Dominant Cr3+ centers in LiNbO3 under hydrostatic pressure

Low-temperature luminescence spectra of stoichiometric Cr: LiNbO₃ and of congruent Cr, Mg: LiNbO₃ were studied. Cr³⁺ impurity ions preferentially occupy Li⁺ sites (Cr_Li) in the LiNbO₃ crystal lattice, while Cr³⁺ ions substituting for Nb⁵⁺ ions (Cr_Nb) occur in addition to Cr_Li centers in codoped Cr, Mg: LiNbO₃ crystals. Application of a high hydrostatic pressure leads to a transformation of (dominant in concentration) Cr³⁺ centers from low-to high-crystal-field centers. Due to a strong pressure-induced blue shift of the ⁴ T ₂ state resulting in crossing with the ² E state, the replacement of the broad band ⁴ T ₂ → ⁴ A ₂ emission by a narrow R-line emission ² E → ⁴ A ₂ occurs in the luminescence spectra of the samples. This effect of level crossing was observed for the dominant Cr _Li ³⁺ and Cr _Nb ³⁺ centers at pressures which correlated well with estimations based on the ⁴ T ₂-² E energy gap (230 and 1160 cm^?1) and on the rate of their pressure-induced change (14.35 and 11.4 cm^?1/kbar, respectively).     

Cr<Superscript>3+</Superscript> spectroscopy study in ZnO-codoped and Zn-in-diffused congruent LiNbO<Subscript>3</Subscript>:Cr crystals

This paper reports on the spectroscopy properties, absorption and luminescence, of Cr³⁺ ions in singly doped, ZnO-codoped, and Zn in-diffused LiNbO₃:Cr crystals. In addition to the broad absorption, inter-ionic transitions ascribed to Cr³⁺ ions located in Li⁺ and Nb⁵⁺ sites; [Cr]_Li and [Cr]_Nb centres two absorption bands at higher energy are reported and ascribed to the charge transfer transitions of the Cr³⁺ ions of the two defect centres. The charge transfer transitions are used as optical probe to study the role of the Zn ions in the Zn in-diffused LiNbO₃:Cr samples. It has been observed that the Zn-in-diffused processes created [Cr]_Nb centres in the diffusion zone. The location of the diffused Zn²⁺ ions is considered to be in Li⁺ site, displacing the Cr³⁺ ions from the Li⁺ sites, [Cr]_Li, to the Nb⁵⁺ positions, [Cr]_Nb.     

The effect of stoichiometry and Mg doping on the Raman spectra of LiNbO3:Mg crystals

LiNbO₃:Mg crystals doped with 0–8 mol. % Mg with stoichiometric, intermediate and congruent compositions were systematically investigated by Raman spectroscopy in backscattering y(zx)y, y(zz)y and z(xx)z geometries. The damping was found to be a very sensitive parameter for the characterization of the crystal composition. The half-widths of E(TO₃)–E(TO₉) and A ₁(TO₁)–A ₁(TO₄) bands having significant composition dependence for the undoped LiNbO₃ crystals show only a weak Mg concentration dependence below the photorefractive threshold, which is a consequence of the counteracting effect of the decreasing Nb_Li and increasing Mg_Li contents. The half-widths of the bands, however, increase linearly with growing Mg content for samples above the threshold, irrespective of the Li/Nb ratio. The change in the Mg concentration dependence at a given Li/Nb ratio determines the same threshold value as that concluded from IR and UV spectroscopic measurements. The half-width of the main A ₁(LO₄) band at 873 cm^-1 increases linearly with growing Mg concentration, but no threshold effect is observed. However, the ratio of the area of the main band and the high-frequency sideband shows a threshold effect that can be interpreted by the existing defect incorporation models. The small Raman band at about 740 cm^-1 attributed earlier to Nb_Li vibration is also detected in above-threshold LiNbO₃:Mg crystals, which can be explained by the vibration of Nb ions in Mg₄Nb₂O₉ defect clusters appearing at high Mg concentrations. PACS 77.84.Dy; 63.20.Mt; 42.70.Mp; 78.30.-j     

