass. At the same time annealing led to the occurrence of local voids on the aluminum/upper oxide interface. Such defects were not observed on the Ag mirrors. As a whole, the Ag mirrors turned out to be more stable to heating as compared to those made of Al.     

Optimal conditions for third-harmonic generation in gas mixtures

The authors consider the effect of the buffer gas nonlinear susceptibility on the third-harmonic generation efficiency in gas mixtures. Experiments were performed in mixtures of krypton with xenon, argon and nitrogen at the wavelength of hydrogen Lyman-α. The optimal compositions for these mixtures were found and the gain in the generation power against the pure gas was estimated.     

Anisotropic photoionization as a mechanism of axial iron center alignment in KTaO3

An EPR study has revealed light-induced recharging and optical alignment of the Fe _Ta ⁴⁺ -V _O tetragonal complexes in KTaO₃. The data on the optical creation and destruction of this center by light of different polarizations and wavelengths are discussed together with similar results obtained for the Fe _K ³⁺ -O_i center. These two centers were established to undergo mutual charge transfer, in which the electron released in the photoionization of the Fe _K ²⁺ -O_i center is trapped by the Fe _Ta ⁵⁺ -V _O center. Irradiation by light with a photon energy below 2.05 eV, which is the ionization threshold of Fe _K ²⁺ -O_i, reverses this process. In both cases, the absorption cross section depends on the orientation of the center axis relative to the light polarization vector. As a result, the Fe _Ta ⁴⁺ -V _O and Fe _K ³⁺ -O_i tetragonal centers in KTaO₃ acted upon by polarized light undergo orientation-sensitive light-induced recharging and the defects with the given charge state are no longer characterized by an equally probable distribution of the orientations of their axes over the three 〈100〉 directions. This mechanism, which does not involve real reorientations of the Fe_Ta-V _O and Fe_K-O_i complexes, gives rise, nevertheless, to the alignment of the centers along (or at right angles to) the light polarization vector.     

Electric field effects in the EPR spectrum of low-spin Ni<Superscript>3+</Superscript> centers in the KTaO<Subscript>3</Subscript> crystal

The electric field effects in the EPR spectra of low-spin (S = 1/2) Ni³⁺ tetragonal centers in KTaO₃ single crystals are investigated. It is revealed that the resonance lines are split and the centers are oriented as a result of the interaction of the external field with the electric dipole moment of the center. The dipole moment of the center is determined to be p = 100 D = 21 e?. An analysis of the set of experimental data obtained permits one to choose correctly the microscopic models for two nickel centers in KTaO₃ crystals among the models discussed in the literature. Original Russian Text ￠ L.S. Sochava, S.A. Basun, V.é. Bursian, A.G. Razdobarin, D.R. Evans, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 12, pp. 2157–2160.     

Ferroelectric-specific stark effect in stoichiometric LiNbO3:Fe at room temperature

By means of EPR spectroscopy of LiNbO(3):Fe at room temperature (RT) it is shown that the Stark effect in ferroelectric crystals can be different from that observed in other materials. Novel properties appear when an external E field reverses the direction of the spontaneous polarization, the direction of the linear Stark shift stays the same with a reversal of the E field. The corresponding spectral line shifts can occur over a long time scale (hours). These properties seem to be a general feature of the Stark effect in ferroelectric crystals when the external E field exceeds the coercive field.     

The Topography of Surface and Light Transmission by Quartz Windows after Exposure in a High-Frequency Discharge in Deuterium and Mixture of Deuterium with Nitrogen

Technical Physics - The effect of a high-frequency discharge on the change in the topography of the surface of KU-1 optical quartz and transmission of visible light (400–1000 nm) is studied....