Influence of vapor transport equilibration on Raman spectra of Er:LiNbO3 crystals

Raman spectra of as-grown and vapor transport equilibration (VTE) treated Er:LiNbO₃ crystals, which have different cut orientations (X-cut and Z-cut), different Er-doping levels (Er:(0.2, 0.4 and 2.0 mol%)LiNbO₃) and different VTE durations (80, 120, 150 and 180 h), were recorded at room temperature in the wavenumber range 50-1000 cm⁻¹ by using backward scattering geometry. The spectra were attributed on the basis of their spectral features and the previous experimental work and the most recent theoretical progress in lattice dynamics on pure LiNbO₃. In comparison with the pure crystal the most remarkable effect of Er-doping on the Raman spectrum is observed for the E(TO₉) mode. It does not appear at 610 cm⁻¹ as the pure crystal, but locates at 633 cm⁻¹. In addition, the doping also results in the lowering of the Raman phonon frequency, the broadening of the Raman linewidth and the changes of the relative Raman intensity of some peaks. The VTE treatment results in the narrowing of the linewidth, the recovery of the lowered phonon frequency and the further changes of relative Raman intensity. The narrowing of Raman linewidth indicates that the VTE processing has brought these crystals closer to stoichiometric composition. The VTE treatment has induced the formation of a precipitate ErNbO₄ in the high-doped Er(2.0%):LiNbO₃ crystals whether X- or Z-cut. For these precipitated crystals, besides above linewidth and phonon frequency features, they also display more significant Raman intensity changes compared with those not precipitated crystals. In addition, a slight mixing between A₁(TO) and E(TO) spectra is also observed for these precipitated crystals. Above doping and VTE effects on Raman spectra were quantitatively or qualitatively correlated with the characteristics of the crystal structure and phonon vibrational system.     

Emission characteristics, crystalline phase and composition of vapor-transport-equilibrated Er:LiNbO3 crystal codoped with 6 mol% MgO

Polarized downconversion, 980-nm-upconversion and near-infrared emission characteristics of vapor-transport-equilibrated (VTEed) bulk Er (0.4 mol%)/MgO (6 mol%)-codoped LiNbO₃ crystals were investigated. The downconversion and upconversion visible emissions display similar VTE effects including the drop of emission intensity and the weakening of polarization dependence. At 0.98 and 1.5 μm regions, the VTE has a weak effect on the emission intensity, but a strong effect on the spectral shape. The crystalline phases in these bulk Er/Mg-codoped VTE crystals are determined by comparing their infrared emission characteristics with those of pure ErNbO₄ powder and locally Er-doped MgO (4.5 mol%):LiNbO₃ crystal. The results show that the Er³⁺ ions present in these bulk Er/Mg-codoped VTE crystals as a mixture of Er:LiNbO₃ and ErNbO₄ phases. The percentages of the ErNbO₄ phase contained in these VTE crystals were evaluated from the 1531 and 1536 nm characteristic absorption areas. The contents of constituent elements were determined by chemical analysis.     

Raman study on vapor-phase equilibrated Er:LiNbO3 and Er:Ti:LiNbO3 crystals

Raman spectra of Er:LiNbO₃ crystal and Ti-diffusedEr:LiNbO₃ strip waveguide, in which the Li/Nb ratio was altered using a vapor-phase equilibration (VPE) technique, were measured at room temperature in the wave-number range 50–3500 cm^-1. Both 488 and 514.5 nm radiations were used to excite Raman scattering, A₁(TO) and E(TO) modes were recorded at backward scattering geometry. The results indicated that the lattice vibrational spectra of the as-grown Er:LiNbO₃ are almost the same as those of pure LiNbO₃ except for the little shift of the peak position and the change of relative intensity of some peaks. In comparison with the spectra of as-grown Er:LiNbO₃ crystal the vapor-phase equilibrated Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals in the lattice vibrational region exhibit the following features: firstly, Raman peaks become narrow, indicating that the VPE process has brought Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals closer to a stoichiometric composition; secondly, relative intensity of some peaks varies with the VPE time; and finally, slight blue shifting in peak position was observed. Some of these features were correlated with the NbO₆ octahedra and with the site distribution of the doped Er ions. In addition, green fluorescence peaks and/or bands associated with the electron transitions ² H _11/2?⁴ I _15/2 and ⁴ S _3/2?⁴ I _15/2 of the doped Er³⁺ were also observed. For 488 nm excitation they appear in the wavenumber range of 1200–3000 cm^-1 and are well separated from lattice vibrational region; for 514.5 nm excitation, however, these fluorescence peaks shift towards the low wavenumber region and overlap partially with the lattice vibrational spectra. Received: 24 May 2000 / Accepted: 29 May 2000 / Published online: 13 September 2000     