OH impurities in co-doped LiNbO3:Cr :ZnO congruent crystals

Infrared optical absorption has been used to study OH⁻impurities into congruent co-doped LiNbO₃:Cr³⁺:ZnO crystals doped with different Zn²⁺ concentration. The OH⁻ IR absorption spectra present three bands that can be associated with different OH⁻ complex centres available in the lattice. For crystals with lower Zn²⁺ concentrations (<4.7%) only one IR absorption band centred at 2867 nm (3490 cm⁻¹) is reported which is associated with the OH⁻ unperturbed vibration. For crystals with higher Zn²⁺ concentrations (>4.7%), two new bands associated with OH⁻vibration in distortion environment are reported. These bands are centred at 2827 nm (3537 cm⁻¹) and 2847 nm (3512 cm⁻¹) and can be associated with OH⁻-Zn²⁺ and Cr³⁺(Li⁺)-OH⁻-Zn²⁺(Int.) complex centres, respectively. Electron paramagnetic resonance (EPR) has been used to identify the Cr³⁺ centres in the lattice of the doped LiNbO₃:ZnO crystals.     

Optical properties of LiNbO3:Cr crystals co-doped with germanium oxide

Lithium niobate (LiNbO₃) crystals doped with chromium ions show a clear green colouring reflecting the absorption profile of the dominating [Cr]_Li defect centres. A significant change in its colouration takes place when it is co-doped with other valency impurities such as Mg²⁺, Sc³⁺ and W⁶⁺, above a certain threshold concentration. This concentration singularity has been attributed to the formation of [Cr]_Nb centres coexisting with the [Cr]_Li centres.In this work, we extended the investigation on the effect of co-dopant ions in Cr:LiNbO₃ to tetravalent cation such as GeO₂. A singularity in the relative intensity of the ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ absorption band was observed for a concentration of ～1.5 mol%, compared with 4.5 mol% for Mg²⁺. The photoluminescence emission spectra also reveal a new emission band, at a lower energy than the [Cr]_Li centre, corresponding to this threshold concentration. A charge compensation model is proposed to explain the role of cation impurities and results are compared with those of other valence impurities.     

Theoretical interpretation of optical and EPR spectra and the substitutional site of ferroelectric LiNbO3:Ni crystal

The optical absorption spectrum, zero-field splitting (ZFS) and EPR g factor of LiNbO₃:Ni²⁺ are explained uniformly on the basis of complete energy matrix diagonalization procedure (CDP) and Zhao's self-consistent field (SCF) d-orbit of free Ni²⁺ ions. The agreement between the calculated results and the experimental data shows quantitatively that impurities Ni²⁺ replace the Nb⁵⁺ rather than Li⁺ sites in LiNbO₃:Ni²⁺.     

Superlinear photovoltaic currents in proton-exchanged LiNbO<Subscript>3</Subscript> waveguides

Photovoltaic currents along the c axis have been measured in α-phase LiNbO₃ proton-exchanged waveguides at several visible wavelengths for a guided-beam configuration. The light-intensity dependence is superlinear and all experimental curves are very well fitted by computer simulations using a two-centre model, with Fe²⁺/Fe³⁺ as primary and Nb_Li ⁴⁺/Nb_Li ⁵⁺ as secondary photovoltaic centres. The superlinear behaviour arises from a much higher effective photovoltaic length of Nb_Li ⁴⁺ (small polaron) compared with that of Fe²⁺. In β₁-phase guides, the photocurrents are much smaller than in α-phase guides and apparently do not show superlinear behaviour. Received: 22 October 2002 / Revised version: 6 January 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +34-91/3978-579, E-mail: m.carrascosa@uam.es     

