Counter-propagating entangled photons from a waveguide with periodic nonlinearity

The conditions required for spontaneous parametric down-conversion in a waveguide with periodic nonlinearity in the presence of an unguided pump field are established. Control of the periodic nonlinearity and the physical properties of the waveguide permits the quasi-phase matching equations that describe counter-propagating guided signal and idler beams to be satisfied. We compare the tuning curves and spectral properties of such counter-propagating beams to those for co-propagating beams under typical experimental conditions. We find that the counter-propagating beams exhibit narrow bandwidth permitting the generation of quantum states that possess discrete-frequency entanglement. Such states may be useful for experiments in quantum optics and technologies that benefit from frequency entanglement.Comment: submitted to Phys. Rev.

Femtosecond laser crystallization of boron-doped amorphous hydrogenated silicon films

Crystallization of amorphous hydrogenated silicon films with femtosecond laser pulses is one of the promising ways to produce nanocrystalline silicon for photovoltaics. The structure of laser treated films is the most important factor determining materials' electric and photoelectric properties. In this work we investigated the effect of femtosecond laser irradiation of boron doped amorphous hydrogenated silicon films with different fluences on crystalline volume fraction and electrical properties of this material. A sharp increase of conductivity and essential decrease of activation energy of conductivity temperature dependences accompany the crystallization process. The results obtained are explained by increase of boron doping efficiency in crystalline phase of modified silicon film

Post-hydrogenation of amorphous hydrogenated silicon films modified by femtosecond laser irradiation

Crystallization of amorphous hydrogenated silicon thin films with femtosecond laser pulses is a currently developable technique for nanocrystalline silicon production for optoelectronics applications. The significant drawback of this technology is the hydrogen losses upon laser treatment of the film, while certain hydrogen concentration is essential to obtain high-quality material. Therefore we aimed to study the effect of post-hydrogenation of laser-modified amorphous silicon films on their hydrogen content and photoelectric properties. Using laser pulses of different fluence we obtained two-phase films with different crystalline volume fraction up to 60%. Post-hydrogenation procedure was found to partially compensate hydrogen out-diffusion and remarkably increase photoconductivity of highly crystallized films. At the same time the contribution of nanocrystalline phase to the total films' photoconductivity substantially increases. The results points out the effectiveness of applied hydrogenation procedure for a production of laser crystallized amorphous silicon films with suitable properties for optoelectronics

Amorphous hydrogenated silicon modified by femtosecond laser radiation for photovoltaics

Thin film technology based on hydrogenated amorphous silicon (a-Si:H) has been playing a significant role in thin-film photovoltaics. However, a-Si:H based solar cells suffer from a low carrier mobility and light-induced degradation. In order to diminish these effects, tandem solar cells based on amorphous and nanocrystalline silicon (nc-Si) are developed. One of the technologically attractive methods for incorporating nc-Si into a-Si:H is laser induced crystallization.Most of the studies on laser crystallization of a-Si:H employ picosecond and nanosecond laser pulses with radiation photon energies higher than the band gap in a-Si:H of 1.6-1.8 eV. In the last few years, femtosecond laser driven crystallization of a-Si:H films attracts considerable interest. Intense femtosecond laser pulses can alter the structure of the film at photon energies smaller than the band gap. Numerous studies on the structural transformation of a-Si:H induced by femtosecond laser pulses have been reported. However there is a lack of data on the electric and photoelectric properties which are vital for the application of femtosecond laser introduced structures in solar cells.Here we present the results of optical, electric and photoelectric properties of a-Si:H films treated by femtosecond laser radiation with different wavelengths and laser fluencies. Specifically the correlation between changes of these properties and the film structure is shown and the problem of hydrogen out-diffusion is discussed

Effect of laser wavelength on structure and photoelectric properties of a-Si:H films crystallized by femtosecond laser pulses

Femtosecond laser processing of hydrogenated amorphous silicon is a perspective method for thin film solar cells production. It allows to make local crystallization and surface texturing of the films which results in the enhancement of their light absorption and stability of parameters. Thickness of modified material depends strongly on a laser wavelength. However laser wavelength affects also other properties of the film. Therefore here we study structure, surface morphology and photoelectric properties of hydrogenated amorphous silicon films treated by femtosecond laser pulses of different photon energies, namely above, around and below the mobility gap of amorphous silicon

Effect of the femtosecond laser treatment of hydrogenated amorphous silicon films on their structural, optical, and photoelectric properties

The effect of the femtosecond laser treatment of hydrogenated amorphous silicon (α-Si:H) films on their structural, optical, and photoelectric properties is studied. Under the experimental conditions applied in the study, laser treatment of the film with different radiation intensities induces structural changes that are nonuniform over the film surface. An increase in the radiation intensity yields an increase in the contribution of the nanocrystalline phase to the structure, averaged over the sample surface, as well as an increase in the conductance and photoconductance of the samples. At the same time, for all of the samples, the absorption spectrum obtained by the constant-photocurrent method has a shape typical for those of amorphous silicon. Obtained results indicate the possibility of α-Si:H films photoconductance increase by femtosecond pulse laser treatment

The Holocene environmental changes revealed from the sediments of the Yarkov sub-basin of Lake Chany, south-western Siberia

Lake Chany is the largest endorheic lake in Siberia whose catchment is entirely on the territory of Russia. Its geographical location on the climate-sensitive boundary of wet and dry landscapes provides an opportunity to gain more knowledge about environmental changes in the West Siberian interior during the Holocene and about the evolution of the lake itself. Sediment cores obtained from the Yarkov sub-basin of the lake in 2008 have been comprehensively studied by a number of approaches including sedimentology and AMS dating, pollen, diatom and chironomid analyses (with statistical interpretation of the results), mineralogy of authigenic minerals and geochemistry of plant lipids (biomarker analysis.). Synthesis of new results presented here and published data provides a good justification for our hypothesis that Lake Chany is very young, no older than 3.6 ka BP. Before that, between 9 and 3.6 ka BP, the Chany basin was a swampy landscape with a very low sedimentation rate; it could not be identified as a water body. In the early lake phase, between 3.6 and 1.5 ka BP, the lake was shallow, 1.2–3.5 m in depth, and it rose to its modern size, up to 6.5 m in depth, during the last millennium. Our data reveal important changes in the understanding of the history of this large endorheic lake, as before it was envisioned as a large lake with significant changes in water level since ca. 14 ka BP. In addition to hydrology, our proxies provide updates and details of the regional vegetation and climate change since ca. 4 ka BP in the West-Siberian forest-steppe and steppe. As evolution of the Chany basin is dependent on hydroclimatic changes in a large region of southern West Siberia, we compare lake-level change and climate-change proxies from the other recently and most comprehensively studied lakes of the region