Search

We review the progress in the development of continuous-wave optical parametric oscillators over the last decade. A recently developed theoretical analysis shows that their stability strongly depends on the group velocity dispersion of the nonlinear material used. Now, these devices generate not only near- and mid-infrared radiation, but also visible and terahertz light. Active locking to external references like atom transitions, resonators, or frequency combs enables mode-hop-free operation up to days. Furthermore, whispering-gallery-resonator-based devices enable the realization of millimeter-sized monolithic resonators with microwatt oscillation thresholds. [ABSTRACT FROM AUTHOR]

Light-induced charge transport in nominally undoped LiNbO crystals essentially differs from that in Fe- and Cu-doped crystals. Furthermore, it is of relevance for numerous effects ranging from optical damage to domain reversal. Here, we review the state of the art of the studies of light-induced charge transport in this important material. This includes a summary of the facts known for doped crystals, a description of the experimental methods relevant to undoped crystals, a summary of the main experimental data for this material, especially the data on the intensity dependences, presentation of the methods of purification from unwanted photo-active electrons, and a discussion of possible mechanisms responsible for the charge-transport peculiarities. [ABSTRACT FROM AUTHOR]

Heating and UV illumination reduce the coercive field of lithium niobate crystals. It is found that both effects add up, leading to the conclusion that the underlying mechanisms are different. We present arguments that heating leads to an increased ionic conductivity that changes defect structures which pin domain walls, while UV illumination enables electrical screening of the pinning defects by photo-excited electrons. [ABSTRACT FROM AUTHOR]

Light absorption measurements of nominally undoped congruent lithium niobate crystals (CLN) as well as 5 mol% magnesium-oxide-doped lithium niobate (MgO:LN) crystals were performed in the light wavelength range of 350 to 800 nm. Absorption spectra reveal that—besides iron (Fe) impurities—chromium (Cr) impurities of less than 0.5 wt. ppm concentration contribute significantly to the total optical absorption in the CLN crystals with a maximum of 0.035 cm−1 around 500 nm. The axial distribution of Cr within a CLN boule is examined, revealing that the bottom part of the boule contains less Cr and therefore light absorption is reduced as well. In the case of the MgO:LN crystals, Cr impurities also contribute significantly to the total optical absorption, which is on the order of 0.025 cm−1 for ordinarily polarized light and 0.015 cm−1 for extraordinarily polarized light around 500 nm. [ABSTRACT FROM AUTHOR]

Using plasmonic resonances of metal films, enhanced transmission of light through sub-wavelength holes has been demonstrated. Here we show that external resonances can be employed as well: the transmission of 1.5-μm wavelength light through 600-nm holes is enhanced by a factor of 20 using a Fabry–Pérot arrangement. The maximal enhancement factor is determined by the limited reflectivity of metal surfaces. It seems promising to combine both effects—plasmonic resonances plus tailored photonic-crystal structures on top of the metal film—in order to realize efficient sub-wavelength light sources as they are required for, e.g., advanced spectroscopy and lithography. [ABSTRACT FROM AUTHOR]

The effective nonlinear coefficient deff of lithium niobate is determined to be 94 pm/V for a process that converts infrared light to 1.35 THz radiation. This value is inferred from the performance of a continuous-wave, singly-resonant optical parametric oscillator, in which the cavity-enhanced signal wave of a primary parametric process acts as a pump wave for a cascaded process, generating terahertz waves. To quantify the nonlinear coefficient, the coupled wave equations including absorption are evaluated. Furthermore, from our measurements we also determine the temperature dependence of the refractive index to be d nTHz/d T=0.0013/K around 1.4 THz. [ABSTRACT FROM AUTHOR]

Time-resolved light-induced birefringence measurements based on a phase compensation technique are used to determine the charge transport properties of congruent, nominally-undoped lithium niobate crystals (LiNbO3). Some of the crystals are conventionally oxidized. The steady-state space-charge field Epv, the bulk photovoltaic coefficient β, and the photoconductivity σph are determined in the intensity range (30–30 000) W/cm2. The photovoltaic coefficient β increases by one order of magnitude over this intensity range, the space-charge field Epv even by two orders of magnitude. We discuss the results in the context of the known one- and two-center charge transport models for LiNbO3. The experimental findings presented here are of relevance for the long-standing problem of optical damage in such crystals, which inhibits their use in high-intensity applications like nonlinear optics. [ABSTRACT FROM AUTHOR]