Spectroscopic analysis of Er transition in Mg/Er-codoped LiNbO3 crystal

We present a Judd-Ofelt spectroscopic analysis on the Mg/Er-codoped congruent lithium niobate (LiNbO₃) crystals. The Judd-Ofelt model is applied to the room temperature unpolarized absorption intensities of Er³⁺ ions on eleven transition bands to determine their intensity parameters: Ω₂=2.36×10⁻²⁰ cm², Ω₄=0.76×10⁻²⁰ cm², Ω₆=0.30×10⁻²⁰ cm² in Er:LiNbO₃ crystal heavily codoped with MgO. The radiative lifetime of ²H_9/2 becomes longer when MgO is added into Er:LiNbO₃ crystal. The experimental lifetimes are obtained using microsecond time-resolved spectra at 400 nm femtosecond pulse excitation to predict radiative quantum efficiency. Combining higher radiative quantum efficiency with longer radiative lifetime, we conclude that Mg/Er-codoped LiNbO₃ crystals are more suitable than Er: LiNbO₃ ones in laser materials.     

Praseodymium-doped Ti:LiNbO3 waveguides

3 by diffusion doping is investigated by means of secondary neutral mass spectrometry and secondary ion mass spectrometry. The diffusion of praseodymium in LiNbO₃ can be described by Fick’s laws of diffusion with a concentration-independent diffusion coefficient and a limited solubility of praseodymium in LiNbO₃ increasing exponentially with rising temperature. The diffusion depends on the Li₂O content of the LiNbO₃ crystal. For LiNbO₃ crystals with a nominal slight difference in the congruent composition, the diffusion constants and activation energies for Z-cut LiNbO₃ are 3.28×10^-5 cm²/s and 2.27 eV, and 1.39×10^-5 cm²/s and 2.24 eV, respectively. Titanium-doped waveguides are formed in Pr:LiNbO₃ and characterised in relation to waveguide loss and absorption in the visible and near infrared. Received: 17 September 1998 / Accepted: 11 November 1998     

OH-absorption properties of the optical damage region in codoped In/Mg:LiNbO3 crystals with various Li/Nb ratios

OH-absorption properties of the optical damage region in a series of codoped In/Mg:LiNbO₃ crystals with various Li/Nb ratios have been investigated. The OH⁻-associated vibrational peak at 3507 cm⁻¹ is confirmed to occur in crystals with Li/Nb ratio of 0.94. For codoped In/Mg:LiNbO₃ crystals with Li/Nb ratio of 1.05 and 1.20, the OH⁻-associated vibrational peaks are detected at 3536 and 3507 cm⁻¹ as well. A new peak at 3518 cm⁻¹ attributed to a (In_Nb)²⁻-OH⁻-(Mg_Nb)³⁻ defect center is revealed in crystals with Li/Nb ratio 1.38. When the “In-Mg threshold” concentration is reached, the optical damage resistance ability of codoped In/Mg:LiNbO₃ crystals is greatly improved.     

Effects of the ordering of structural units of the cationic sublattice of LiNbO3:Zn crystals and their manifestation in Raman spectra

We have studied the Raman spectra of congruent crystals of lithium niobate (LiNbO₃) that were doped with Zn²⁺ ions in the range of concentration of 0–1.59 mol %. We have revealed a region of a more ordered structure such that the order of sequence of basic ions, impurity ions, and vacancies along the polar axis of the cationic sublattice is more regular, while the oxygen octahedra are close to ideal. In this case, crystals have a higher optical quality and are more stable with respect to optical damage. An increased ordering of the structure is realized because small amounts of Zn²⁺ cations displace Nb_Li defects and order the alternation of cations and vacancies along the polar axis and make the crystal less defect with respect to Li⁺ vacancies. Our results are important for industrial production of optically perfect lithium-niobate crystals by doping a congruent crystal with small concentrations of Zn²⁺ ions, since, in this case, technological regimes of crystal growth almost do not differ from regimes of growing of nominally pure congruent crystals.     