Emission characteristics of Er in vapor-transport-equilibrated Er/Zn-codoped LiNbO3 crystals

Polarized visible and infrared emission characteristics of Er³⁺ ions in vapor-transport-equilibration (VTE)-treated LiNbO₃ crystals codoped with different concentrations of Zn and Er were investigated in comparison with corresponding as-grown crystals. The results show that the VTE treatment leads to substantial spectral changes of Er³⁺ emissions at 0.65, 0.98 and 1.5 μm regions, and the spectral changes in the 0.98 and 1.5 μm regions appear to be Zn-concentration-dependent. It is concluded in combination with X-ray powder diffraction results and optical absorption characteristics reported previously that the VTE treatment resulted in crystalline phase transformation with respect to Er³⁺ ions from original LiNbO₃ to ErNbO₄ phase in all crystals studied. The formation of the ErNbO₄ phase and the Zn²⁺ codopants are responsible for the VTE-induced substantial spectral changes. The emission characteristics of the ErNbO₄ precipitates in the Zn/Er-codoped crystals are found to be very different from those of the ErNbO₄ precipitates in the only Er-doped crystal in the infrared region, and the difference is attributed to the influence of the Zn²⁺ codopant on the Er³⁺ ion environment. The mechanism of the crystalline phase transformation is qualitatively explained from the viewpoint of the declined solubility of Er³⁺ ion in a Li-rich LiNbO₃ crystal and from the phase diagram of Li₂O-Nb₂O₅ system.     

Light-induced absorption instability in weakly reduced congruent Er:LiNbO3 crystal

Light-induced absorption (LIA) characteristics in weakly reduced (or strongly annealed) congruent and/or vapor transport equilibration (VTE)-treated Er-doped LiNbO₃ crystals have been investigated in comparison with their corresponding as-grown ones and undoped crystals by using a polarized 632.8-nm beam as probe light and another polarized 632.8- or 488-nm beam as pump light. Under He-Ne pump, the LIA was observed only in strongly reduced pure VTE LiNbO₃ crystal. Under 488-nm pump, LIA is still not observed in the doped or undoped as-grown crystals. The weakly reduced VTE-treated Er(0.2 mol %):LiNbO₃ crystal displays weak and stable LIA. On the other hand, the corresponding weakly reduced congruent crystal displays a rather unpredictable light-induced absorption instability phenomenon. The instability was shown by the random competition of the LIA and light-induced transparency (LIT). When both crystals were further reduced, the VTE sample still shows stable LIA but with increased LIA, while the LIA in the congruent sample also becomes stable enough. The instability was experimentally proved to be associated with the presence of the Er³⁺ ion that performs the role of an extrinsic defect instead of photoluminescence. A three-level model is suggested that consists of a deep level (the bipolaron) and two shallow levels: the small polaron level and the level with respect to the Er³⁺ ion. The model has been employed to qualitatively explain the LIA characteristics of the weakly reduced congruent Er:LiNbO₃ crystal, including the the instability, the effect of the state of reduction, the pump intensity and the pump–probe polarization dependences. The inhomogeneity of the defects caused by the weak reduction and the simultaneous participation of the two shallow levels in the light-induced electron-transport process result in the random competition between LIA and LIT, and consequently result in the LIA instability. PACS 42.70.Hj; 42.70.Ln; 42.70.Mp; 42.65.Hw; 81.05.-t     

Structural and optical characterization of Er3+/Yb3+-doped LiNbO3

We report the dependence of the unit-cell parameters and the extraordinary and ordinary refractive indices of Er³⁺/Yb³⁺-codoped LiNbO₃ crystals. Both properties depend in a non-monotonic manner on the Er³⁺/Yb³⁺ content. A singularity was observed at concentrations of 1.1-1.2 mol. % in the crystal (0.6-0.7 mol. % in the melt). In the same way the Er and Yb concentration influences the periodically poled lithium niobate formation. The observed behavior of refractive indices and unit-cell parameters of Er³⁺/Yb³⁺-codoped LiNbO₃ crystals could be explained in terms of the RE³⁺-ion concentration affecting the Li-vacancy concentration and the RE³⁺-ion positions in the crystal. Received: 21 May 2001 / Revised version: 22 August 2001 / Published online: 23 October 2001     

