Hyperfine structure of LiNbO3:57Fe(III)

Lithium niobate monocrystals exhibit many interesting physical phenomena such as ferro-, piezo- and pyroelectricity, low acoustical losses, small optical absorption in the visible region and, especialy after doping with small amounts of transition metal ions, strong photorefraction and photovoltaic effects [1–3]. According to the multitude of the properties there are numerous applications of LiNbO₃ for e.g. light modulation, Q-switch, second harmonic generation, fibre optics, acoustic transducers, pyroelectric detectors and holographic data storage. In order to understand in detail the microscopic mechanisms of the bulk photovoltaic effect and the photorefraction, which are strongly enhanced by doping the crystals with iron, an exact knowledge of the not yet unambigously known lattice site and the charge compensation [4,5] as well as the electronic structure of the iron impurities is indispensable. Here the Mössbauer investigation of the hyperfine interaction of⁵⁷Fe ions probing the crystal field may contribute to clarification.As can be seen from the isomer shift iron enters the LiNbO₃-lattice as Fe(II) or Fe(III) [5]. By annealing the samples in oxydizing atmospheres (e.g. p24 hours in air p600C) all iron is transformed to Fe(III).     

Superlinear photovoltaic currents in LiNbO<Subscript>3</Subscript>: analyses under the two-center model

Superlinear photovoltaic currents in LiNbO₃ are theoretically studied by using a two active center model, with Fe²⁺/Fe³⁺ as primary center and Nb_Li⁴⁺/Nb_Li⁵⁺ as secondary center. Analytical instead of numerical results are provided, including close-form expressions for most common experimental situations. Recent photovoltaic parameters obtained for -phase proton-exchanged LiNbO₃ waveguides (very similar to the substrate) are used for applying the model and comparing with published experimental results. Thoroughly studied aspects are: the redistribution of donor/acceptor states for each species as a function of the light intensity, their contribution to the photovoltaic current density, the effect of the temperature, and the role of the center concentrations and their reduction state. This provides a detailed understanding of the photovoltaic current function shapes versus light intensity, predicts new features of interest for experimentalists and suggests further experiments to better determine the material parameters. Photovoltaic measurements and modeling appear a simpler and safer way of understanding the role of the two-center photovoltaic effect in photorefractive phenomena as well as for determining important photorefractive parameters. PACS 42.70.Nq; 72.40.+w.     

Thermal lens spectrometry in pyroelectric lithium niobate crystals

In this work, the light-induced lens effect due to thermal and/or photorefractive processes was studied in pyroelectric (undoped and Fe²⁺-doped) lithium niobate crystals (LiNbO₃) using thermal lens spectrometry with a two-beam (pump–probe) mode-mismatched configuration. The measurements were carried out at two pump beam wavelengths (514.5 and 750 nm) to establish a full understanding of the present effects in this material (thermal and/or photorefractive). We present an easy-to-implement method to determine quantitative values of the pyroelectric coefficient (dP _s/dT), its contribution to the thermal effect and other thermo-optical parameters like thermal diffusivity (D), thermal conductivity (K) and temperature coefficient of the optical path length change (ds/dT). These measurements were performed in LiNbO₃ and LiNbO₃:Fe (0.1 ppm Fe²⁺) crystals with c axis along the direction of laser propagation.     

Space charge field limitations in photorefractive LiNbO3:Fe crystals

We use holographic techniques for the investigation of strongly oxidized LiNbO₃:Fe crystals with small Fe²⁺ concentrations and compare the results with theoretical predictions. Experimental evidence is presented for enhanced phase shifts between light intensity pattern and refractive index grating and for limitations of optically induced space charge fields in photovoltaic crystals due to the low concentration of filled traps. Our findings do not support the model of a nonlocal photovoltaic effect in LiNbO₃.     

