Laser fluence dependence on emission dynamics of ultrafast laser induced copper plasma

The characteristic emission features of a laser-produced plasma depend strongly on the laser fluence. We investigated the spatial and temporal dynamics of neutrals and ions in a femtosecond laser (800 nm, ~40 fs, Ti:Sapphire) induced copper plasma in vacuum using both optical emission spectroscopy (OES) and spectrally resolved two-dimensional (2D) imaging over a wide fluence range of 0.5-77.5 J/cm2. 2D fast gated monochromatic images showed a distinct plume splitting between the neutrals and ions, especially at moderate to higher fluence. OES studies at low to moderate laser fluence confirm intense neutral line emission over ion emission, whereas this trend changes at higher laser fluence with dominance of the latter. This evidences a clear change in the physical processes involved in the femtosecond laser-matter interaction at high input laser intensity. The obtained ion dynamics resulting from OES and spectrally resolved 2D imaging are compared with charged particle measurement employing Faraday cup and Langmuir probe; results showed good correlation