Features of infralow-frequency polarization relaxation processes in LiNbO3 single crystals

The dielectric response of lithium niobate single crystal in ac fields with amplitudes E of 3.74 to 13.1 kV cm^?1 is investigated at frequencies of 1 and 10 Hz in the temperature range of 70 to 200°C. The increase in effective dielectric permittivity ?_eff′ and the effective dielectric losses ?_eff″ is found to be associated with the formation of short-radius Nb _Li ⁴⁺ polarons at temperatures T > 130°C and their contribution to polarization relaxation in a LiNbO₃ crystal.     

Effect of external electric field on R line absorption spectra in LiNbO3:Cr3+ crystals

The effect of an external electric field on the R absorption lines of LiNbO₃:Cr³⁺ crystals has been studied by a high-sensitivity differential technique at 77 K. Linear Stark effect has been observed in a field directed along the trigonal crystal axis. No effect of the field was observed when directed perpendicular to this axis. These characteristics of the Stark effect provide a convincing argument for the electric dipole moments of Cr³⁺ centers being oriented along the trigonal C ₃ axis of the crystal and indicate that the chromium centers in LiNbO₃ have C ₃ symmetry. Fiz. Tverd. Tela (St. Petersburg) 39, 2053–2056 (November 1997)     

A study on the influence of ytterbium and impurities on lattice parameters and the phase transition temperature of Czochralski-grown LiNbO3

LiNbO₃ crystals were grown from a congruent melt by the Czochralski method. Special care was taken with respect to purity and an analysis of impurity levels. Present crystals are as pure as 3N-4N. In particular, doping by Yb³⁺ was investigated. An upper limit for the inclusion of Yb³⁺ in LiNbO₃ may be set at 2.5 mol% for a congruent melt. Present results demonstrate the complex interaction between intrinsic and extrinsic defects in LiNbO₃. The variation of lattice parameters supports evidence for the Li⁺ vacancy model and an occupation of Li sites for Yb³⁺. Data on purities, laser properties and analytical methods provide a short review on a field, where analytical results gained by different techniques often may disagree.     

Light-induced charge transfer and kinetics of the NIR absorption of Nb4+ polarons in SBN crystals at low temperatures

Sr_1-xBa_xNb₂O₆ (SBN) crystals with open tungsten-bronze structure show enhanced photorefractive properties with doping of impurities such as Ce, Cr, Rh etc. Under illumination with Kr⁺ laser (647 nm) or Ar⁺ laser light (488 nm or 514 nm) or UV light at low temperature, pure and doped SBN crystals show a broad polaron absorption band around 0.7 eV (6000 cm^-1). The first step of a theoretical model involves the excitation of electrons by illumination from Cr³⁺/Ce³⁺ to higher excited states or the conduction band. The excited electrons can then be trapped by Nb⁵⁺ to form Nb⁴⁺ polarons and further on can directly tunnel through or hop over the potential barrier (with a value Δ≈0.15±0.02 eV) to recombine with Cr⁴⁺/Ce⁴⁺ ions. The experimental intensity dependence, temperature dependence, and decay process of the light-induced Nb⁴⁺ polarons can be fitted with the help of this model. Small, but systematic, differences lead to the additional assumption of different recombination rates of polarons at distinct distances from the Cr⁴⁺/Ce⁴⁺ recombination centers and therefore many parallel decay channels are active where each decay channel obeys a monoexponential decay law. A stretched exponential decay function is employed to fit in this case the decay process of the Nb⁴⁺ polarons at different temperatures and under illumination with different intensities. Due to the high dielectric constant value (ε₃₃ and ε₁₁ have values in the 10²-10³ range) at low temperature, the long range Coulomb attraction (to Ce³⁺ _Sr/Ba) or repulsion (from Cr³⁺ _Nb) of the electronic polaron is suppressed. The leading role in the attraction and the following trapping of the electronic Jahn–Teller polaron, both on Cr³⁺ _Nb and Ce³⁺ _Sr/Ba centers, is played by the indirect dipole–dipole interaction via the soft TO-mode. Received: 27 November 1998 / Revised version: 22 January 1999 / Published online: 7 April 1999     

