X-ray diffraction study on precipitate of Er:LiNbO3 induced by vapor transport equilibration

An X-ray powder diffraction study was performed on vapor transport equilibration (VTE) treated Er:LiNbO₃ crystals with different doping levels (0.2, 0.4 and 2.0% Er per cation site), different cut orientations (X- and Z-cuts) and different VTE durations (120, 150 and 180 h). Their diffraction characteristics were compared with those of pure congruent LiNbO₃ and as-grown Er:LiNbO₃. The most significant characteristic is the appearance of additional weak and broad peaks around the 2θ angles 30° and 59° in the diffraction patterns of both X- and Z-cut 2.0 mol% doped VTE crystals, confirming that they precipitated. A further comparison of their diffraction data with the powder diffraction files indicated that the new phase in these precipitated crystals is ErNbO₄, which has an approximate concentration of 1.0%, 1.065%, 1.485% for 120, 150 and 180 h crystals, respectively. The crystalline grain sizes of the new phase are 132.2∼184.1?. The unit cell parameters of the as-grown and VTE crystals were also determined from diffraction data; the variation from pure LiNbO₃ to as-grown Er:LiNbO₃ was qualitatively explained according to the crystal structure of LiNbO₃ and using the concept of ionic radius. VTE brings the crystal closer to a stoichiometric composition, thus causing the contraction of the lattice constants. Finally, a tentatively qualitative explanation for precipitate formation is given on the basis of crystal structure. Received: 2 August 2000 / Accepted: 29 March 2001 / Published online: 20 June 2001     

Raman characterization of H:LiNbO3 waveguides

In this work we present polarized Raman measurements of z-cut multimode H:LiNbO₃ waveguides in the κ_i- and β_i-phase states. A spectrum with unexpected form and behaviour, originating from the layer at the very surface, was observed in samples with various degrees of Li–H substitution. Possible reasons for the presence of this strange spectrum are considered. Received: 16 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

Raman Investigation of BaWO4-II Phase under Hydrostatic Pressures up to 14.8GPa

The BaW04-17 phase is synthesized at 5.0 GPa and 610~C with a cubic-anvil apparatus and identified by XRD. Raman scattering measurement is carried out to investigate the phase behaviour of a pure BaW04-Ⅱ phase （space group P21/n, Z = 8） under hydrostatic pressures up to 14.8 GPa at ambient temperature. In each spectrum recorded for this phase, 27 Raman modes are observed, and all bands shift toward higher wavenumber with a pressure dependence ranging from 3.8 to 0.2 cm- 1/GPa. No pressure-driven phase transition occurs in the entire pressure range in this study. Our results indicate that the previously reported high pressure phase of Ba WO4 at pressure above about 10 GPa and room temperature （Errandonea et al. Phys. Rev. B 73（2006）224103） is not the BaW04-Ⅱ phase.     

E(transverse optical)-mode properties in the LiTaO3: Nd crystal at room temperature

E(TO)-mode properties in LiTaO₃:Nd crystal were examined by analyzing the Raman spectra measured.E(TO) modes appear in the transverseA ₁ spectrum. Their intensities obviously increase in theE andE+A ₁ mixed-symmetry spectra but decrease in theE spectrum which shows new vibrational modes. In particular, in the transverse-E spectrum of y(xz) geometry, the properties ofE(TO) modes are similar to those of pure LiTaO₃ of the same geometry, whereas in the transverse-E spectrum in x(yz) geometry these modes are turned intoA ₁ (TO) modes. We attribute these properties to both the surface strain produced by mechanical polishing of the sample and the microstructural change of the LiTaO₃ crystal resulting from Nd doping.     

