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.