Studies of absorption spectra and the photovoltaic effect in LiNbO3:Mg:Fe crystals

The absorption spectra, photoconductivities and photovoltaic currents of LiNbO₃:Fe crystals with different Mg doping levels and Li/Nb ratios in the oxidized state have been investigated at room temperature. The Fe²⁺ ions in LiNbO₃:Mg:Fe with Mg content above a critical value are more easily oxidized than in crystals with Mg content below a critical value. The photoconductivity of LiNbO₃:Mg:Fe crystals with Mg content above a critical value is one order of magnitude higher than those with Mg content below a critical value, however, the photovoltaic current of the former is one order of magnitude lower than the latter. The differences are postulated to be due to different sites of Fe in these two classes of crystals.     

The off-axis pyroelectric effect observed for lithium tetraborate

We find a pyroelectric current along the 〈110〉 direction of stoichiometric Li₂B₄O₇ so that the pyroelectric coefficient is nonzero but roughly 10⁻³ smaller than along the 〈001〉 direction of spontaneous polarization. Abrupt decreases in the pyroelectric coefficient along the 〈110〉 direction can be correlated with anomalies in the elastic stiffness contributing to concept that the pyroelectric coefficient is not simply a vector but has qualities of a tensor, as expected. The time dependent surface photovoltaic charging suggests that an inverse piezoelectric effect occurs at the (110) surface but not the (100) surface. Both effects along the 〈110〉 direction or at the (110) surface are distinct the conventional as a bulk pyroelectric effect.     

Strong permanent reversible diffraction gratings in copper-doped lithium niobate crystals caused by a zero-electric-field photorefractive effect

Permanent reversible diffraction gratings with refractive-index modulations of up to 10^-4 and with a grating vector perpendicular to the polar axis are realized in LiNbO₃:Cu crystals by making use of high temperature recording and charge compensation. The index changes do not result from light-induced space-charge fields and the linear electro-optic effect. They are linked to fixed high-contrast narrow-band absorption gratings of Cu⁺/Cu²⁺ ions via the Kramers–Kronig relations. PACS 42.65; 42.70; 72.40; 78.20     

Study of near-infrared nonvolatile two-center holographic recording in LiNbO3:Fe:Rh crystal

The near-infrared nonvolatile holographic recording has been realized in a doubly doped LiNbO₃:Fe:Rh crystal by the traditional two-center holographic recording scheme, for the first time. The recording performance of this crystal has been investigated by recording with 633 nm red light, 752 nm red light and 799 nm near-infrared light and sensitizing with 405 nm purple light. The experimental results show that, co-doped with Fe and Rh, the near-infrared absorption and the photovoltaic coefficient of shallow trap Fe are enhanced in this LiNbO₃:Fe:Rh crystal, compared with other doubly doped LiNbO₃ crystals such as LiNbO₃:Fe:Mn. It is also found that the sensitizing light intensity affects the near-infrared recording sensitivity in a different way than two-center holographic recording with shorter wavelength, and the origin of experimental results is analyzed.     

Charge transport processes in LiNbO3:Fe at high intensity laser pulses

Light-induced refractive index changes in LiNbO₃:Fe crystals are investigated at high light intensities (>10⁹ Wm^–2). Holographic gratings are recorded and erased with frequency-doubled pulses of a Q-switched Nd:YAG laser. We find new intensity dependent contributions to the holographic sensitivity, to the photoconductivity, and to the saturation value of refractive index change. Light-induced absorption changes are also detected. These results indicate that the Fe²⁺/Fe³⁺ charge transport model, well established for low intensities, has to be modified for high intensities by assuming additional centers which trap and supply electrons.     

Amplified backward scattering in LiNbO3:Fe

We study the characteristics of the amplified back-scattered light in a LiNbO₃:Fe crystal cut normal to the optic axis when it is illuminated with a linearly polarized parallel beam of monochromatic light. The scattered light forms different patterns depending on the direction of vibration of the incident beam. The incident beam and the back-scattered beams interfere and generate numerous ‘noisy’ gratings in the volume of the crystal which diffract and amplify selectivity the weak scattered beams. This study is of practical importance since a normally cut LiNbO₃:Fe crystal may be used as a self-starting non-linear adaptive mirror in laser cavities.     