Ferroelectric domain inversion in near-stoichiometric lithium niobate for high efficiency blue light generation

LiNbO₃ single crystals with a composition close to stoichiometry ([Li]/[Li+Nb]=0.496), 16 mm in diameter and 40 mm in length were grown by the Czochralski method using K₂O flux. The domain reversal characteristics of near-stoichiometric LiNbO₃ single crystals were investigated. The switching field required for 180° ferroelectric domain reversal in the near-stoichiometric crystal at room temperature was 7.5 KV/mm. This is about one third of the switching field required for conventional LiNbO₃ crystals. Domain reversal (180°) in near-stoichiometric LiNbO₃ samples of 1.0 mm thickness has been achieved. Samples have been evaluated by second harmonic generation and conversion efficiencies of up to 32% have been obtained. Received: 8 November 2000 / Accepted: 29 January 2001 / Published online: 20 June 2001     

Photorefractive properties of LiNbO3:Zn crystals related to the defect structure

LiNbO₃:Zn single crystals and powders were studied by precise X-ray diffraction methods. Structural refinement yielded new models of the intrinsic defect structure valid for different Zn concentration ranges. For concentrations up to 7 at. % in the crystal, Zn ions incorporate onto Li sites; at higher concentrations Zn ions are found on both Li and Nb sites. Photorefractive properties of LiNbO₃:Zn are discussed in the context of the deduced defect models. A smooth increase in the photoconductivity up to 7 at. % Zn is accounted for by a decrease of Nb antisites. Steeper growth of the photoconductivity at higher Zn concentrations is related to vanishing Li vacancies. Received: 17 November 2000 / Revised version: 23 January 2001 / Published online: 20 April 2001     

Investigation of optical homogeneity and photorefractive properties of lithium-niobate single crystals by the Raman-spectroscopy and laser-conoscopy methods

Photorefractive properties and structural and optical homogeneity of (1) LiNbO₃:Cu crystals ([Cu] = 0.015 mas %) grown from a congruent melt, (2) nominally pure stoichiometric crystals grown from a melt with 58.6 mol % of Li₂O (LiNbO_3stoich), and (3) nominally pure congruent crystals (LiNbO_3congr) have been studied using the Raman-spectroscopy method with excitation in the UV, visible, and near-IR ranges; the laser-conoscopy method; and the electron paramagnetic resonance-spectroscopy method. In optically uniaxial LiNbO₃ crystals, a weak optical biaxiality has been revealed, which is attributed to an insignificant deformation of the optical indicatrix. This deformation can be caused both by the initial structural inhomogeneity of crystals and by the photorefractive effect. It has been shown that, under the action of light, charge exchange of copper cations Cu²⁺ → Cu⁺ takes place in the crystal LiNbO₃:Cu ([Cu] = 0.015 mas %). The LiNbO₃:Cu crystal exhibits photorefractive properties not only because of the occurrence of intrinsic defects with electrons localized at them, as is the case with the LiNbO_3stoich and LiNbO_3congr crystals, but also due to the charge exchange of copper cations under the action of the laser radiation.     

Single-frequency Ti:Er:LiNbO3 distributed Bragg reflector waveguide laser with thermally fixed photorefractive cavity

The first single-frequency Ti:Er:LiNbO₃ distributed Bragg reflector waveguide laser with two thermally fixed photorefractive gratings as resonator mirrors is reported. The optically pumped (λ_p=1480 nm,120-mW incident power) laser emits up to 1.1 mW at λ_s=1561.1 nm. The threshold pump power is 70 mW. Received: 7 June 2001 / Published online: 30 October 2001     

