Femtosecond laser pulse filamentation versus optical breakdown in H<Subscript>2</Subscript>O

The competition between femtosecond laser pulse induced optical breakdown and femtosecond laser pulse filamentation in condensed matter is studied both experimentally and numerically using water as an example. The coexistence of filamentation and breakdown is observed under tight focusing conditions. The development of the filamentation process from the creation of a single filament to the formation of many filaments at higher pulse energy is characterized systematically. In addition, strong deflection and modulation of the supercontinuum is observed. They manifest themselves at the beginning of the filamentation process, near the highly disordered plasma created by optical breakdown at the geometrical focus. Received: 9 July 2002 / Revised version: 15 November 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/6562-623, E-mail: wliu@phy.ulaval.ca     

Investigation of the transformation of the spectrum of femtosecond laser radiation on filamentation in gas medium

The dependence of the parameters of femtosecond laser radiation (55 fs, 3.7 mJ, 800 nm) in a filament on the distance along the filament propagation axis, the diaphragm diameter, and the type and pressure of gas used for filamentation has been studied experimentally.     

Optimization of a femtosecond pulse self-compression region along a filament in air

We have identified the pulse self-compression region in a filament produced by 55 fs, 4 mJ, 805 nm radiation propagating in air without geometrical focusing. In our experiment the pulse self-compression region is attained by the propagation distance, where the shortest wavelength in the supercontinuum blue wing reaches a minimum, and the growing conversion efficiency to white light has a large gradient. Numerical tracking of the pulse along the filament shows a single-peak 9 fs pulse with a flat spectral phase at the optimum compression distance. PACS 42.65.Jx; 42.65.Re; 42.65.Ky     

Effect of beam diameter on the propagation of intense femtosecond laser pulses

This paper describes the effects observed during the propagation of intense femtosecond laser pulses in air following the modification of the laser beam diameter with a pair of convex–concave lenses placed/mounted in a telescopic configuration. We observed that by reducing the diameter of the beam the detected back-scattered nitrogen fluorescence from the filaments becomes more stable on a shot-to-shot basis while, for a larger beam size, greater fluctuations are observed that are not correlated to shot-to-shot fluctuations in the laser pulse energy. This result leads to a new method to control the fluorescence signal which can be very important in remote-sensing applications. PACS 42.65.Jx; 42.68.Ay; 42.68.Wt     

Bis-arylsulfonamide Derivatives of 2-Amino-5(6)-(4-aminophenylthio)benzimidazole

Three different procedures have been developed for the preparation of bis-arylsulfonamide derivatives of 2-amino-5(6)-(4-aminophenylthio)benzimidazole, and biological activity of the products have been studied.     

Modern state of underground storage of liquid radioactive waste

Underground storage of liquid radioactive waste (LRW) has been successfully carried out in Russia over the past 40 years. Stages of development of the technology, its realization and chemical transformations of LRW components are considered and discussed, as well as the modern approach to underground storage. Control of the state of waste products and migration radionuclides and toxic compounds remains the key issue of providing safety of LRW storage. Radiation thermal and biochemical transformations of components of waste form new chemical compounds and, first of all, gases. The formation of gases is one of the important factors determining the safety of underground LRW storage.     

Structural reasons for the perpendicular anisotropy in ferrospinel single-crystal films

Results of investigations into the perpendicular anisotropy of ferrospinel single crystal films are analyzed. The exchange interaction constants and the contribution of the exchange energy, energy of crystallographic anisotropy, and perpendicular anisotropy to the domain wall energy density are estimated. It is demonstrated that the parameters characterizing the perpendicular anisotropy depend on the chemical composition, degree of structure imperfection, and technological conditions of synthesis. The conclusion is drawn that the perpendicular anisotropy is caused by the magnetic constants of the material, macro- and microstresses, and anisotropy of the crystal shapes.