Centers In LiNbO3 Research Articles

Real-Time Identification of Oxygen Vacancy Centers in LiNbO3 and SrTiO3 during Irradiation with High Energy Particles

Oxygen vacancies are known to play a central role in the optoelectronic properties of oxide perovskites. A detailed description of the exact mechanisms by which oxygen vacancies govern such properties, however, is still quite incomplete. The unambiguous identification of oxygen vacancies has been a subject of intense discussion. Interest in oxygen vacancies is not purely academic. Precise control of oxygen vacancies has potential technological benefits in optoelectronic devices. In this review paper, we focus our attention on the generation of oxygen vacancies by irradiation with high energy particles. Irradiation constitutes an efficient and reliable strategy to introduce, monitor, and characterize oxygen vacancies. Unfortunately, this technique has been underexploited despite its demonstrated advantages. This review revisits the main experimental results that have been obtained for oxygen vacancy centers (a) under high energy electron irradiation (100 keV–1 MeV) in LiNbO3, and (b) during irradiation with high-energy heavy (1–20 MeV) ions in SrTiO3. In both cases, the experiments have used real-time and in situ optical detection. Moreover, the present paper discusses the obtained results in relation to present knowledge from both the experimental and theoretical perspectives. Our view is that a consistent picture is now emerging on the structure and relevant optical features (absorption and emission spectra) of these centers. One key aspect of the topic pertains to the generation of self-trapped electrons as small polarons by irradiation of the crystal lattice and their stabilization by oxygen vacancies. What has been learned by observing the interplay between polarons and vacancies has inspired new models for color centers in dielectric crystals, models which represent an advancement from the early models of color centers in alkali halides and simple oxides. The topic discussed in this review is particularly useful to better understand the complex effects of different types of radiation on the defect structure of those materials, therefore providing relevant clues for nuclear engineering applications.

Investigations on the EPR parameters and defect structures due to Jahn-Teller effect for the Cu2+ and Ni+ centers in LiNbO3

The EPR parameters of the impurity Cu2+ and Ni+ in LiNbO3 are theoretically studied from the perturbation formulas for 3d9 ions under rhombically elongated and compressed octahedra, respectively. In both centers, the impurity Cu2+ and Ni+ ions was ascribed to occupy the host trigonally-distorted octahedral Li+ sites and the octahedrons experience the Jahn-Teller distortion from trigonal to rhombically -elongated and -compressed, respectively. The elongated [CuO6]10− cluster (and compressed [NiO6]11− cluster) are ascribed to suffering the axial elongation of 0.102 Å (and compression of 0.062 Å) along z-axis and the planar bonds are found to experience the relative variation of 0.023 Å (and 0.032 Å), respectively, due to the Jahn-Teller effect. The calculated EPR parameters based on the above lattice distortions agree reasonably with the experimental data, and the local structures of the impurity centers are discussed.

Determination of g-tensors of low-symmetry Nd3+ centers in LiNbO3 by rectification of angular dependence of electron paramagnetic resonance spectra

Two procedures for facilitation of line tracing and deciphering of complicated spectra of electron paramagnetic resonance (EPR) were developed: a correction of microwave frequencies for every orientation of external magnetic field on the base of known values of g-tensor components for a reference paramagnetic center and followed rectification of measured angular dependences using plots of effective deviation of g2-factors of observed lines from effective g2-factors of the reference center versus angles or squared cosines of angles describing magnetic field orientations. Their application to EPR spectra of nearly stoichiometric lithium niobate crystals doped with neodymium allowed identifying two axial and six different low-symmetry Nd3+ centers, to determine all components of their g-tensors, and to propose common divacancy models for a whole family of Nd3+ centers.

Theoretical studies on the local structure and spin Hamiltonian parameters for the orthorhombic Cu2+ center in LiNbO3

The local structure and spin Hamiltonian parameters (the g factors and the hyperfine structure constants) for the orthorhombic Cu2+ center in LiNbO3 are theoretically studied from the perturbation formulas of these parameters for a 3d9 ion in an orthorhombically elongated octahedron. This center is ascribed to Cu2+ occupying the Nb5+ site in LiNbO3, associated with one nearest neighbour oxygen vacancy VO along the Z axis. The planar bond lengths are found to suffer the relative variation of about 0.16Å by compressing and stretching the Cu2+–O2− bonds along the X and Y axes, respectively, due to the Jahn–Teller effect and the charge mismatching substitution of Nb5+ by Cu2+. Meanwhile, the effectively positive VO can make the central Cu2+ displace away from the VO along the Z axis by about 0.3Å. The theoretical spin Hamiltonian parameters based on the above local distortions show good agreement with the experimental data.

