Critical density effects in femtosecond ablation plasmas and consequences for high intensity pulsed laser deposition

Laser ablation plumes produced by a single pulse from an ultrafast laser consist, in the far field where film deposition occurs, of mostly neutral atoms, a percentage of ionized species, and, very often, condensed clusters. In certain situations adding energy to the plume may be of interest for a deposition, and methods for increasing the charged fraction need to be considered. This paper examines these issues and demonstrates a method for overcoming the plasma critical density limitations encountered for absorption of a single pulse. Precisely controlled time-delayed secondary pulses are used to change the average charge state, temperature, and plasma density of the far field plume, with implications for thin film deposition and nano-cluster formation. A plasma-jet nozzle effect is proposed to explain condensed cluster formation of germanium. Results are also presented in relation to enhanced isotope enrichment for boron. (C) 2002 Elsevier Science B.V. All rights reserved.