Enhancement of nonvolatile blue photorefractive properties in LiNbO<Subscript>3</Subscript>:In:Fe:Cu crystals

The nonvolatile photorefractive characteristics of LiNbO₃:Fe:Cu and In-doped LiNbO₃:Fe:Cu crystals are investigated. The stronger nonvolatile blue photorefraction observed can be ascribed to its remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers, which is one or two orders of magnitude higher than those obtained in conventional two-color recordings under the same recording conditions. Furthermore, it is interesting that, compared with LiNbO₃:Fe:Cu, the recording properties, such as the saturation refractive index change, nonvolatile sensitivity and response time at 488 nm wavelength are enhanced in LiNbO₃:In:Fe:Cu crystals under the same recording conditions. The so-called damage-resistant dopants such as In³⁺ ions in red photorefraction are not damage resistant at 488 nm wavelength but they enhance the blue photorefraction. PACS  42.40.Ht; 42.40.Lx; 42.70.Ln     

Preparation and holographic storage properties of tri-doped Mg:Mn:Fe:LiNbO3 crystals

Doping MgO, MnO and Fe₂O₃ in LiNbO₃ crystals, tri-doped Mg:Mn:Fe:LiNbO₃ single crystals were prepared by the conventional Czochralski method. The UV-vis absorption spectra were measured and the shift mechanism of absorption edge was also investigated in this paper. In Mg:Mn:Fe:LiNbO₃ crystal, Mn and Fe locate at the deep level and the shallow level, respectively. The two-photon holographic storage is realized in Mg:Mn:Fe:LiNbO₃ crystals by using He-Ne laser as the light source and ultraviolet as the gating light. The results indicated that the recording time can be significantly reduced for introducing Mg²⁺ in the Mg:Mn:Fe:LiNbO₃ crystal.     

Infrared holographic recording in LiNbO3:Cu

Holograms may be recorded in photorefractive LiNbO₃:Cu with pulsed infrared light (wavelength =1064 nm, Q-switched Nd:YAG laser), if the crystals are previously or simultaneously illuminated with a green (=532 nm) light pulse. We study refractive index changes and time constants of as-grown and thermally treated crystals with different copper concentrations. A model explaining this effect is discussed.     

High-performance pyroelectric single crystals for uncooled infrared detection applications

Uncooled pyroelectric infrared detectors based on ferroelectric single crystals 0.74Pb(Mg_1/3Nb_2/3)O₃–0.26PbTiO₃ (PMN–0.26PT) were fabricated. The performances of pyroelectric detectors dependence on detector fabrication temperature, absorption layer, and element thickness were compared. The room-temperature voltage responsivity (R_v) of 200 V/W and specific detectivity (D^*) of 10⁸ cm Hz^1/2/W at 12.5 Hz have been achieved. The results reveal that the better pyroelectric response can be expected by controlling temperature below 70 °C during the fabrication of the pyroelectric detectors, selecting absorption layer with high absorption coefficient, and decreasing the thickness of the elements.     

Characterization of stoichiometric LiNbO3 grown from melts containing K2O

The growth of LiNbO₃ single crystals from a melt with the Li/Nb ratio of 0.946, to which 6 wt.% K₂O has been added, leads to stoichiometric specimens, essentially free of potassium, with (50±0.15) mol% Li₂O in the crystal. This is established by studying the composition dependence of the following properties: linewidths of the electron paramagnetic resonance (EPR) of Fe³⁺, energy of the fundamental absorption edge, Raman linewidths of phonon modes, and dispersion of the optical birefringence. Comparison of the results with relevant calibration scales leads to the above composition. In all cases the Li₂O content was found to be closer to 50% than that of a LiNbO₃ crystal vapor-phase equilibrated to 49.9mol% Li₂O. The photorefractive effect at light intensities I10⁷ W/m² is suppressed in this stoichiometric material. The features of the ternary system K₂O-Li₂O-Nb₂O₅, which are possibly responsible for the unexpected growth of stoichiometric LiNbO₃ from the indicated melts, are discussed.     

