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Laser Filamentation
The phenomenon of filamentation is a vibrant topic where pulses of picosecond and femtosecond duration are employed. When focusing an intense laser in gas, liquids or solids two main effects take place. Due to the high intensity the medium does not react in a linear fashion anymore. The radial intensity distribution causes the medium to change the reflractive lolally. This effect is called self-focusing and is governed by the Kerr resonse (Kerr effect) of the medium. The beam collapses on itself and focuses stronger and stronger. At a certain limit the medium is ionised and leads to plasma formation. The free electrons change the refractive index and cause the beam to defocus. These two effects eqalise each other and form the so called filament. This balance is broken when the laser power is lower than the critical power for self-focusing. Since high intense lasers were up to recently only available at 800 nm (Titanium:Sapphiere) it was extensively studied at this wavelength .