Low symmetry centers in LiNbO3 doped with Yb and Er

Electron paramagnetic resonance spectroscopy studies of LiNbO3 single crystal doped with 1wt% of Yb3+ and 0.1 wt% Er are reported. Additionally, Raman spectra of the following crystals are presented: LiNbO3:Nd, Yb (0.5wt%, 0.7 wt%), LiNbO3:Nd, Mg (2wt%, 6wt%), and LiNbO3:Er (0.3 wt%). Raman spectra have revealed bands in the 50–220cm−1 range, suggesting the presence of localized phonons. The localized phonons may be considered as indirect evidence of local perturbations around Yb/Er ions, possibly due to formation of Yb/Er ion pairs. EPR spectra are interpreted basing on this presumption using a spin Hamiltonian for the Yb3+ dissimilar ion pairs. This model explains the observed spectral features, apparently due to the C1 symmetry of Yb ions. In the case of the LN:Er sample, the angular dependence of EPR lines enabled distinguishing the presence of several non-equivalent centers. After deconvolution of the main EPR line into several Lorentzian components, the Er3+ center with the lowest C1 point group symmetry was resolved and values of the g tensor were estimated.

ESR study of the Mn2+ center in LiNbO3

Fine structure and hyperfine structure of Mn2+ in LiNbO3 are analyzed with electron spin resonance (ESR) at room temperature. It is concluded that the zero-field splitting of energy levels of electron spin angular moment between ±12〉 and ±32〉 is Δε1=-587×10-4cm-1, and that between ±32〉 and ±52〉 is Δε2=-2633×10-4cm-1. The anisotropic La nde factors are g∥=21810，and g⊥=20937 The fine structure co nstant is D=-536×10-4cm-1, and hyperfine structure constants ar e A∥=8836cm-1 and A⊥=8120×10-4cm-1. The interaction action of the fine structure is larger than that of the hyperfine structure. In addition, it is worth paying special attention to the fact that two forbidden transitions -32〉12〉 and -12〉52〉 are found.

EPR parameters and defect structures for two trigonalEr3+centers inLiNbO3and MgO or ZnO codopedLiNbO3crystals

In this paper, the perturbation formulas of EPR parameters (g factors g(parallel to),g(perpendicular to) and hyperfine structural constants A(parallel to) and A(perpendicular to)) for the lowest Kramers doublet Gamma(6) or Gamma(7) of I-4(15/2) of 4f(11) ion in trigonal symmetry are established. In these formulas, the second-order perturbation contributions are considered in addition to the first-order perturbation contributions considered in the previous works. Based on these formulas, the EPR parameters and the defect structures of two trigonal Er3+ centers in LiNbO3 and MgO or ZnO codoped LiNbO3 crystals are studied. It is found that in order to reach a good fit between calculations and experiments, the Er3+ ions in centers I and II do not occupy exactly the Li+ and Nb5+ sites, respectively, but are displaced along the C-3 axis away from the center of oxygen octahedron by about 0.039 nm for center I and towards the center of octahedron by about 0.030 nm for center II. The reasonableness of the local lattice distortions and the contributions from the second-order perturbation terms are discussed.

Optical characterization of Cr3+ centers in LiNbO3

Low-temperature luminescence spectra of stoichiometric Cr:LiNbO3, congruent Cr:LiNbO3 and congruent Cr,Mg:LiNbO3 were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr3+ impurity ions that preferentially occupy Li+ sites (CrLi) in the Cr:LiNbO3 crystals. Low-field centers related to Cr3+ substitution of Nb5+ (CrNb) occur in addition to CrLi in co-doped Cr,Mg:LiNbO3 samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr3+ centers from low- to high-field type due to strong pressure-induced blue shift of the 4 T 2 state, resulting in its crossing with the 2 E state of Cr3+. This level-crossing effect was observed for the dominant Cr3+ Li and Cr3+ Nb centers at pressures that correlate well with estimations based on the 4 T 2-2 Eenergy gap (230 cm-1 and 1160 cm-1) and on the rate of their pressure-induced change (14.35 and 11.4 cm-1/kbar, respectively). We also studied inhomogeneous broadeningof the 2 E?4 A 2transitions at ambient pressure for the minor high-field “defect” Cr3+ Li centers in congruent LiNbO3. A fine structure in the spectral response of these centers was observed. The obtained results are discussed on the basis of a microscopic hierarchic model for perturbed Cr3+ ions in the LiNbO3 lattice.