Synthesis and Raman characterization of mono-sized single-wall carbon nanotubes in one-dimensional channels of AlPO4-5 crystals

Mono-sized single-wall carbon nanotubes were formed in one-dimensional channels of AlPO₄-5 single crystal (AFI) by pyrolysis of tripropylamine (TPA). Raman spectra have been measured for the TPA-AFI crystals thermally processed at different conditions. TPA molecules are carbonized at 400 °C, and carbon nanotubes were formed at 500 °C or above. The radial-breathing mode, which is special for carbon nanotube geometry, was observed. Three Raman-active modes with symmetry A _1g, E _1g, and E _2g were identified by detailed symmetrical analysis for the polarized-Raman spectra. Received: 29 October 1998 / Accepted: 29 March 1999 / Published online: 24 June 1999     

High-pressure Raman spectroscopy of antimony: As-type, incommensurate host-guest, and bcc phases

The vibrational dynamics of elemental solids that form incommensurate host-guest structures are of fundamental interest. High-pressure Raman scattering has been used to examine the vibrational spectrum of the group-V element Sb up to 33 GPa. A_1g and E_g phonons of the ambient pressure rhombohedral A7 phase display a marked decrease with pressure, i.e., prior to the transition to the tetragonal host-guest Sb-II phase at 8.6 GPa, via the monoclinic host-guest Sb-IV phase. The Raman spectrum of the incommensurate host-guest Sb-II phase, has five bands between 80 cm⁻¹ and 200 cm⁻¹ that increase with pressure. For the bcc structure stable above 28 GPa, we observe one weak disorder-induced band that increases with pressure.     

Anharmonic behaviour of BaFCl using Raman scattering

Raman scattering from oriented single crystals of BaFCl was recorded at various temperatures from 20 to 1073 K for the first time. The Raman spectra, corrected for phonon population, were fitted to the sum of four Lorentzian peaks. The peak frequencies and full width at half maximum (FWHM) of the peaks were obtained from the fit. The FWHM is accounted for by cubic and quartic anharmonic processes. The quartic anharmonicity of the mode increases with the mode frequency. The quartic anharmonicity of the fluorine mode is exceptionally high. The peak frequencies decrease linearly with the increasing temperature. Fluorine mode frequencies decrease more than the internal mode frequencies do. The LO-TO splitting of the fluorine modes and that of the internal modes increases with temperature indicating the increase of the ionic bonding character. The results are discussed. Received: 12 May 1997 / Accepted: 25 July 1997     

Comparison of the influence of the fictive and the annealing temperature on the UV-transmission properties of synthetic fused silica

′ and NBOH). Samples with high OH content exhibit gradual recovery from the absorption band within several minutes after exposure to the KrF laser radiation. The formation of the KrF laser-induced 210 nm absorption band depends on the fictive temperature and on the OH content. Low fictive temperature, as a measure for the number of intrinsic defects, retards E^′ generation at the beginning of intense KrF excimer laser irradiation when the majority of defects are generated from precursor defects. However, for longer irradiation periods with pulse numbers of the order of 10⁵ pulses, a high OH content is the beneficial parameter. The accompanying atomic hydrogen is essential for the suppression of the 210 nm absorption band. This happens by transformation of the E^′ centers into Si-H defects. In contrast to a generally held view, annealing (decreasing of the fictive temperature) of fused silica does not always reduce UV induced defect generation. For example, annealing of the samples in an argon atmosphere causes a significantly higher 210 nm absorption increase during KrF excimer laser irradiation (240000 pulses) compared to nonannealed samples. Two spectroscopic methods to determine the OH content of fused silica were applied: Raman and infrared spectroscopy, which in this work lead to differing results. The energetics of the 210 nm absorption band generation and bleaching is summarized by a diagram explaining the interaction of the 248 nm laser radiation with fused silica. Received: 2 June 1997/Accepted: 13 June 1997     

Amorphization and grain size effect on milled PbTiO3 studied by Raman scattering and visible photoluminescence emission

Raman scattering and photoluminescence (PL) studies on milled PbTiO₃ are presented in this paper. The results suggest that the visible PL emission could be related to both the localized states in the interface between the amorphous layer and the crystalline core and the amorphous layer itself. The Raman spectrum of PbTiO₃ milled for a long time showed the vibrational density of states, and a detailed analysis of the soft mode allowed us to conclude that the PbTiO₃ crystalline core did not experience any structural phasetransition. Received: 2 February 2001 / Accepted: 23 July 2001 / Published online: 17 October 2001     

Low-loss, damage-resistant optical waveguides in Zn-diffused LiNbO3 by a two-step procedure

