When a powerful femtosecond laser pulse propagates in an optical medium, self-focusing occurs. Normally, it is the most powerful part (slice) of the pulse that self-focuses first during its propagation. Self-focusing is balanced by the creation of plasma in the self-focal volume, which defocuses the pulse. This balance leads to a limitation of the peak intensity (intensity clamping). The series of self-foci from different slices of the front part of the pulse give rise to the perception of a so-called filament. The back part of the pulse undergoes self-phase modulation and self-steepening resulting in a strong spectral broadening. The final pulse is a white-light laser pulse (supercontinuum). The physics of such (long distance) filamentation and the self-transformation process are reviewed both in air and in condensed matters. The self-transformation leads to a shorter pulse and is currently being studied for efficient pulse compression to the single and (or) few-cycle level. The efficient generation of a third harmonic in the filament is due to a new phenomenon called self-phase locking. The potential applications in atmospheric sensing and lightning control will be briefly discussed. The capability of melting glass leading to index change will be underlined. The paper will end with an outlook into the future of the field. PACS Nos.: 42.65, 42.65Jx, 42.25, 42.79Qx

Our experiment shows that external focusing strongly influences the plasma density and the diameter of femtosecond Ti-sapphire laser filaments generated in air. The control of plasma filament parameters is suitable for many applications such as remote spectroscopy, laser induced electrical discharge, and femtosecond laser material interactions. The measurements of the filament showed the plasma density increases from 10(15)cm(-3) to 2 x 10(18)cm(-3) when the focal length decreases from 380 cm to 10 cm while the diameter of the plasma column varies from 30 microm to 90 microm. The experimental results are in good qualitative agreement with the results of numerical simulations.

Tunable and stable ultrashort laser pulses in the visible spectrum are generated with high efficiency by four-wave mixing process during the filamentation of near-infrared and infrared laser pulses in gases. It is shown that these tunable ultrashort pulses have a very low energy fluctuation and an excellent mode quality due to the processes of intensity clamping and self-filtering in the filament.

We demonstrate for the first time the possibility to generate long plasma channels up to a distance of 1 km, using the terawatt femtosecond T&T laser facility. The plasma density was optimized by adjusting the chirp, the focusing and beam diameter. The interaction of filaments with transparent and opaque targets was studied.