Raman scattering study of reduced Er:LiNbO3 crystals

Raman spectra of as-grown and reduced (or annealed) Er:LiNbO₃ crystals, which have different cut orientations, varied Li/Nb ratio, and different Er-doping levels of 0.2, 0.4, 0.6, 1.0, and 2.0 mol%, were recorded at room temperature over a wavenumber range of 50–1000 cm^-1 by use of backward scattering geometries. The spectra are assigned on the basis of their Raman scattering features and previous relevant work. A weak but well-resolved peak around 633 cm^-1 appears in the E(TO) spectra that were recorded under the configuration of X(ZY)X̄(for an X-cut sample) or Y(ZX)Ȳ(for a Y-cut sample) for all crystals studied. The appearance of this peak in the E(TO) spectrum provides further evidence for a previous attribution of this peak to E(TO₉) mode. Some additional peaks distributed in the low wavenumber region ranging from 101–137 cm^-1 are attributed to Er³⁺ fluorescence with a wavelength range of 490.41–491.3 nm. The reduction effects include a significant drop of the Raman scattering intensity and a slight narrowing instead of a broadening in the linewidth. The reduction procedure hardly affects the spectral shape and the wavenumber of most of the phonons. The anneal effect is similar to the reduction effect and both effects are not as obvious as the vapor transport equilibration (VTE) effect. In addition, the present Raman scattering result provides evidence for our earlier reported individual result on light-induced diffraction from strongly reduced Er:LiNbO₃ crystals. PACS 42.70.Hj; 81.05.-t; 63.20.-e; 78.30.-j.     

The correlation of doping ions to holographic storage properties of Mg:Mn:Fe:LiNbO3 crystals

The congruent Mn(0.1 wt%):Fe(0.03 wt%):LiNbO₃ crystals doped with different concentration of MgO(0,1,3,6 mol%) have been grown by Czochralski method in air atmosphere. Some crystal samples were reduced in Li₂CO₃ powder. The defects and doping ions location in crystals were investigated by UV-Vis. absorption spectrum as well as infrared transmittance spectrum analysis. In two wave coupling experiments we determined the writing time, maximum diffraction efficiency and the erasure time of four crystal samples with He-Ne laser at 633 nm. The results indicated that Mg(3 mol%):Fe:Mn:LiNbO₃ was the most proper holographic recording media material among four crystals in the paper.     

The influence of Zn2+ ion on the 1.5 μm laser properties of LiNbO3 crystal heavily doped with Er3+ ion

The spectral characteristics of 1.54 μm emission in a series of Zn/Er:LiNbO₃ crystals with heavy Er content and variable Zn content were reported. The inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the concentration of Er³⁺ ion in the crystal. The absorption and emission spectra of Zn/Er:LiNbO₃ crystals were measured. Based on Judd–Ofelt theory, the spectral parameters such as intensity parameters Ω_t (t=2, 4 and 6), transition strengths, radiative transition probabilities, radiative lifetime and fluorescence branching ratio have been obtained in Zn/Er:LiNbO₃ system. The emission cross section corresponding to ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ion was obtained according to Füchtbauer–Ladenburg theory. The gain cross section of Er:LiNbO₃ crystal codoped with 6 mol% Zn²⁺ ions were also discussed in this work.     

Influence of post-grown Li-rich and Li-poor vapor transport equilibration on composition, OH− absorption and optical-damage threshold of Mg (5 mol%) : LiNbO3 crystals

Li-rich (Li-poor) vapor transport equilibration (VTE) treatments on a number of Z-cut 0.47 mm thick congruent MgO (5 mol% in melt) : LiNbO₃ crystals were carried out at 1100°C over different durations ranging in 1–172 (40–395) h. Neutron activation analysis shows that neither Li-rich nor Li-poor VTE-induced Mg and Nb loss from the crystal occurred. The Li₂O content in the crystal was measured as a function of VTE duration by the gravimetric method. The Li-rich/Li-poor VTE effects on OH⁻ absorption were studied in comparison with the as-grown crystal. The study shows that the Li-rich VTE results in OH⁻ absorption band annihilation. After further oxidation treatment the band reemerges and peaks at the same wavenumber as that of the as-grown crystal (∼3535.6 cm⁻¹), showing that the MgO concentration in the Li-rich VTE crystal is still above the optical-damage threshold. The Li-poor VTE causes OH⁻ band shift to 3486.3–3491.6 cm⁻¹, indicating that the MgO concentration in all Li-poor VTE crystals is all below the optical-damage threshold. Further successive Li-rich VTE and oxidation treatments on the Li-poor VTE-treated crystal lead the band to shift back to 3535.6 cm⁻¹, showing that the post Li-rich VTE brought the Li-poor VTE-treated crystal above the optical-damage threshold again. It is found that the peaking position, band width, peaking absorption and band area of the absorption at ∼3486 cm⁻¹ all increase monotonously with the decrease of the Li₂O content arising from prolonged Li-poor VTE, and quantitative relationships to the Li₂O content are established for the latter two parameters. The VTE effects on the OH⁻ absorption are conducted with the VTE-induced OH⁻ content alteration and charge redistribution.     

