Theoretical and experimental studies on UV filaments

Summary form only given. High power laser pulses are known to self-focus during their propagation through air due to the nonlinear index of refraction. At the critical power the self-focusing overcomes diffraction and leads to a collapse of the pulse. In the case of 248.6 nm pulses, the self-focusing is entirely balanced by the creation of electrons due to multiphoton ionization which is a purely intensity dependent mechanism. Our experimental studies with 1 ps, 200 /spl mu/J filaments also reveal that about 40 /spl mu/J of energy are consumed per meter of propagation. Since this is a substantial portion of the filament energy, the filament breaks up after about 1 m. Considering that the balance between self-focusing and plasma defocusing is strictly intensity dependent, one should be able to trap more energy into a single filament by increasing its pulsewidth. The upper pulsewidth limit is set by the onset of inverse bremsstrahlung and becomes significant at about 60 ns. This shows the need for a theoretical investigation of nanosecond UV filaments. Our theory assumes a steady state condition, corresponding to a balance between photoionization and recombination.