We demonstrate holographic fixing by means of a thermal cycling procedure in pure and doped photorefractive KNbO3 crystals. At elevated temperatures (70–100°C) the photoinduced grating is compensated for by a secondary grating that is formed by charge carriers whose thermal activation energy is ~1.0 eV. The secondary carriers are stable at room temperature and form the fixed grating after cooling the crystal. They are believed to be represented by ions or ion vacancies. By using our fixing mechanisms the holographic storage time in KNbO3 can be increased by a factor of 106.

Thermal hologram fixing in pure and doped KNbO 3 crystals

Hybrid lead halides CH3NH3PbX3 (X = I, Br, and Cl) have emerged as a new class of semiconductors for low-cost optoelectronic devices with superior performance. Since their perovskite crystal structure may have lattice instabilities against polar distortions, they are also being considered as potential photo-ferroelectrics. However, so far, research on their ferroelectricity has yielded inconclusive results and the subject is far from being settled. Here, we investigate, using a combined experimental and theoretical approach, the possible presence of electric polarization in tetragonal and orthorhombic CH3NH3PbBr3 (T-MAPB and O-MAPB). We found that T-MAPB does not sustain spontaneous polarization but, under an external electric field, it is projected into a metastable, ionic space-charge electret state. The electret can be frozen on cooling, producing a large and long-lasting polarization in O-MAPB. Molecular dynamics simulations show that the ferroelastic domain boundaries are able to trap charges and segregate ionic point defects, thus playing a favorable role in the stabilization of the electret. At lower temperatures, the lack of ferroelectric behavior is explained using first principles calculations as the result of the tight competition among many metastable states with randomly oriented polarization; this large configurational entropy does not allow a single polar state to dominate at any significant temperature range.

Long-lived electrets and lack of ferroelectricity in methylammonium lead bromide CH3NH3PbBr3 ferroelastic single crystals.

Changes, provoked by poling in the corona discharge and isothermal annealing, in the elemental composition of the surface layer and in the electret properties of PbMg13/Nb23/O3 (PMN) single crystals have been studied. Air oxidation has been found to enrich the surface with Mg. This confirms the authors finding that, upon air oxidation, the surface of multicomponent oxide compounds is enriched with the element whose heat of oxide formation is greater relative to one oxygen molecule. Sizeable variations of the PMN surface composition correlate with changes in the electret potential and in its relaxation time. The temperature critical for the changes to occur in elemental composition and electret properties has been found to be 500 degrees C.

Electret state and surface composition of the PbMg13/Nb23/O3 single crystal

The photorefractive effect, an indirect nonlinear optical effect mediated by the material photoconductivity, offers the unique opportunity to obtain high sensitivity light-light interaction already at moderate intensity levels below 1 W/cm2 typical of unfocused continuous wave laser beams. We distinguish two kinds of photorefractive effects. In the conventional one the photoconductivity arises from carriers photoexcited from midgap levels, while in the interband photorefractive effect the carriers are produced by band-to-band transitions through the absorption of higher energy photons. In this work we concentrate on two important aspects of photorefractive nonlinear optics that were not addressed previously. We show that interband photorefractive effects offers new interesting opportunities for optical parallel processing and clarify the role of the various anisotropic material properties on the strength of conventional and interband photorefractive gratings. As compared to the conventional one, the interband photorefractive effect leads to a much faster response speed of the order of few μs at the 1 W/cm2 intensity level and to dynamic holographic gratings which are very robust against simultaneous illumination with light having sub-band gap energy. The usefulness of this effect for optical signal processing and for wave manipulation is shown by demonstrating: (a) a high resolution optically addressed spatial light modulator operating at a refresh cycle of 70 μs based on KNbO3; (b) an optical joint Fourier transform correlator based on Sn2P2S6 which can operate at the record frame rate of 10 kHz; (c) light induced dynamic waveguides in KNbO3 that can be reconfigured in real time. We present here the first study elucidating the role of the complete set of anisotropic material properties on the resulting coupling constant among the optical waves. Anisotropic crystals are in general the materials of choice for obtaining large refractive index changes for both, conventional and interband photorefractive effects. This anisotropy has an enormous influence on the magnitude of the nonlinear interaction. As an important new result, it is shown theoretically and experimentally that an anisotropy of the photoexcitation process with respect to light polarization dramatically influences the two-wave mixing coupling gain landscape in wavevector space. By proper material engineering, this effect can be used for the enhancement of the maximum gain in a given photorefractive medium. Diffraction of light at volume gratings is obviously also influenced by an anisotropy of the medium and/or of the grating itself. The well known isotropic coupled wave theory of Kogelnik (H. Kogelnik, Bell Syst. Tech. J. 48, 2909 (1969).) is extended here to the anisotropic case. This leads to the first complete set of analytic expressions valid in general geometries and that describe mixtures of anisotropic phase and absorption gratings with arbitrary phase relationship. This approach is valid also for non photorefractive media. A predictive understanding of the diffraction properties of emerging holographic materials with strong optical anisotropy can be gained. These include for instance organic crystals, liquid crystals, and polymer dispersed liquid crystals.

