Laser-induced cavities with a controllable shape in nanoporous glass.

The formation of birefringent structures inside nanoporous glass by femtosecond laser pulses was investigated. The laser-modified region is shown to be a cavity whose shape depends on the number of pulses. The shape of the void cross section varied from circle to ellipse when increasing the number of pulses from one to three. A layer of non-porous dense glass was revealed around the cavity. The cross section of this layer is nearly circular, regardless of the cavity shape and number of pulses in the investigated range. The mechanism of elongated cavity formation based on aniostropic light scattering on the spherical cavity is proposed.

3-bit writing of information in nanoporous glass by a single sub-microsecond burst of femtosecond pulses.

We have found that a single sub-microsecond burst of femtosecond laser pulses produces a sub-micrometer cavity possessing the homogeneous birefringence with slow-axis orientation perpendicular to polarization of the laser beam in high-silicate nanoporous glass. Retardance and the root mean square of slow-axis orientation are investigated in dependence on the energy and the number of pulses in the burst. A burst of just three pulses with 98 ns inter-pulse intervals has been shown to induce homogeneous birefringence, and a burst of four pulses has provided birefringence with retardance of 35 nm, which is sufficient for reliable readout of the information recorded with multilevel encoding in slow-axis orientation. A text file has been recorded and read out in an array of birefringent cavities, each carrying 3 bits of information. The sub-microsecond burst of femtosecond pulses paves the way for a multiple increase of the rate of digital information recording with multilevel encoding in glass.

High temperature magnetism and microstructure of ferromagnetic alloy Si1-x Mn x.

The results of a detailed study of magnetic properties and of the microstructure of SiMn films with a small deviation from stoichiometry are presented. The aim was to reveal the origin of the high temperature ferromagnetic ordering in such compounds. Unlike SiMn single crystals with the Curie temperature ~30 K, considered Si1-x Mn x compounds with x = 0.5 +Δx and Δx in the range of 0.01-0.02 demonstrate a ferromagnetic state above room temperature. Such a ferromagnetic state can be explained by the existence of highly defective B20 SiMn nanocrystallites. These defects are Si vacancies, which are suggested to possess magnetic moments. The nanocrystallites interact with each other through paramagnons (magnetic fluctuations) inside a weakly magnetic manganese silicide matrix giving rise to a long range ferromagnetic percolation cluster. The studied structures with a higher value of Δx ≈ 0.05 contained three different magnetic phases: (a)-the low temperature ferromagnetic phase related to SiMn; (b)-the above mentioned high temperature phase with Curie temperature in the range of 200-300 K depending on the growth history and