Fundamentals and applications of plasma filaments

When sufficiently powered femtosecond laser pulses are launched into the atmosphere, white light emitting plasma filaments or even bundles of those are generated along the beam. Under well chosen pulse conditions these bundles may extend even over kilometre lengths. Their formation is based on a fascinating interplay of non-linear optical processes like Kerr lensing, plasma defocusing and self phase modulation. Filaments exhibit along their trajectory extraordinary properties, from which fascinating applications emerge. They emit, for example, directional white light in a wide spectral range from the IR to the UV, which can efficiently (> 60%) be extracted to produce ultra-short pulses (< 5fs). Further they allow the remote and simultaneous analysis of a rich variety of gaseous atmospheric constituents (fs-LIDAR), and when they hit solid or liquid targets, they emit intensive characteristic plasma light, which allows the remote identification of soil, vegetation, waters, etc. (F-LIBS). Until energy levels of 5mJ single filaments are formed. Their diameter is about 100 µ and their length reaches up to 100 metres. So at sufficient repetition rates (> 100 kHz) they cut materials or tissue at meters distance, without focussing the beam and without melting or burning the material. Most interesting properties result from the plasma character of such filament bundles in air. Amazingly they can even be heard by ear, which provides a good and simple plasma monitor. More important, however, is the phenomenon, that the air along filament bundles becomes electrically conductive. The effect allows not only to guide and control electric discharges and currents, it provides a realistic chance to control lightings. With the advent of non-metallic airplanes this aspect has become most important for air traffic safety, namely for situations, when airplanes are obliged to land across thunderclouds. We have successfully demonstrated an influence of fs-laser induced filaments on a lightning. We are, however, still far off from reliably controlling them. So, in the future we wish to explore all critical parameters and develop concepts, how the requirements be met. Another still fully unexplored, but quite relevant effect resulting from the plasma character of filaments is the formation of fog traces and droplets along their path in humid air. The basic effect is quite similar to the formation of charge traces in a Wilson-type fog chamber; but there are also striking differences: As our recent laboratory and field measurements clearly showed, laser-induced water condensation and droplet formation was not only observed in super saturated air but even when the air humidity was the below its saturation threshold. The effect is very promising and can well become a new and important tool in atmospheric research, namely the better understanding and possibly even local control of cloud formation. First, however, the effect must fully be characterized and understood.