Photostimulated electron transfer and its action on paramagnetism of Sp<Subscript>3</Subscript>Cr(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The effect of irradiation by ultraviolet light on the effective magnetic moment of a paramagnetic single crystal based on photochrome spiropyran (Sp) and chromium oxalates Sp₃Cr(C₂O₄)₃ molecules is detected. It is shown that the deviation of the temperature dependence of the magnetic moment from the Curie law is caused not by the exchange interaction, but by electron redistribution between Cr³⁺ and Cr⁴⁺ ions and spiropyran molecules Sp⁰ and Sp⁺. Analysis of the angular dependence of EPR spectra makes it possible to determine the contribution of Cr³⁺ ions to the magnetic properties of the crystals and to determine the crystal field parameters D = 0.619 cm⁻¹ and E = 0.024 cm⁻¹. Irradiation of hydrated samples by ultraviolet light leads to intensity redistribution of EPR lines attributed to Cr³⁺ and Sp⁰. Thermally stimulated paramagnetism of triplet states of spiropyran ions Sp⁺ and the SpI salt is observed.     

Influence of Mg doping on the behaviour of polaronic light‐induced absorption in LiNbO3

Transient light‐induced absorption changes α_li(t), caused by optically generated small polarons, are investigated in LiNbO₃:Mg below and above the optical‐damage‐resistance threshold (ODRT). The lifetime of α_li(t) is reduced by three orders of magnitude above the ODRT while a significantly enhanced amplitude is observed in the infrared. Our observations are in full accordance with the predictions of microscopic models for the ODRT, namely the removal of Nb_Li antisite defects upon incorporation of Mg ions, and an enhanced dark conductivity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical properties of LiNbO3:Mg(5.21 mol %) and LiNbO3:Fe(0.009 mol %):Mg(5.04 mol %) crystals

Using methods of electronic spectroscopy, laser conoscopy, photoinduced (photoreactive) light scattering, and Raman light-scattering spectroscopy, we have studied the optical homogeneity, optical transmission, and photorefractive properties of single crystals LiNbO₃:Mg(5.21 mol %) and LiNbO₃:Fe(0.009 mol %):Mg(5.04 mol %) that were grown from congruent melts. We have ascertained that doping with “nonphotorefractive” Mg²⁺ cations causes suppression of the photorefractive effect in a lithium-niobate crystal. Upon double doping (Fe:Mg), if the concentration of Mg²⁺ cations exceeds the threshold concentration, the photorefractive effect is almost not observed and the presence of “photorefractive” Fe cations does not affect the photorefractive effect as strongly as in congruent crystals doped with Fe.     

Gap levels of Ti<Superscript>3+</Superscript> on Nb or Li sites in LiNbO<Subscript>3</Subscript>:(Mg):Ti crystals and their effect on charge transfer processes

The Ti dopant occupying Li or Nb sites and the charge transfer processes induced by thermochemical reduction and optical bleaching treatments have been investigated in LiNbO₃ systems using optical absorption and EPR. The Ti³⁺ centers, built preferentially at Nb sites in heavily Mg-codoped crystals, are shown to have absorption bands at 1.62±0.08 eV and 2.65±0.25 eV, which are similar or slightly redshifted compared to Ti³⁺ centers at Li sites in LiNbO₃:Ti crystals. The Ti_Nb^4+/3+ gap level plays an important role in the trapping of electron-polarons in LiNbO₃, double-doped with Mg and Ti; in particular, an enhanced optical detrapping sensitivity for pumping in the 1.3–2.8 eV range is observed, which may be relevant for applications in integrated optics. Evidence indicating the possible existence of bipolarons involving the Ti dopant is presented. PACS 77.84.Dy; 71.55.Ht; 71.38.-Mx; 78.40.Ha; 76.30.Fc