In the present work the fabrication of high-quality LiNbO₃ waveguides, based on Zn diffusion from vapor phase using a two-step diffusion process, has been demonstrated. The optical waveguides fabricated using this method have low propagation losses (less than 0.5 dB/cm measured at 633 nm), as a consequence of the high surface quality obtained. High photorefractive damage resistance is reported, and it has been related to the Zn incorporation into the LiNbO₃ crystals. Additional advantages of this waveguide fabrication method due to the low temperature of the process (<900 °C) are to prevent Li out-diffusion and to preserve ferroelectric domain structures. Received: 2 March 2000 / Accepted: 21 June 2000 / Published online: 13 September 2000     

Structural and optical properties of nanophase zinc oxide

Nanophase zinc oxide samples were synthesized by a two-step solid-state reaction method. The phase structure and microstructure were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The vibrational Raman spectra were compared with those from the bulk and their grain size dependence was also examined. Their photoelectric behavior was studied by X-ray photoelectron spectroscopy (XPS). The peaks at 1044.5 and 1021.4 eV were recorded as corresponding to the respective binding energies of Zn 2p_1/2 and Zn 2p_3/2, and the photoelectron spectrum of O 1s in the as-prepared powder was located at 531.2 eV. A strong visible emission centered at 580 nm was clearly observed in the nanosized zinc oxide at room temperature. Photoluminescence (PL) spectra were investigated as a function of grain size after different heat treatments. The origin of the luminescence is attributed to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. Received: 30 June 2001 / Accepted: 20 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

Low temperature Raman study on the site symmetry of ZnSe:P

Raman spectra of phosphorous doped ZnSe are recorded from 9 to 300 K. In addition to TO mode at 208 cm⁻¹ and LO mode at 253 cm⁻¹, a new mode around 240 cm⁻¹ is observed between 55 and 270 K. The spectra at lower and higher temperatures do not show the new mode. This new mode confirms that there is a reduction of (Se site) symmetry from T_d to C_3v when P substitutes for Se. This is due to Jahn Teller distortion.     

Effect of stresses on the OH libration bands in proton-exchanged LiNbO3 optical waveguides

i -H_xLi_1-xNbO₃ phases generated on x-cut LiNbO₃ the new anisotropic libration mode at 890–940 cm^-1 is observed in addition to the known isotropic mode at 960–970 cm^-1, whereas the same phases on z-cut exhibit only latter isotropic libration mode. The observed phenomena are explained as a result of anisotropic stresses generated in the x-cut waveguides, whereas the stresses in the structures on z-cut are isotropic. The OH-bond librations are clearly anisotropic in the α, κ₁, and κ₂ phases generated on both z- and x-cuts, which indicates a fundamental difference of the local symmetry of the proton’s position in the β_i phases compared to the α, κ₁, and κ₂ H_xLi_1-xNbO₃ phases. Received: 4 May 1998/Revised version: 26 June 1998     

A comparative study of dielectric and Raman spectroscopy of Pb(Y b1/2Ta1/2)O3 and Pb(Y b1/2Nb1/2)O3

A comparative study of the properties of two highly ordered lead based complex perovskites Pb(Y b_1/2Ta_1/2)O₃ and Pb(Y b_1/2Nb_1/2)O₃ has been carried out through x-ray diffraction, dielectric and Raman scattering measurements. These two compounds differ significantly in their structure, dielectric response and phonon vibration although the ionic radii and valencies are same for Ta and Nb. The room temperature x-ray diffraction pattern and Raman spectra show that the symmetry of lead ytterbium tantalate is lower than that of lead ytterbium niobate. The Raman spectra of Pb(Y b_1/2Ta_1/2)O₃ also indicates the presence of local distortion in the lattice which may be one of the factors responsible for the existence of a secondary transition.     