Lithium niobate (LiNbO3), also called the ‘silicon of photonics’, is indispensable in advanced photonics and nonlinear optics. For many applications, however, the material is too polluted by transition metals, which are unavoidable at the parts per million level. These impurities serve as sources and traps for photoelectrons, causing optical damage and hampering the usability of LiNbO3. Efforts have therefore been made to get rid of the photoexcitable electrons. Here we introduce a method termed ‘optical cleaning’. We show theoretically and experimentally that, if the material is heated to moderate temperatures, allowing ions to migrate and to maintain charge neutrality, an appropriately moving light beam pushes photoexcitable electrons out of the illuminated region like a brush, and provides exponential cleaning. This promises purification levels that are beyond the reach of current technologies. [ABSTRACT FROM AUTHOR]

We demonstrate that for a given pump source, there is an optimum pump threshold to achieve the maximum single-frequency output power in singly resonant optical parametric oscillators. Therefore, cavity losses and parametric amplification have to be adjusted. In particular, continuous-wave output powers of 1.5 W were achieved with a 2.5 cm lithium niobate crystal in comparison with 0.5 W by a 5 cm long crystal within the same cavity design. This counter-intuitive result of weaker amplification leading to larger powers can be explained using a model from L.B. Kreuzer (Proc. Joint Conf. Lasers and Opt.-Elect., p. 52, ). Kreuzer also states that single-mode operation is possible only up to pump powers which are 4.6 times the threshold value. Additionally, implementing an outcoupling mirror to increase losses, single-frequency waves with powers of 3 W at 3.2 µm and 7 W at 1.5 µm could be generated simultaneously. [ABSTRACT FROM AUTHOR]

A new class of photo-refractive (PR) composite materials based on poly( N-vinylcarbazole) (PVK) (co-)sensitized with nano-crystalline lithium niobate (LiNbO3) is presented. The steady-state performance, as well as the kinetics of the photo-refractive grating formation, was investigated by degenerate four-wave mixing (DFWM) and two-beam-coupling (TBC) experiments. We found an optimum content of LiNbO3 nano-particles of only 10−3 wt.%. Even at concentrations as low as 10−7 wt.% a notable effect of the nano-crystals was detected. This yields materials with improved PR performance compared to that of the standard PR polymer material sensitized by TNF only. The role of LiNbO3 nano-particles is discussed in detail: The particles support generation and transport of the free-charge carriers. Furthermore, they increase the electron trap density. [ABSTRACT FROM AUTHOR]

Holographic phase conjugation is analyzed as a method to create a photo-refractive lens with high numerical aperture. For this purpose a sub-wavelength hole is drilled into a metal surface directly on top of an iron-doped lithium niobate crystal. An interference pattern generated by the light coming from this point source and a plane reference wave is recorded. By using the phase-conjugated reference wave for read-out, a light wave being focused onto the former point source is reconstructed. In principle, a focusing system close to the theoretical diffraction limit could be implemented by this method. The performance of this arrangement is mainly determined by properties of the lithium niobate crystal, especially the crystal symmetry. Experimentally, the tight holographic focusing is demonstrated. The focus width of the reconstructed wave is shown to be below 1.2 μm, which is our spatial resolution. The diffraction efficiency obtained, however, is just 3×10−5 compared to 3×10−2 in the plane-wave case. This can be explained by experimental reasons, the inhomogeneous light intensity and limitations originating from the crystal symmetry. We estimate that the diffraction efficiency for phase conjugation through a sub-wavelength hole can be improved by three to four orders of magnitude by addressing the above-mentioned issues. [ABSTRACT FROM AUTHOR]

Photorefractive index gratings are written into iron-doped lithium niobate crystals using femtosecond laser pulses and cw light, both having a wavelength of 532 nm. The saturation value of the refractive index changes in as-grown samples is found to decrease with increasing peak pulse intensity. Furthermore, in oxidized crystals, writing with femtosecond pulses is much faster than with cw light and retains about the same writing speed than in as-grown crystals. We propose a charge transport model that addresses the special case of recording with high intensity femtosecond pulses. [ABSTRACT FROM AUTHOR]

