Abstract

Recently, it has been reported that intense femtosecond pulses can self-channel through air over distances of tens of meters.¹ High- intensity ultrashort pulses induce the nonlinear Kerr effect in air, leading to self-focusing and thereby to an increase of the pulse intensity in a confined region. Eventually, the peak intensity exceeds the ionization threshold of air, and the generation of free electrons then produces a negative contribution to the refractive index. Long- range channeling (or filamentation) is a consequence of the balancing of two effects: self-focusing due to the Kerr effect and defocusing due to the presence of the free electrons. The propagation process of the filaments can thus be regarded as selfguiding. This can be contrasted with the situation in which a laser beam can undergo channeling in a preformed plasma.²

© 1995 Optical Society of America