Effect of reflections on phase hologram recording in amorphous As-S-Se films

Experimental and theoretical studies of phase hologram recording in amorphous As-S-Se films with slightly wedge-shaped thickness profile are presented. It is shown that Fabry-Perot resonator effect caused by the interference of multiply reflected light beams inside the sample strongly changes the values of diffraction efficiency and its growth rate as well as the exposure time dependences of diffraction efficiency and transmissivity making them site-dependent and sample-dependent. Absorptivity and recording light intensity inside the sample are also significantly changed. The obtained results can be used to explain the holographic and optical experiments also in other materials with Fabry-Perot resonator effect such as other transparent thin films and photorefractive crystals.     

Degradation of polyolefines during various recovery processes

The purpose of the presented research was the investigation of the stability and differences of degradation of polyolefines during various recycling processes. In modeling the recycling process during melting, extrusion with a one-screw extruder was used. Recycling through selective dissolution was modulated by two different solvents (xylene and a definite mixture of n-alkanes). Materials used for the investigations were polypropylene (PP), low-density polyethylene (LDPE) and high-density polyethylene (HDPE) (Ziegeler-Natta technology with vanadium catalyst). Changes in the chemical structure of polymers were measured with infrared spectroscopy and differential scanning calorimetry (DSC). Flow properties were characterized by melt flow index, and mechanical characteristics by tension. Experimental results show that for PP and HDPE, utilizing all investigated recycling technologies, chain scission prevailed over branching. For the LDPE chain branching was obtained. By the same token, differences in crystallinity (and as follows, in molecular mass) between the same materials, recycled by extrusion and selective dissolution, was obtained. During selective dissolution changes of properties and morphology in dependence of the solvent used were observed with the trend being that the amount of the admixture of n-alkane used in this investigation was more considerable with regard to the amount of material destruction as compared to xylene. Any reduction of the mechanical properties of any of the investigated polymers as a result of the various methods used was comparable.     

Modified hydrotalcites application as precursors for (Na,K)Mg/Al spinel-type compounds formation

In this work, a synthesis route of (Na,K)Mg/Al spinel-type compounds, which combines hydrothermal synthesis at low temperatures (<200 °C) and solid-state sintering (>800 °C) methods, is presented. It was examined that NaOH and KOH additives induce the reaction between initial Mg and Al components and the formation of hydrotalcite during hydrothermal treatment. It should be noted that after 1 h of calcination of synthetic precursors at 850 °C spinel-type compounds are formed only in the samples with alkali addition. Meanwhile in the pure system only traces of the mentioned compounds are observed at 900 °C. Moreover, the increase in solid-state sintering temperature and duration lead to the higher-crystallinity (Na,K)MgAl₂O₄ spinel-type compounds. It should be noted that textural properties of formed (Na,K)Mg/Al spinel-type compounds depend on the chemical composition of precursors. The synthetic and calcined products are characterised by XRD, STA, FT-IR analyses and BET method.     

Investigations of As-S-Se thin films for use as inorganic photoresist for digital image-matrix holography

As-S-Se chalcogenide thin films are successfully employed in classical and dot-matrix holography as inorganic photoresists for obtaining a relief-phase hologram. However using these films for image-matrix hologram recording has not been studied due to some features of image-matrix technology. For the applied research of the optical properties of As-S-Se films an experimental device of digital image-matrix holographic recording based on 100 mW 405 nm semi-conductor laser and Spatial Light Modulator (SLM) has been created. The device has the following main parameters: 140 × 105 μm frame size; laser intensity during exposure 10 W/cm². With the help of this device diffraction grating and security holograms were recorded on As-S-Se thin films. The work reported herein presents results of an experimental study of how diffraction efficiency (DE) of the received relief-phase holographic gratings depends on an exposure and period. Diffraction grating profiles and speed of etching corresponding to different exposure doses are shown. Hologram samples with DE = 65% have been received which allows for using chalcogenide film as alternative to organic photoresists in applied dot-matrix and image-matrix holography.     

A contribution to the sintering model of viscoelastic liquid drops with solid kernels

A model for filled polymer sintering has been proposed. In this model the sintering process is reduced to coalescence of two viscoelastic drops with solid kernels. The mathematical calculations are based on the Ostwald empirical relation and the previous models for sintering of pure polymers. The results show that in the first approximation for calculations of filled polymer sintering we can use the same methods as for unfilled ones.Institute of Polymer Mechanics, Latvian Academy of Sciences, Riga, LV-1006 Latvia. Published in Mekhanika Kompozitnykh Materialov, Vol. 33, No. 4, pp. 564–569, July–August, 1997.     

Second harmonic generation in selenium-metal structures

Results of second harmonic (SH) generation in amorphous and crystalline selenium films and selenium-metal (Ga, Zn, In, Sb, Bi) sandwich structures induced by titan-sapphire femtosecond laser (wavelength λ – 690–1040 nm) are presented. It is found that the highest intensity of SH is provided by fundamental wave at wavelength 1000 nm and it reaches maximum in approximately 100 s. The intensity of transmitted SH depends on film thickness while that of reflected does not. The highest SH intensities in selenium-metal structures are provided by Se–Ga and Se–Zn films. A possible mechanism for central symmetry breaking in amorphous films also is proposed.