Optical emission and dynamics of aluminum plasmas produced by ultrashort and short laser pulses

Abstract

Optical emission and dynamics of ionized and neutral species present in plasmas generated by irradiating an aluminum target with ultrashort (100 fs) and short (7 ns) laser pulses is investigated, in the background pressure range of 10−2 to 760 Torr. Emission spectra exhibit lines from neutrals (Al I) as well as singly and doubly ionized ions (Al II, Al III). Maximum emission is seen in the pressure range of 10 to 20 Torr, around a distance of 1 mm from the target surface. While thermal ablation and laser–plasma interaction are prevalent in short pulse laser ablation (SLA), Coulomb explosion is prominent in ultrashort pulse laser ablation (ULA) because of the high optical intensity of the fs pulse. Ion population is an order of magnitude higher for SLA compared to ULA. Time of flight (TOF) studies reveal both fast and slow moving species in SLA, while this distinction is not quite obvious in ULA. Fast neutrals observed in SLA are formed by the recombination of fast ions with electrons. Space-charge effect in the expanding plasma and the associated Coulomb force results in the acceleration of charged particles, with ions reaching peak velocities up to 30 km s−1 in SLA. Velocities decrease at higher background pressures because of plume confinement. TOF data of Al neutrals fitted to shifted Maxwell–Boltzmann distributions indicate a general thermal nature, while the fast species in SLA appear to show a non-thermal behavior. Time-resolved ICCD images of the plasma depict a relatively rapid initial expansion for the SLA plume. These investigations provide valuable insights into the composition and expansion of Al plasmas produced by ultrashort and short laser pulses respectively, which might be of importance for a number of applications including EUV and X-ray generation, pulsed laser deposition, cluster production, and nanoparticle formation and growth.