Optical wave manipulation and signal processing in anisotropic photorefractive materials

Electret state in PZT-based ceramics

The electret state in a model group of oxides with the perovskite structure has been studied. The magnitude of the electret charge has been found to depend on the concentration of intrinsic point defects in the structure, i.e. of the A- and O-ion vacancies. A correlation has been revealed between the magnitude and stability of the electret charge, on the one hand, and the mutual position of allowed energy bands of the elements contained in the oxides on the other.

Electret state in oxides of the perovskite family

Introduction. Testicular germ cell tumor is a relatively rare disease. Its high social significance is due to the fact that this pathology occurs in young patients. The standard schemes of polychemotherapy determine the potential possibility of effective treatment for most of the patients even with an advanced disease. Several circulating markers (alpha-fetoprotein, human chorionic gonadotropin and lactate dehydrogenase) are being used for therapy monitoring, but the low diagnostic specificity of these molecules determines the need to develop new approaches. Over the past years, circulating microRNA, for instance miR-371a-3p, appeared to be promising marker for testicular germ cell tumor monitoring. However, to develop and to implement in practice the microRNA-based diagnostic technologies, it’s necessarily to understand the features of the microRNA expression alterations specific for different histological types of testicular germ cell tumor.The study objective – to evaluate changes in the expression of several potential marker microRNA molecules (miR-302/ miR-367, miR-371/miR-373) in testicular germ cell tumor samples of various histological types.Materials and methods. Testicular germ cell tumor samples (n = 61), including seminomas, embryonic carcinomas, post-pubertal teratomas, yolk sac tumors, chorioncarcinomas, and corresponding normal tissue samples (n = 61) were included in the study. The analysis of selected miRNA expression was performed by reverse transcription and polymerase chain reaction.Results. We identified the changes in the expression profile of the miR-302/miR-367 cluster typical for semines, embryonic carcinomas, post-pubertal teratomas, yolk sac tumors and chorioncarcinomas, as well as changes in the expression profile of the miR-371/miR-373 cluster, universal for all histotypes except chorioncarcinomas. Inhibition of miR-10b and miR-145 expression in semines, embryonic carcinomas, and post-pubertal teratomas was demonstrated.Conclusion. Activation of miR-302b, miR-302d, miR-371a expression and inhibition of miR-10b, miR-145 expression in the tissue of the most common variants of testicular germ cell tumor is a characteristic feature of these tumors. The detected changes are significant and can lead to corresponding changes in the profile of circulating microRNAs.

Expression of miR-302a, miR-302b, miR-302c, miR-302d, miR-367, miR-371, miR-372, miR-373, miR-10b, miR-21 and miR-93 in cells of different histotypes of testicular germ cell tumors

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Electret state in oxides of the perovskite family

Expression of miR-302a, miR-302b, miR-302c, miR-302d, miR-367, miR-371, miR-372, miR-373, miR-10b, miR-21 and miR-93 in cells of different histotypes of testicular germ cell tumors