Abstract

Abstract
We report, for the first time to our knowledge, the excellent optical performance of long-period gratings written in three standard libers and in two nonstandard fibers, namely, S-doped and N-doped fibers, using the flexible and low-cost electric are technique. The fundamental mechanism for their formation is proposed. The gratings thermal behavior and the effect of fiber preannealing prior to writing have been investigated. We have demonstrated a good stability of these gratings at high temperatures.

Abstract
We propose to use nondegenerate cascaded four-wave mixing processes for efficient high order harmonic generation in plasmas and dielectrics. A simple model is used to estimate the temporal duration and maximum order of the generated harmonics for both a typical laser-produced plasma and a glass slide. In the plasma case, we expect considerable pulse compression, with the production of sub-femtosecond pulses starting from the 8th harmonic.

Abstract
A new method of high-harmonic generation by intense and short laser pulses is proposed. This method is based on the idea that, in quite general situations, a cascade of four-wave-mixing processes can occur. A theoretical model is established and exact analytic solutions are obtained. Examples of possible experiments in low-charged plasmas and in glass are discussed in detail, including the phase-matching conditions, the angular spread of the secondary beams and the pulse durations. Numerical examples of the possible generation of sub-femtosecond pulses are also given.

Abstract

Abstract
X-ray emission spectra in the 5-22 nm range were recorded from planar and structured (meshlike groove surface) gold targets at 45 degrees and 90 degrees to the laser axis. A laser beam of 10-ns duration with E-L less than or equal to 700 mJ and 1.06-mum wavelength was used for the experiment. Experimental results indicate an enhanced X-ray yield from a structured target as compared to a planar target under identical experimental conditions. Increased X-ray emission is attributed to plasma confinement and the possibility of conversion of kinetic energy into localized thermal energy of the plasma. Results are analyzed explicitly on the incident laser energy.