The mode-hop behavior and the power characteristics of a laser diode with wavelength-selective optical feedback are experimentally investigated. The feedback is provided by external volume holographic gratings, also called ‘Bragg mirrors’, at normal incidence. We demonstrate that a Bragg mirror forces a laser diode to operate only within a narrow wavelength range, and that the emission wavelength of the laser diode is stabilized against variations of the injection current. Moreover, we present periodic undulations of the power characteristics of the laser, depending on the driving current. They can be qualitatively explained with a simple model which takes into account that the threshold gain in the laser system strongly depends on the wavelength. [ABSTRACT FROM AUTHOR]

Transmission gratings are recorded with nanosecond light pulses in highly Fe-doped (0.5 to 3 wt%) as-grown and oxidized lithium niobate crystals. In as-grown samples, the diffraction efficiency decays exponentially after recording, and an exponential increase of the decay rate with decreasing mean distance between the Fe centers takes place. Such a behavior is typical for tunneling of electrons between Fe centers. A different and highly peculiar dark evolution of the gratings, which includes a strong transient increase of the diffraction efficiency during about 100 s after illumination, is observed in strongly oxidized crystals. This behavior is explained within a simple relaxation model which accounts for the pyroelectric effect. The long-time decay of the diffraction efficiency again follows an exponential law. In strongly oxidized crystals, the decay rate shows, however, a surprising decrease with decreasing mean distance between the Fe centers. This peculiarity is shown to be consistent with the present notion of hopping charge transport in highly doped strongly compensated semiconductors. [ABSTRACT FROM AUTHOR]

We report on the periodic polarization of α-phase proton- exchanged LiNbO3 planar waveguides down to 5 μm period lengths using structured electrodes and electrical fields. Unlike a majority of previous work, room-temperature electric field poling has been carried out after waveguide fabrication. Chemical etching of the large guiding surface has revealed homogeneous and high-quality domains with duty cycles close to 0.5. Moreover, it was verified by diffraction techniques and etching that the domain structure is preserved more than 2 μm along the thickness of the waveguide. The excellent optical performances of single-domain α-phase waveguides were not affected by the periodic poling. [ABSTRACT FROM AUTHOR]

The photorefractive properties of highly iron-doped lithium niobate crystals in as-grown and completely oxidized states are investigated. Iron concentrations range from 0.5 to 3 wt. %. Measurements of light-induced refractive-index changes, two-beam coupling gain, bulk-photovoltaic current densities, and conductivities are performed with visible light. Absorption spectra and the bulk photovoltaic currents are used to determine the Fe2+ concentration. The dark conductivity confirms the tunneling model for charge-carrier migration. Beam-coupling experiments as well as photocurrent measurements with highly oxidized samples indicate that the one-center model is valid in such highly doped crystals, but that hole conductivity dominates the charge transport. [ABSTRACT FROM AUTHOR]

Light pulses of 240 fs duration and 388-nm wavelength generate in lithium niobate crystals transient absorption. The absorption changes are observed with delayed ultra-short pulses (wavelength 776 nm) and with continuous-wave light (wavelength 785 nm) over 10 decades from 400 fs up to 10 s after the pump pulse. The decay is well described by one stretched-exponential function over the entire temporal range. We attribute the absorption changes to small bound polarons because of the temporal, spectral, and polarization fingerprints. The polarons are formed in less than 400 fs after the pump illumination, ruling out that phonons with wavenumbers below 100 cm-1 are the main cause for polaron generation and indicating that the breathing mode of the oxygen octahedra is of relevance. An upper limit of the generated polaron concentration is estimated as ≈4.4×1018 cm-3 with an intensity of ≈220 GW/cm2. [ABSTRACT FROM AUTHOR]

We report on a metal-polymer compound material with optical properties that can be reversibly switched all-optically. The key element is a metallic photonic crystal slab with an additional layer of photoaddressable material that provides a large variable birefringence and sharp resonances. Pump-probe experiments show a shift of the photonic crystal resonances that depends on the pump polarization and on the exposure. Comparison of the results with calculations from a scattering-matrix theory allows one to determine the refractive index changes for different polarization geometries and to model our compound material quantitatively. [ABSTRACT FROM AUTHOR]