The use of photoinduced light scattering for the evaluation of photoelectric fields in Lithium Niobate Crystals

The photovoltaic and diffusion fields in nominally pure single crystals of stoichiometric composition (R = Li/Nb = 1) grown from the melt with 58.6 mol % of Li₂O (LiNbO₃ stoich), in the nominally pure single crystals of congruent composition (LiNbO₃), and in congruent single crystals doped with Cu²⁺, Zn²⁺, and Gd³⁺ are found from the parameters of the photoinduced light scattering indicatrix obtained with the use of a 60-mW He-Ne laser.     

Phase gratings in the visible and near-infrared spectral range realized by metastable electronic states in Na2[Fe(CN)5NO] · 2H2O

Phase and amplitude gratings in the visible and near-infrared spectral range can be written in SodiumNitro-Prusside (SNP), Na₂[Fe(CN)₅NO] · 2H₂O, single crystals by optical excitation of infinitely long-living metastable electronic states, localized in the [Fe(CN)₅NO]^2– anions. Hence, its photorefractive effect does not depend on dopants or defects. The refractive index is modulated by more thann = 1 × 10^–3 in the red (632.8 nm) andn = 5 × 10^–4 in the near-infrared region (1047 nm). The absorption coefficient is modulated by about = 100 m^–1 at 632.8 nm and 40 m^–1 at 1047 nm. The wavelength dependence ofn can be explained by strong absorption bands in the ultraviolet considering Kramers-Kronig dispersive analysis. The time constant of the write-read-erase processes and the diffraction efficiency depend on light intensity, wavelength and polarization of the light with respect to the crystallographic axes. After excitation of the metastable states the indicatrix is modulated only along thea- andb-axis of the orthorhombic system.     

Enhanced photoluminescence from Cr<Superscript>3+</Superscript> centers in α-sapphire coated with LiNbO<Subscript>3</Subscript>(:Fe) and LiTaO<Subscript>3</Subscript> films

A LiNbO₃(:Fe) or LiTaO₃ film is sandwiched between a (012)-oriented -sapphire wafer and an amorphous Al₂O₃ or SiO₂ film using pulsed laser deposition. After annealing at 1000 °C in O₂, the film becomes a c-oriented single-domain ferroelectric. This sandwich structure shows an enhanced photoluminescence from trace amounts of Cr³⁺ centers in the host -sapphire (R-line emission at 691 nm). Spectral analyses suggest that both strong space-charge and photorefractive effects of the LiNbO₃(:Fe) or LiTaO₃ film cause a change in the crystal field of the host -sapphire, which increases the transition probability of Cr³⁺ and thus leads to an enhancement of the R-line intensity. The result has prospective applications in laser and optical integrated devices. PACS 77.55.+f; 78.20.-e; 78.55.-m     

Defect structure and optical damage resistance of Hf:Fe:LiNbO3 crystals

A series of Hf:Fe:LiNbO₃ crystals were grown by the Czochralski technique with various doping concentrations of HfO₂. Their defect structures were analyzed by the UV-visible absorption spectra and infrared absorption spectra. The optical damage resistance of Hf:Fe:LiNbO₃ crystals was measured by the photo-induced birefringence change and the transmitted light spot distortion method. The results show that the optical damage resistance ability of Hf:Fe:LiNbO₃ crystals enhances remarkably with the HfO₂ concentration increasing when the HfO₂ concentration is lower than its threshold concentration (4 mol%). However, when the HfO₂ concentration exceeds its threshold concentration, the optical damage resistance ability of the crystals returns to decrease. This unusual behavior is explained by using the photovoltaic field produced in the crystals.     

One-dimensional steady-state bright photovoltaic solitons in LiNbO3:Fe crystal with background illumination

We investigate theoretically and experimentally one-dimensional bright photovoltaic solitons in LiNbO₃:Fe crystal by use of the background illumination. We find that, in LiNbO₃:Fe crystal, bright photovoltaic solitons can be obtained with background illumination for κ>1, where κ is the ratio of the background illumination photovoltaic constant to the soliton optical beam's photovoltaic constant. For κ<1, dark photovoltaic solitons are generated. On the other hand, our experiments show good agreement with theoretical prediction for the soliton existence curve in a special intensity ratio.