ludwigite crystals in the temperature range from 15 to 300 K. At room temperature ludwigite has an orthorhombic structure belonging to space group D 9 2h . Attempts made to record polarized Raman spectra were successful, and we were able to identify most of the phonons belonging to A 1g species. A careful analysis of the spectra shows that the observed Raman modes do not exhibit variations either in wavenumber or intensity or both with temperature, indicating that the D 9 2h phase does not undergo any structural modification in the temperature range investigated. Also, we observed for Fe 3 O 2 BO 3 ludwigite the appearance of a continuous scattering background in the phonon spectrum for temperatures below 200 K . This background, which is due to electronic transitions between Fe 2+ and Fe 3+ ions, is related to the charge delocalization effect.
The transient grating technique was used to study the optical properties of hydrogenated amorphous silicon films. A carrier lifetime τe=3.3 μs, a diffusion coefficient D=4.4×10−2 cm2/s, and effective third order nonlinear susceptibility χ(3)=(5.1±1.0)×10−7 esu were measured for λpump=532 nm. The electronic and thermal contributions of the grating were estimated by diffracting a continuous wave HeNe laser beam. For intensities 1.4–4.1 mW/cm2, ∼68% of the response at 633 nm is of electronic origin. A slowly rising diffraction component was also observed.
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