Irradiation of lithium-niobate crystals (LiNbO3) with fast, high-energy 3He ions changes the refractive index in the interaction region where the ions speed through the material. Thus an inhomogeneous flux density profile can be used for a tailored modification of the optical properties of LiNbO3 crystals, without employing ion implantation. A new method to fabricate embedded, polarization sensitive channel waveguides in LiNbO3 utilizing accelerated 3He ions with an energy of 40 MeV is demonstrated. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Spectral investigation of europium-doped flourophosphate glass shows photosensitivity in the range of 244 nm to 351 nm. Holographic investigations of the dependence of the achievable refractive-index changes indicate a strong impact of the doping concentration. The refractive-index changes increase with europium concentrations up to 1.0×1020 Eu/cm3; for higher doping levels the effect becomes smaller. Based on these results, refractive-index changes in excess of 10-3 for optimized material and holographic recording are predicted. About 23% of the initially recorded holographic grating decay in a double-exponential way with time constants of 7 h and 240 h, but the other 77% of the primary refractive-index changes are persistent. The spatial resolution of this material is better than 1 µm. [ABSTRACT FROM AUTHOR]

A new deflection phenomenon of light has been discovered in ferroelectric crystals that provides information about domain structures, their nucleation, and their dynamics: Illumination of a multi-domain crystal with a single light beam yields light patterns, if external electrical fields are applied. Deflection angles of up to ±8° are observed. We attribute this effect to light deflection from the boundaries of the domains. Besides providing a new method to study domains of any ferroelectric material, the method can be of practical importance for electro-optic beam scanning. [ABSTRACT FROM AUTHOR]

Examines the feedback operation in the case of reflection geometry, specially for iron-doped lithium niobate crystals. Formation of the feedback equations; Numerical analysis of the operation regimes for lithium niobate; Comparison between theory and experiment.

Focuses on atomic nanofabrication with complex light fields. Method of neutral atom lithography; Modulation of an atomic beam; Laser cooling.

The electrooptic effect in lithium niobate crystals (LiNbO[sub 3] ) for extremely high externally applied electric fields of up to 65 kV/mm is investigated. Homogeneous electrooptic refractive-index changes of up to 4.8×10[sup -3] are found for ordinarily polarized light. No quadratic electrooptic effect is observed. An upper bound for the quadratic electrooptic coefficient of |s[sub 13] |≤2.3×10[sup -21] m[sup 2] /V[sup 2] is determined. Electrooptic, angular, and wavelength tuning of the Bragg condition of a thermally fixed hologram are demonstrated. [ABSTRACT FROM AUTHOR]

Ferroelectric domain reversal via electric field poling of congruently melting lithium niobate (LiNbO[sub 3] ) crystals is investigated. An electro-optic interferometric observation technique reveals spatial and temporal dynamics of the poling process. Starting from seeds, the domains grow until the entire crystal has a switched polarization. During the switching process the boundaries are preferentially aligned along the crystallographic axes. The coercive field between two sequenced domain inversions is transiently reduced after a poling event, and recovers exponentially with a time constant of about half a minute. No light-induced change of the recovery time constant, neither with green nor with ultraviolet light, is observed. The results are of relevance for domain engineering of LiNbO[sub 3] crystals. [ABSTRACT FROM AUTHOR]

The impact of protons and copper on the dark conductivity of copper-doped photorefractive lithium niobate crystals is investigated by observation of the dark decay of holograms. The dark conductivity is significantly smaller than that of iron-doped lithium niobate. A non-mono-exponential hologram decay is observed, which needs to be considered for multiplexing of holograms. [ABSTRACT FROM AUTHOR]

Abstract. We investigate the role of carrier mobility in holographic recording in LiNbO[sub 3] crystals. Both normal holographic recording (single wavelength, single trap) and two-center recording are considered, and the differences between the performances of the two methods are explained. We show that increasing mobility by using stoichiometric crystals or by doping with Mg does not improve sensitivity considerably, but does reduce M/# by at least one order of magnitude. [ABSTRACT FROM AUTHOR]

Abstract. X-Ray-induced absorption changes in Cu-doped lithium niobate crystals (LiNbO[sub 3]) are investigated. All induced photochromic effects are fully reversible, i.e., illumination with white light annihilates the absorption changes. With increasing doping level, a decrease in the saturation value of the absorption changes is found. Changes of the Li concentration of the crystal to a more stoichiometric composition via Li indiffusion further decreases the photochromic effect. For sufficient X-ray doses, the saturation value is independent of the radiation energy in the measured range from 50-150kV. The spectral distribution of the X-ray-induced absorption changes and the slow dark decay indicate that the additional absorption results from an increase of the Cu[sup +] concentration. [ABSTRACT FROM AUTHOR]