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 (CrLi) in the LiNbO3 crystal lattice, while Cr3+ ions substituting for Nb5+ ions (CrNb) occur in addition to CrLi 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 3+Li and Cr 3+Nb 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).

Electronic structure of deep centers in LiNbO3

A model is proposed for photo-and thermally induced processes in lithium niobate crystals. The model is based on the presence of structural defects formed as a result of departure of the crystal composition from stoichiometry. Defects of the type NbLi-NbLi are introduced and the influence of oxidation-reduction heat treatment on the optical characteristics of the crystals is analyzed.

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 C3 axis of the crystal and indicate that the chromium centers in LiNbO3 have C3 symmetry.

Electric field effect on the b-absorption lines of cr3+impurity centers in linbo3

Abstract A Stark effect sensitive differential spectroscopy technique was employed to study the behaviour of the B-absorption lines of Cr3+ impurity centers in LiNbO3. At 77 K a pronounced linear field effect on the B1 and B2 lines occurs for electric fields E || C3 axis and is absent for E ┴ C3, indicating a trigonal C3 symmetry for the Cr3+ position. The magnitude of the linear Stark effect appeared to be ≈ 2.3×10−5cm−1(V/cm)−1 Unexpected opposite signs for the Stark shifts of the B1 and B2 absorption lines were also observed.

Characterization of Cr3+ centres in LiNbO3 using fluorescence line narrowing

The optical spectroscopic properties of Cr3+ in LiNbO3 have been investigated using optical absorption, fluorescence, fluorescence line narrowing (FLN), selective excitation and radiative lifetime measurements. The present results are compared with data obtained using other techniques including electron spin resonance (ESR) and electron nuclear double resonance (ENDOR), and shown to be consistent with there being five Cr3+ centres. The predominant centre is due to substitution at the Li+ site, which provides the weak crystal field in which Cr3+ ions emit into a broad 4T2 to 4A2 band. Using FLN, two centres experiencing a stronger crystal field are shown to be due to substitution at this same site, probably perturbed by the location of Nb5+ antisites and Nb5+ vacancies in the next-nearest-neighbour cation shell. The addition of Mg2+ ions to LiNbO3 appears not to create new Cr3+ complexes: rather it modifies the nature of the disorder, thereby changing the relative concentrations of the different centres. To this extent it becomes possible to identify two other lightly occupied Cr3+ centres where substitution is at the Nb5+ site. This total of five Cr3+ centres in LiNbO3 gives a consistent interpretation of all the available spectroscopic evidence, including ESR and ENDOR, while pointing to the difficulties of theoretical work in sites where the differences in the energy levels of Cr3+ ions are too small to be distinguished by the available models.

Intrinsic photorefractive centers in LiNbO3:Fe

In LiNbO3:Fe,Me crystals (Me = K, Mg, Zn) elementary holograms are recorded and erased with frequency-doubled, ordinarily polarized pulses of a Q-switched Nd:YAG laser. At high light intensities (I > 109 Wm−2) holographic sensitivities and saturation values of refractive index changes grow with increasing intensity and a contribution to photo-conductivity appears increasing quadratically withI. The intensity-dependent parts of the holographic quantities are diminished by increasing the lithium content of the samples or by doping the crystals with magnesium or zinc. Experimental results can be described by a two-center charge transport model, which is the Fe2+/3+ model supplemented by shallow traps, namely niobium ions occupying lithium sites.

Non equivalent active centres and their lattice location in LiNbO3 minilasers

This paper describes how the combined use of optical spectroscopy and RBS/Channelling techniques is useful to investigate the optically active M3+ centres in LiNbO3:M crystals (M = Pr, Nd, Eu, Ho, Er, Tm and Yb). Particular emphasis is made on the LiNbO3(MgO):Nd laser system. Possible structures for M3+ centres are discussed in terms of different off-centred positions from regular lattice sites for the M3+ ions. The effect of MgO codoping (necessary for laser applications) on the M3+ centres is also analyzed.

