K. Lengyel

Nonlinear refraction and absorption of Mg doped stoichiometric and congruent LiNbO3

The light induced change of refraction is studied in pure and Mg doped LiNbO3 with congruent and stoichiometric compositions by the Z-scan method using all-lines visible argonion laser, up to MW/cm2 intensity level. In Mg-doped congruent and stoichiometric crystals with Mg concentrations above threshold a positive change in the refractive index was found, in contrast to all other cases where beam fanning and negative change of the refractive index were observed. The beam distortion in the samples doped above threshold was related to thermal lensing, while below it to the photorefractive effects. It was also shown that for thermal lensing nonlinear absorption plays a dominant role. The Z-scan method was found to be an alternative technique to decide whether the Mg dopant level is above or below the photorefractive threshold. The damage resistance of the Mg doped samples above threshold was higher for the stoichiometric crystal than for the congruent one and increased with the amount of the built-in Mg conc...

A computational study of intrinsic and extrinsic defects in LiNbO3

Lithium niobate is a material with many important technological applications as a result of its diverse physical properties. Using a recently derived interatomic potential, intrinsic defect energies have been calculated leading to conclusions about the defect properties of the material that are compared with experimental conclusions. The incorporation of dopant ions into the structure is also considered, and solution energies are calculated, which enable predictions to be made about which ions are most easily added and which solution energy schemes are favoured energetically.

IR absorption spectroscopy of water in CsLiB6O10 crystals

AbstractPolarized absorption spectra of CsLiB 6 O 10 crystals have been measured between )186 and 25 C in the 1500–7500cm 1 wavenumber range. A series of absorption bands have been observed with intensities depending on the thermalhistory of the crystals. Three of them have been assigned as the fundamental H–O–H bending mode (d ¼ 1650 cm 1 ),and symmetric and asymmetric OH stretching modes of the bound H 2 O molecule (m s ¼ 3413 cm 1 and m a ¼ 3581 cm 1 ,respectively). Additional features in the 3800–4400, 4700–5300 and 6500–7100 cm 1 ranges have been interpreted asovertones and combinations of the fundamental H 2 O vibrations. The above assignments have been verified by detectingthe isotopic replica of the bands in a crystal treated in D 2 O vapour atmosphere. The possible sites for the H 2 O moleculein the lattice have been deduced from polarization angle dependence measurements. 2003 Elsevier B.V. All rights reserved. PACS: 42.70.Mp; 61.72.)y; 78.30.HvKeywords: FTIR spectroscopy; H

Growth, defect structure, and THz application of stoichiometric lithium niobate

Owing to the extraordinary richness of its physical properties, congruent lithium niobate has attracted multidecade-long interest both for fundamental science and applications. The combination of ferro-, pyro-, and piezoelectric properties with large electro-optic, acousto-optic, and photoelastic coefficients as well as the strong photorefractive and photovoltaic effects offers a great potential for applications in modern optics. To provide powerful optical components in high energy laser applications, tailoring of key material parameters, especially stoichiometry, is required. This paper reviews the state of the art of growing large stoichiometric LiNbO3 (sLN) crystals, in particular, the defect engineering of pure and doped sLN with emphasis on optical damage resistant (ODR) dopants (e.g., Mg, Zn, In, Sc, Hf, Zr, Sn). The discussion is focused on crystals grown by the high temperature top seeded solution growth (HTTSSG) technique using alkali oxide fluxing agents. Based on high-temperature phase equilibria studies of the Li2O–Nb2O5–X2O ternary systems (X = Na, K, Rb, Cs), the impact of alkali homologue additives on the stoichiometry of the lithium niobate phase will be analyzed, together with a summary of the ultraviolet, infrared, and far-infrared absorption spectroscopic methods developed to characterize the composition of the crystals. It will be shown that using HTTSSG from K2O containing flux, crystals closest to the stoichiometric composition can be grown characterized by a UV-edge position of at about 302 nm and a single narrow hydroxyl band in the IR with a linewidth of less than 3 cm−1 at 300 K. The threshold concentrations for ODR dopants depend on crystal stoichiometry and the valence of the dopants; Raman spectra, hydroxyl vibration spectra, and Z-scan measurements prove to be useful to distinguish crystals below and above the photorefractive threshold. Crystals just above the threshold are preferred for most nonlinear optical applications apart holography and have the additional advantage to minimize the absorption even in the far-infrared (THz) range. The review also provides a discussion on the progress made in the characterization of non-stoichiometry related intrinsic and extrinsic defect structures in doped LN crystals, with emphasis on ODR-ion-doped and/or closely stoichiometric systems, based on both spectroscopic measurements and theoretical modelling, including the results of first principles quantum mechanical calculations on hydroxyl defects. It will also be shown that new perspective applications, e.g., the generation of high energy THz pulses with energies on the tens-of-mJ scale, are feasible with ODR-doped sLN crystals if optimal conditions, including the contact grating technique, are applied.

