title: Plasma density inside a femtosecond laser filament in air: strong dependence on external focusing.
abstract: Our experiment shows that external focusing strongly influences the plasma density and the diameter of femtosecond Ti-sapphire laser filaments generated in air. The control of plasma filament parameters is suitable for many applications such as remote spectroscopy, laser induced electrical discharge, and femtosecond laser material interactions. The measurements of the filament showed the plasma density increases from 10(15)cm(-3) to 2 x 10(18)cm(-3) when the focal length decreases from 380 cm to 10 cm while the diameter of the plasma column varies from 30 microm to 90 microm. The experimental results are in good qualitative agreement with the results of numerical simulations.
authors.affiliation: Centre d'Optique, Photonique et Laser and Département de Physique, de Génie Physique et d'Optique, Université Laval, Québec, Canada, G1K 7P4.
authors.forename: Francis
authors.initials: F
authors.lastname: Théberge
authors.forename: Weiwei
authors.initials: W
authors.lastname: Liu
authors.forename: Patrick Tr
authors.initials: PT
authors.lastname: Simard
authors.forename: Andreas
authors.initials: A
authors.lastname: Becker
authors.forename: See Leang
authors.initials: SL
authors.lastname: Chin