Abstract

Recent investigations of the propagation of high- power femtosecond infrared (IR) laser pulses in air show that the dynamical interaction between nonlinear self-focusing and plasma defocusing causes an initial beam to breakup spatially into several filaments, or light strings, which maintain themselves over long distances via a turbulent optical light guide. It has been observed experimentally that femtosecond light strings in turn produce plasma channels by multi-photon ionization (MPI) along their direction of propagation with lengths ranging from tens of centimeters to several meters, and diameters around a hundred microns,^1,2 For high-intensity (e.g. 10¹⁴ Wlcm²) femtosecond 1R laser input pulses, MPI is the dominant mechanism of plasma creation. If these channels attain a finite dipole moment then this could explain the recently observed electromagnetic pulse (EMP) from light string induced plasmas.¹ The lifetime of the observed plasma channels is around a nanosecond in keeping with the duration of the observed EMP. One anticipates that the central frequency of the EMP will be related to the plasma frequency of the channel, but the frequency has not been determined experimentally as of yet.

© 2001 Optical Society of America