We propose and demonstrate a pulse compression technique using a hollow fiber with a pressure gradient. This technique improves the spatial and spectral qualities of femtosecond laser pulses spectrally broadened by self-phase modulation, and allows an increase of the pulse energy for pulse compression. Using chirped mirrors for dispersion compensation, we have successfully compressed the pulse to less than 10 fs with an energy as high as 5 mJ.

Infrared and far infrared transmission spectra have been measured for the ferromagnetic Ga1-x MnxAs (x=0.034 and x=0.050) between 10 and 12000 cm-1. A broad peak is observed around 1600 cm-1, and this energy is close to the deep acceptor level of Mn impurity in GaAs. A finite featureless absorption, spreading between this peak and the band gap, due to inter valence band transitions is also observed. At the same time, a non-zero absorption is observed in the lowest frequency studied, which points to the absorption by mobile holes related to the metallic DC conductivity. The frequency dependence in this range, which differs markedly from the Drude model, demonstrates a strong influence of electrostatic and magnetic disorder on the optical conductivity. In particular, the existence of a strong coupling between the Mn spins and the holes explains the considerable increase of the far infrared absorption below the ferromagnetic ordering temperature.

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The propagation dynamics of intense femtosecond laser pulses in argon have been investigated theoretically and the results are compared with experimental data. It was found that in the initial stage the pulse propagates with the focal point moving ahead of the original one. The central beam of the trailing part experiences defocusing owing to ionization by the leading part and then regains self-focusing provided by power from the outer part. On propagating further, a quasistable balance is established between self-focusing and defocusing due to ionization-induced nonlinearity and diffraction, causing the beam to propagate in a self-guided mode. Furthermore, it was shown that the front of the split pulse decays faster, while the trailing edge experiences self-focusing and self-defocusing until a self-guided propagation mode is achieved. Multiple pulse splitting and shortening as a result of the dynamics near the focal point were also observed.

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We prove that the probability of finding a scattered quantum-mechanical particle at large times in a truncated cone is identical with the scattered flux, integrated over time, across a distant spherical surface subtending this cone. The theory applies to potentials with arbitrary local singularities and decaying faster than r −2/3 at large distances.