Spectroscopic characterisation of LaGaO3:Er3+ crystals

LaGaO₃ crystals doped with Er³⁺ ions were grown by the Czochralski method and their optical properties were examined. The Er³⁺ energy levels have been determined from the low-temperature absorption and emission spectra. The results of Judd–Ofelt analysis are presented and compared with experimental data. The emission cross sections are determined for the ⁴ I _13/2→⁴ I _15/2 (1.55 μm) and ⁴ I _11/2→⁴ I _13/2 (2.85 μm) transitions of erbium. Received: 6 December 1999 / Revised version: 10 February 2000 / Published online: 27 April 2000     

Site-selective spectroscopy of Er3+:Ti:LiNbO3 waveguides

Starting from previous investigations in LiNbO₃ bulk crystals, we studied the optical properties of Er³⁺ ions in Ti:LiNbO₃ channel waveguides and investigated the waveguide-specific lattice environment of the Er³⁺ ions (“sites”) caused by the doping method used and the presence of a large number of Ti⁴⁺ ions. For that purpose the method of combined excitation–emission spectroscopy was applied for the first time to waveguides at low temperatures. Comparing the spectroscopic results obtained for the green, red, and near-IR luminescence (λ≈550, ≈650 and ≈980 nm) under direct (450 nm), 2-step (980 nm), and 3-step (1.5 μm) laser excitation, we found several distinguishable Er³⁺ sites which in terms of energy levels and relative numbers are similar to those in bulk material, but exhibit significantly different up-conversion efficiencies and strongly inhomogeneously broadened transitions. Moreover, we were able to distinguish isolated and cluster Er³⁺ sites by their characteristic excitation and emission transition energies and studied the respective excitation/relaxation channels. The cluster sites are most efficient in the up-conversion process, especially under 3-step excitation. Using accepted microscopic models for Er³⁺ and Ti⁴⁺ incorporation into the LiNbO₃ crystal lattice, the site distribution and up-conversion mechanisms are elucidated and their consequences for laser applications in different spectral regions are discussed. Received: 16 November 2000 / Published online: 21 March 2001     

Towards new lasers in Ti:Er:LiNbO3 waveguides: a study of the excited Er3+ states

Detailed excited state absorption measurements under pumping at 980 nm and 1.5 μm together with conventional absorption and emission spectroscopy is employed to investigate optical transitions of Er³⁺ in Ti:LiNbO₃ channel-waveguides. The experimental data were evaluated using the Judd–Ofelt method giving parameters close to those in the bulk. The good agreement between theoretical prediction and measurements allows us to calculate cross section, lifetimes, and branching ratios. Based on these results we developed and tested a model which is able to predict the conditions for which laser operation at 550 nm and 2.7 μm will be possible under 980 nm pumping. Received: 9 December 1998 / Revised version: 8 January 1999 / Published online: 24 March 1999     

Theoretical investigations of homo- and heteronuclear bridged fullerene oligomers

\chem{[C_{60}]_{\chemindex{n}}} -oligomers (, , and ) as well as dimers partially bridged by oxygen atoms or a group. The applicability of the scheme for finding the ground state of these systems was carefully checked against more sophisticated methods including an all-electron, self-consistent (SCF) scheme based on the local-density-approximation (LDA), and the recently developed generalized-gradient approximation (GGA). We present Raman intensities for the vibrations of the oligomers, obtained in the framework of the bond polarization model, and compare with recent experimental data. Received: 26 September 1996/Accepted: 16 December 1996     

Absorption gratings in ferroelectric Bi4Ti3O12

4 Ti₃O₁₂. Basic properties of absorption gratings in this crystal are investigated with beam-coupling experiments. Depending on the grating spacing, wavelength, and intensity of the writing beams, an absorption grating in our sample can be either in phase or shifted by 180° with respect to the light pattern. The formation mechanism of absorption grating is discussed and proposed to originate from a shallow-trap effect. Received: 10 February 1997     

Spectral line broadening mechanism of Er3+ transitions in Er:Ti:LiNbO3 channel waveguides

In order to elucidate the interaction effects among the various defects present in a LiNbO₃-based integrated optical device, we investigated the change of the optical properties of Er³⁺ ions under the application of an external electric field and hydrostatic pressure. We obtained for stoichiometric bulk material a complete picture of the field-induced spectral shifts as a function of transition and site. As a first important application of these results we were able to clarify the mechanism of spectral broadening of the Er³⁺ transitions in Ti:Er:LiNbO₃ channel waveguides. By selecting different waveguide modes for excitation and using highly selective double-resonance excitation with two lasers, we found that the [Ti⁴⁺] concentration gradient caused by the indiffusion results in an internal E-field gradient. This translates, due to the averaging within the guided mode, into mode-dependent spectral line broadening. Received: 24 May 2001 / Published online: 23 October 2001