M. Imlau

Lifetime of small polarons in iron-doped lithium–niobate crystals

Pulsed illumination of lithium–niobate crystals with green light excites electrons from deep traps into the intrinsic defect NbLi5+ (Nb on Li site in the valence state 5+) and creates NbLi4+ centers (small polarons). The electrons trapped in this more shallow center increase the light absorption in the red and near infrared. The dark decay of the polaron concentration is observed by monitoring the relaxation of these absorption changes. Iron-doped lithium–niobate crystals with different concentrations of NbLi are investigated for various illumination conditions and temperatures. The relaxation shows a stretched-exponential behavior which is in disagreement with the predictions of the standard rate-equation-based model. The observed lifetimes of the polarons range from tens of nanoseconds to some milliseconds. Computer simulations reveal that all results can be explained considering distance-dependent excitation and recombination rates, i.e., the lifetime of an individual polaron depends on the distance to...

The modulated structure of Ba0.39Sr0.61Nb2O6. I. Harmonic solution

The structure of a crystal of Sr(0.61)Ba(0.39)Nb(2)O(6) has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P4bm(pp1/2,p - p1/2), unit-cell parameters a = 12.4566 (9), c = 7.8698 (6) A, modulation vectors q(1) = 0.3075 (6) (a* + b*), q(2) = 0.3075 (6) (a* - b*). The data collection was performed on a KUMA-CCD diffractometer and allowed the integration of weak first-order satellite reflections. The structure was refined from 2569 reflections to a final value of R = 0.0479. The modulation affects mainly the positions of the O atoms, which are displaced by as much as 0.5 A, and the site 4c that is occupied by Sr and Ba atoms. Only a simplified model, in which this atomic position is occupied by an effective atom Sr/Ba, could be refined from the data set. The modulation of displacement parameters has been used to account for the modulated distribution of Sr and Ba. The whole refinement uses only first-order modulation waves, but there are strong indications that for a complete solution the use of higher-order satellites and a more complicated model is necessary.

Anisotropic holographic scattering in centrosymmetric sodium nitroprusside

Anisotropic holographic light scattering has been discovered in centrosymmetric single crystals of sodiumnitroprusside, Na2[Fe(CN)5NO]⋅2H2O, in which the photorefractive response is based on the excitation of metastable electronic states. Besides a bright homogeneous scattering corona generated by isotropic scattering, a sharp scattering ring builds up with light polarized orthogonal to that of the pump beam and the corona. This anisotropic scattering resulting from interference of scattered waves with the pump beam occurs although there is no linear electro-optic effect in centrosymmetric crystals.

Optical nonlinearities of small polarons in lithium niobate

An overview of optical nonlinearities of small bound polarons is given, which can occur in the congruently melting composition of LiNbO3. Such polarons decisively influence the linear and nonlinear optical performance of this material that is important for the field of optics and photonics. On the basis of an elementary phenomenological approach, the localization of carriers in a periodic lattice with intrinsic defects is introduced. It is applied to describe the binding energies of four electron and hole small polarons in LiNbO3: small free NbNb4+ polarons, small bound NbLi4+ polarons, small bound NbLi4+:NbNb4+ bipolarons, and small bound O− hole polarons. For the understanding of their linear interaction with light, an optically induced transfer between nearest-neighboring polaronic sites is assumed. It reveals spectrally well separated optical absorption features in the visible and near-infrared spectral range, their small polaron peak energies and lineshapes. Nonlinear interaction of light is assigned...

Temperature dependent determination of the linear electrooptic coefficient r33 in Sr0.61Ba0.39Nb2O6 single crystals by means of light-induced scattering

Abstract The temperature dependence of the light-induced polarization–isotropic scattering in strontium–barium–niobate (SBN) doped with 0.66 mol% cerium is studied in a temperature range covering the ferroelectric phase and the relaxor phase transition. We introduce a method to determine the electrooptic coefficient r 33 and the effective density of the photorefractive centers N eff from the angular distribution of the scattered light. The temperature dependence of these two parameters is discussed with respect to the phase-transition behavior.

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 thanΔn = 1 × 10−3 in the red (632.8 nm) andΔn = 5 × 10−4 in the near-infrared region (1047 nm). The absorption coefficient is modulated by aboutΔα = 100 m−1 at 632.8 nm and 40 m−1 at 1047 nm. The wavelength dependence ofΔn 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 efficiencyη 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.

Transition Metal Compounds Towards Holography

We have successfully proposed the application of transition metal compounds in holographic recording media. Such compounds feature an ultra-fast light-induced linkage isomerization of the transition-metal–ligand bond with switching times in the sub-picosecond regime and lifetimes from microseconds up to hours at room temperature. This article highlights the photofunctionality of two of the most promising transition metal compounds and the photophysical mechanisms that are underlying the hologram recording. We present the latest progress with respect to the key measures of holographic media assembled from transition metal compounds, the molecular embedding in a dielectric matrix and their impressive potential for modern holographic applications.

Local electric-field-driven repoling reflected in the ferroelectric polarization of Ce-doped Sr0.61Ba0.39Nb2O6

We present pyroelectric measurements with the relaxor-ferroelectric strontium barium niobate in the phase transition regime. It is demonstrated that domains poled at high temperatures are more stable than those oriented at room temperature. This yields a higher phase-transition temperature when heating the sample and a strong repoling during the cooling process. We explain this difference within the framework of the random field model for relaxor ferroelectrics.

Pronounced photosensitivity of molecular [Ru(bpy) 2 (OSO)] + solutions based on two photoinduced linkage isomers

Photosensitive properties of [Ru(bpy)(2)(OSO)] PF(6) dissolved in propylene carbonate originating from photoinduced linkage isomerism have been studied by temperature and exposure dependent transmission and UV/Vis absorption spectroscopy. An exceeding photochromic photosensitivity of S = (63 +/- 10) x 10(3) cm l J(-1) mol(-1) is determined. It is attributed to a maximum population of 100% molecules in the photoinduced isomers, a unique absorption cross section per molecule and a very low light exposure of Q(0) = (0.25 +/- 0.03) Ws cm(-2) for isomerism. Relaxation studies of O-bonded to S-bonded isomers at different temperatures verify the existence of two distinct structures of linkage isomers determined by the activation energies of E(A,1) = (0.76 +/- 0.08) eV and E(A,2) = (1.00 +/- 0.14) eV.

Beam fanning reversal in the ferroelectic relaxor Sr0.61Ba0.39Nb2O6 at high external electric fields

Beam fanning has been studied in the cerium-doped ferroelectric relaxor strontium–barium–niobate (Sr0.61Ba0.39Nb2O6:Ce, SBN:Ce) under the application of high external electric fields. Spatial reversal in the distribution of the scattered light is achieved both below and above the phase transition. It is shown that the photorefractive response in SBN strongly depends on the state of the polar structure of the crystal, which can be controlled by external fields. Coherent illumination of the crystal greatly facilitates the repoling process, causes a considerable refinement of the domain order in the ferroelectric phase and assists the stabilization of the polar structure in the relaxor regime. The model of scattering centers associated with refractive index anomalies located on domain walls is applied to receive detailed information about the distribution of initial seed scattering in the crystal bulk and the efficiency of nonlinear amplification of the scattering.