Abstract. Holographic gratings are recorded and thermally fixed in iron-doped photorefractive lithium-niobate crystals. A spatially modulated concentration of filled and empty electron traps (Fe[sup 2+] and Fe[sup 3+]) yields a modulated dark conductivity. As a consequence, dark diffusion currents and dark drift currents arise. Space-charge fields and electro-optic refractive-index changes build up. An additional refractive-index grating, which may originate from a modulated proton concentration, as well as a pronounced absorption grating arising from the modulated Fe[sup 2+] concentration are also observed. The dark development has practical advantages: thermally fixed holograms can be used in devices without the need to develop them freshly from time to time by illumination. Although with dark development diffraction efficiencies up to 50% are demonstrated, in general the efficiencies are smaller compared to those achieved by development with light. [ABSTRACT FROM AUTHOR]

Abstract. Nominally pure and rhodium-doped barium-calcium-titanate crystals of the congruently melting composition Ba[sub 0.77]Ca[sub 0.23]TiO[sub 3] (BCT) are grown by the Czochralski technique. These crystals are characterized by holographic and conventional electrical experiments. The doped samples have an absorption maximum at 630 nm. They are hole conductive, and for intensities between 0.1 and 1 W cm[sup -2] all properties are explained within a one-center charge-transport model. Two-beam-coupling gain factors up to 8 cm[sup -1] are obtained, and doping with rhodium increases the effective trap density. Although diffusion is the dominant charge-driving force, bulk photovoltaic currents also occur which are especially pronounced in the range between 460 and 640 nm. [ABSTRACT FROM AUTHOR]

Abstract. Light-induced refractive-index changes, bulkphotovoltaic current densities, and photoconductivities of photorefractive iron-doped lithium niobate crystals (iron concentrations between 0.02 and 0.17 wt. %) are investigated in detail using visible and near-infrared light. It turns out that the one-center model predicts the material performance correctly for small iron concentrations (C[sub Fe]< 0.06 wt. % Fe[sub 2]O[sub 3]), only. A strong increase of the photoconductivity for higher doping levels limits the space-charge fields. Refractive-index changes up to 7 x 10[sup -4] for green and 2.8 x 10[sup -4] for nearinfrared ordinarily polarized light are obtained. The corresponding hologram multiplexing numbers are 11 for green and 5 for near-infrared light. [ABSTRACT FROM AUTHOR]

Reports on how photorefractive materials are being widely investigated for applications in holographic data storage. How the process of homogeneous illumination results in light diffraction and reconstructs the information encoded in the original interference pattern; Problems with this method; Use of lithium niobate crystals in an effort to solve the problems.

Abstract. The concept is discussed of wavelength demultiplexing with volume-phase holograms that are superimposed in a photorefractive lithium niobate crystal, and first experimental results are presented. From theoretical considerations, it turns out that in the telecommunication wavelength region around 1550 nm, simultaneous and polarization-independent demultiplexing of 40 channels can be achieved with losses of less than 1 dB, a channel spacing of 0.8 nm and a cross-talk of less than -35 dB. Superposition of many holograms with high diffraction efficiency requires thermal fixing. Proof-of-concept experiments at room temperature are performed where holographic gratings are written by plane waves of green light. Rotation stages enable precise adjustment of orientation and spatial frequency of the gratings. Demultiplexing of a beam with light of the wavelengths 1558.0 and 155 8.8 nm is demonstrated, where the losses are about 15 dB and cross-talk suppression up to 20 dB is achieved. PACS: 42.40; 42.70; 72.40 [ABSTRACT FROM AUTHOR]

The charge transport processes in different photorefractive oxide crystals are reviewed and consequences for applications are outlined. – The light-induced charge transport processes in LiNbO[sub 3] , LiTaO[sub 3] , BaTiO[sub 3] , Ba[sub 1-x] Sr[sub x] TiO[sub 3] ( [ABSTRACT FROM AUTHOR]

Presents the light-induced charge transport processes in photorefractive barium titanate doped with rhodium and iron. Measurement of absorption, light-induced absorption changes and photo and dark conductivity of barium titanate; Result of the oxidized sample; Identification of the impurities and valence states involved.