Optical behaviour of VTE treated near stoichiometric LiNbO3 crystals

Near stoichiometric LiNbO₃ crystal wafers of thickness up to 2 mm were prepared by vapour transport equilibration technique (VTE) at various process temperatures. Crystals were characterised by measurement of the UV absorption edge, refractive index, second harmonic generation (SHG) efficiency, and conoscopy pattern analysis. The comparison of VTE treated crystals show that the blue shift in cut off wavelength occurred with the increasing process temperature (i.e. increasing Li/Nb ratio). The refractive indices were found decreasing with increasing process temperature of VTE samples. The SHG efficiency increases in the range of 1.98-2.3 times for the VTE processed samples with respected to congruent crystals. Conoscopy pattern reveals the optical homogeneity of the VTE treated crystal.     

Influence of post-growth treatment on the holographic storage properties of In:Fe:LiNbO3

The congruent In (3 mol%):Fe (0.03 wt%): LiNbO₃ crystal has been grown by Czochralski method in air. Some crystal samples were reduced in Li₂CO₃ powder, and others were oxidized in Nb₂O₅ powder. The defects and ions location in crystal were investigated by infrared (IR) transmission spectrum. The photorefractive properties were measured by two-wave coupling and light-induced scattering resistance experiments. In the oxidized sample, the photovoltaic effect was the dominant process during recording. However, for the as-grown sample as well as the reduced, the photorefractive effect was governed by the diffuse field and the photovoltaic field, together. In addition, the reduction treatment made the photoconductivity increase, which resulted in shorter erasure time and lower diffraction efficiency, but higher light-induced scattering resistance ability. The oxidation treatment caused the inverse effect.     

Electro-optic properties in undoped and Cr-doped LiNbO3 crystals

3 along the ferroelectric c axis in pure and chromium-doped crystals. We have studied four series of samples with various crystal compositions and doping concentrations to separate the influence of the intrinsic defects related to the non-stoichiometry and the effect of the introduction of chromium. In pure and slightly doped crystals, the electro-optic coefficient r_c shows a non-monotonous dependence on the controlled crystal composition, which reflects a similar behaviour of the corresponding dielectric permittivity. In the highly Cr-doped crystals r_c displays a large decrease with increasing Cr concentration whatever the crystal composition. The results have been discussed by means of the LiNbO₃ structure and the substitution site dynamics. Received: 14 November 1997/Accepted: 2 January 1998     

Nonvolatile photorefractive holographic recording in Sc:Ce:Cu:LiNbO3

In this paper experimental studies of nonvolatile photorefractive holographic recording in Ce:Cu:LiNbO₃ crystals doped with Sc(0,1,2,3 mol%) were carried out. The Sc:Ce:Cu:LiNbO₃ crystals were grown by the Czochralski method and oxidized in Nb₂O₅ powders. The nonvolatile holographic recording in Sc:Ce:Cu:LiNbO₃ crystals was realized by the two-photon fixed method. We found that the recording time of Sc:Ce:Cu:LiNbO₃ crystal became shorter with the increase of Sc doping concentration, especially doping with Sc(3 mol%), which exceeds the so-called threshold, and there was little loss of nonvolatile diffraction efficiencies between Sc(3 mol%):Ce:Cu:LiNbO₃ and Ce:Cu:LiNbO₃ crystals.     

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.     

Nonvolatile hologram storage properties of tri-doped Zn:Ce:Cu:LiNbO3 crystals

Congruent Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was grown by the Czochralski method in air. The occupation mechanism of the Zn²⁺ was discussed by an infrared transmittance spectrum. The nonvolatile holographic recording in Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was measured by two-photon fixed method. Zn(7 mol%):Ce:Cu:LiNbO₃ single crystals present the faster recording time and higher light-induced scattering resistance ability comparing with Ce:Cu:LiNbO₃ single crystals.