ODMR-investigations of transition metals in LiNbO3

Abstract By means of optical detection of magnetic resonance (ODMR) the paramagnetic part of the magnetic circular dichroism (MCD) of Ti3+-and Cr3+- centers in LiNbO3 is unambiguously identified. The MCD-spectra of these impurities are compared with the absorption bands of the investigated crystals and possible contributions from the same centers in other charge states or from other defects can be identified or discarded. By this method we find, that in our LiNbO3:Ti samples the absorption in the visible part of the spectrum is mainly due to intervalence transfer transitions of Ti3+ A comparison with data from Al2O3:Ti supports this statement. In the case of LiNbO3:Cr, we prove that new absorption bands arising from codoping with Mg are due to new Cr3+ - centers on a Nb site, which have been discovered recently by conventional EPR and ENDOR.

Electron spin resonance and electron-nuclear double-resonance investigation of a new Cr3+ defect on an Nb site in LiNbO3:Mg:Cr

For LiNbO3 crystals heavily doped with Mg and codoped with Cr a single isotropic ESR line with g=1.971+or-0.002 and a minimal linewidth of 1.8+or-0.2 mT has been observed. With ENDOR it could be shown unambiguously that the new defect is Cr3+ substituting for Nb. The 53Cr hyperfine and quadrupole constants are A/sub ///=50.8+or-0.2 MHz, Aperpendicular to =51.0+or-0.2 MHz and P=0.10+or-0.05 MHz. The crystal field at the chromium site is smaller by more than an order of magnitude than for previously reported Cr3+ centres in LiNbO3, indicating that those centres are not on the Nb site.

THE PROPERTIES OF THE SURFACE LAYER OF LiNbO3 CRYSTAL AFTER LOW-ENERGY ION BOMBARDMENT

We observed experimentally that at least two optical absorption peaks appear between 3500-6500? after bombardment of LiNbO3 crystal surface by low-energy ion. Also the relative dark conductivity increase by two order of magnitudes, the fact suggests that the additional optical absorption and conductiviting active center creates after ion-bombardment. During bombardment, owing to the prior sputtering of some atom and atom groups in the crystal, a substantial deviation of stoichiometry occur in the surface layer, which has a direct effect upon the optical and electrical properties of LiNbO3 crystal.The measurements of XPS (X photoelectron energy spectroscopy) prove that the structure of this surface layer conforms to the vacancy model of LiNbO3 defect structure. We propose a possible mechanism of the formation of this kind of active center in LiNbO3. By this model, we can explain qualitatively the result obtained from LiNbO3 crystals after bombardement by the mixed ions of Ar+/O+.

Efficient hologram recording in LiNbO3 : Fe using optical pulses

A Q-switched frequency-doubled Nd : YAG laser was used to record holograms in LiNbO3 : Fe. For 1% diffraction efficiency, only 50 mJ/cm2 was needed, as compared to 500 mJ/cm2 for cw recording. Induced absorption, comparable in magnitude with the linear absorption at 647 nm, was observed during hologram recording. This indicates the existence of secondary absorption centers in LiNbO3 : Fe. Their contribution to the enhanced photorefractive effect is discussed.

Photorefractive centers in LiNbO3, studied by optical-, Mössbauer- and EPR-methods

The photorefractive properties of LiNbO3∶Fe and LiNbO3∶Cu have been studied in combination with optical absorption-, Mossbauer- and EPR-measurements. The charge states of Fe in successively reduced LiNbO3∶Fe have been investigated with respect to the influence on the photorefractive sensitivity and saturation value of the refractive index change. The results of this experiment demonstrate clearly the close correlation between the concentration of Fe2+ impurities and the optical absorption band around 2.6 eV in LiNbO3∶Fe, which is known to give rise to an anisotropic charge transport upon optical excitation. The resulting photocurrents determine the photorefractive sensitivity mainly in the initial state of halographic exposure. With increasing conversion from Fe3+ to Fe2+ the photorefractive sensitivity saturates and the saturation value of the refractive index change decreases remarkably.