Abstract. Spatially uniform heating or cooling of a photorefractive strontium-barium niobate crystal yields large pyroelectric fields. Two-beam coupling measurements and observation of beam intensity profiles behind the crystal reveal that drift of charge carriers in the pyroelectric field strongly enhances the photorefractive effect. Illumination by a continuous wave laser beam causes large pyroelectric fields via absorption and heating. The drift in pyroelectric fields must be taken into account as a transport mechanism if pyroelectric crystals are used. This mechanism is promising for novel photorefractive devices. PACS: 42.65.Hw; 77.70.+a; 78.20.Jq [ABSTRACT FROM AUTHOR]

Iron–doped photorefractive barium titanate crystals (BaTiO[sub 3] ) are annealed in a hydrogen atmosphere at various temperatures. After these reducing treatments, absorption, light–induced absorption changes, two–beam coupling direction, photo electron paramagnetic resonance (photo EPR), dark and photoconductivity as well as bulk photovoltaic current density are investigated. The samples are electron conductive and the charge transport is governed by only one level, which is identified by photo EPR as Fe[sup 2+] /Fe[sup 3+] . The photoconductivity exceeds the dark conductivity for intensities above 1 kWm[sup -2] . A relation between the absorption constant and the Fe[sup 2+] concentration is derived. From the known charge transport parameters the advantageous photorefractive properties of optimized reduced BaTiO[sub 3] :Fe are deduced; possible response times in the millisecond range at an intensity of 10 kWm[sup -2] are estimated. [ABSTRACT FROM AUTHOR]

A new charge-transport model assuming one center in three different valence states is discussed for explanation of the photorefractive effect. Quantitative description of experimental results in KNb0:Fe by the model is demonstrated. Many similarities with the so-called 'two-center' model are found although the microscopic explanation of the light-induced charge transport is rather different. [ABSTRACT FROM AUTHOR]

Holographic gratings are recorded in LiNbO: Fe using frequency-doubled pulses of a Q-switched Nd : YAG laser (light wavelength λ = 532 nm, pulse duration t = 20ns). We monitor diffraction efficiencies of the holograms during and several milliseconds after exposure. Variations of Fe concentration, light intensity, and fringe spacing show that different types of gratings are involved: photochromic, thermooptic, pyroelectric, and space-charge gratings. The influence of the internal pyroelectric field on the light-induced charge transport is discussed in terms of a two-center model. Pyroelectric and bulk photovoltaic effect are shown to partially compensate each other. In LiNbO: Fe, the photovoltaic effect is the dominant charge-driving force for our recording conditions. [ABSTRACT FROM AUTHOR]

Phase gratings are recorded holographically with nanosecond light pulses by the interference of two plane waves in colored liquid samples using the thermooptic effect. The samples consist of toluene and distilled water which are immiscible and have different thermal conductivities. The exponential decay of the gratings caused by the thermal conductivity is monitored by diffraction of continuous wave light, and the thermal diffusivity is determined. Scanning of the read-out beam provides spatial resolution. [ABSTRACT FROM AUTHOR]

The dc conductivity of as-grown and oxidized iron-doped lithium niobate crystals is measured by impedance spectroscopy in the temperature range 30–700 °C. Electron and ion conduction at lower and higher temperatures are clearly distinguished, providing relevant information for understanding and optimizing the recently developed thermo-electric oxidization that makes crystals robust against optical damage. [ABSTRACT FROM AUTHOR]

Stable suspensions of iron- or copper-doped lithium niobate nanocrystals in heptane are prepared by high-energy ball milling of iron- and copper-doped bulk crystals for 25 h. The distribution of particle sizes is determined by means of dynamic light scattering with more than 90% of the crystals in the range of 10–25 nm. The optical absorption of the suspensions suggests a changed environment of the dopants. [ABSTRACT FROM AUTHOR]

A new method is presented, allowing the nearly complete oxidization of lithium niobate crystals (LiNbO3), doped with large amounts of iron oxide (0.05–3 wt. % Fe2O3) utilizing annealing at 700 °C in the presence of externally applied electric fields. The treatment results in a concentration ratio of Fe2+ and Fe3+ ions of less than 2×10-3. Strong oxidization of iron in LiNbO3 reduces the photorefractive effect and is therefore of particular interest for nonlinear-optical applications. [ABSTRACT FROM AUTHOR]