Photorefractive damage resistance threshold in stoichiometric LiNbO 3 :Zr crystals

Several optical methods including ultraviolet absorption, infrared absorption of the hydroxyl ions, Raman spectroscopy, and the Z-scan method have been used to determine the damage resistance threshold in 0-0.72 mol. % Zr-containing, flux-grown, nearly stoichiometric LiNbO₃ single crystals. All spectroscopical methods used indicate that samples containing at least ≈0.085 mol. % Zr in the crystal are above the threshold while Z-scan data locate the photorefractive damage threshold between 0.085 and 0.314 mol. % Zr.

Analyses of the optical and magneto-optical spectra of Tb3Ga5O12

Absorption spectra of the FJ7, D4,3,25, G6,5,45, and L10,95 multiplet manifolds of Tb3+ (4f8) in D2 sites in cubic garnet Tb3Ga5O12 (TbGaG) are investigated at sample temperatures between 1.8K and room temperature. Absorption measurements extend from 5000to340nm. From analyses of temperature-dependent (hot-band) absorption spectra, many of the crystal-field split energy (Stark) levels of the LJ2S+1 multiplet manifolds of Tb3+ are identified and confirmed from analyses of the fluorescence spectra observed between 485 and 680nm, representing transitions from the D45 to the FJ7 manifolds. Each manifold is split by the crystal field into 2J+1 Stark levels. Some of these manifolds, including the ground-state manifold F67, consist of Stark levels that are accidentally degenerate, or nearly so, making transitions to or from these levels appear as unresolved spectra, even at the lowest temperature investigated (1.8K). To resolve these spectra, we have investigated the Zeeman and magneto-optical spectra for repres...

Kinetics of OH− ions in nearly stoichiometric LiNbO3 crystals

Abstract Kinetics of the complex OH− stretching vibrational band in a nearly stoichiometric LiNbO3 crystal (Li/Nb ≈ 0.988) has been studied in order to explore the relation between the thermal fixing process of holograms and the hydroxyl ion absorption. By a careful analysis of the spectra we have observed that the intensity of the absorption band components at a given temperature changes with time suggesting that protons migrate among the different sites until they reach a thermodynamic equilibrium. The time dependence of the intensities has been measured at different temperatures between 40–120 °C after a heat treatment at 250 °C. From the measured data the time constants and the corresponding activation energies (Eav ≈ 1.1 ± 0.1 eV) were determined and compared with those of the thermal fixing process. The results obtained confirm that the hologram fixing process is governed by proton migration in the crystal.

Vibrations of H+(D+) in stoichiometric LiNbO3 single crystal

A first principles quantum mechanical calculation of the vibrational energy levels and transition frequencies associated with protons in stoichiometric LiNbO(3) single crystal has been carried out. The hydrogen contaminated crystal has been approximated by a model one obtains by translating a supercell, i.e., a cluster of LiNbO(3) unit cells containing a single H(+) and a Li(+) vacancy. Based on the supercell model an approximate Hamiltonian operator describing vibrations of the proton sublattice embedded in the host crystal has been derived. It is further simplified to a sum of uncoupled Hamiltonian operators corresponding to different wave vectors (ks) and each describing vibrations of a quasi-particle (quasi-proton). The three dimensional (3D) Hamiltonian operator of k=0 has been employed to calculate vibrational levels and transition frequencies. The potential energy surface (PES) entering this Hamiltonian operator has been calculated point wise on a large set of grid points by using density functional theory, and an analytical approximation to the PES has been constructed by non-parametric approximation. Then, the nuclear motion Schrödinger equation has been solved by employing the method of discrete variable representation. It has been found that the (quasi-)H(+) vibrates in a strongly anharmonic PES. Its vibrations can be described approximately as a stretching, and two orthogonal bending vibrations. The theoretically calculated transition frequencies agree within 1% with those experimentally determined, and they have allowed the assignment of one of the hitherto unassigned bands as a combination of the stretching and the bending of lower fundamental frequency.

Structure of OH− defects in LiNbO3

The geometry of the most stable configuration of OH− defects in nearly stoichiometric, hydrogen-contaminated, LiNbO3 has been determined from first principles theoretical calculations. In the most stable configuration the proton is located near a bisector of an oxygen triangle and is tilted by 4.3 degrees out of the oxygen plane towards a lithium vacancy. The equilibrium length of the OH− bond has been found to be 0.988 A.

Combination transitions due to stretching and librations of OH − ions in LiNbO 3

A new absorption band has been detected at 4009 cm⁻¹ in stoichiometric LiNbO₃ single crystals. Based on present and earlier experiments and recent calculations, this band has been assigned as a combination transition involving the OH stretching and an OH librational mode. The librational mode participating in this combination is of lower fundamental frequency than that contributing to the combination band at 4415 cm¹ observed by Gröne and Kapphan [J. Phys. Condens. Matter 7, 3051 (1995)].