Dynamics of femto- and nanosecond laser ablation plumes investigated using optical emission spectroscopy

Abstract

We investigated the spatial and temporal evolution of temperature and electron density associated with femto- and nanosecond laser-produced plasmas (LPP) from brass under similar laser fluence conditions. For producing plasmas, brass targets were ablated in vacuum employing pulses either from a Ti:Sapphire ultrafast laser (40 fs, 800 nm) or from a Nd:YAG laser (6 ns, 1064 nm). Optical emission spectroscopy is used to infer the density and temperature of the plasmas. The electron density (ne) was estimated using Stark broadened profiles of isolated lines while the excitation temperature (Texc) was estimated using the Boltzmann plot method. At similar fluence levels, continuum and ion emission are dominant in ns LPP at early times (<50 ns) followed by atomic emission, while the fs LPP provided an atomic plume throughout its visible emission lifetime. Though both ns and fs laser-plasmas showed similar temperatures (∼1 eV), the fs LPP is found to be significantly denser at shorter distances from the target surface as well as at early phases of its evolution compared to ns LPP. Moreover, the spatial extension of the plume emission in the visible region along the target normal is larger for fs LPP in comparison with ns LPP.

Emission features and expansion dynamics of nanosecond laser ablation plumes at different ambient pressures

“…The distinct features of ns laser ablation spectra compared to ultrafast laser ablation plume spectra are the presence of ionic lines along with intense continuum at early times of plume generation (<100 ns). 34 In ns LPP, the leading edge of the laser pulse ablate the target and produce a vapor plume which contains electrons, ions, and excited neutrals, but the rest of the laser pulse is then utilized for heating and further ionizing of the plume leading to intense continuum generation at the earliest times. Along with continuum, the emission intensity from the all lines is increased significantly at atmospheric pressure compared with expansion into vacuum environment as shown in Fig.…”

“…When a nanosecond laser with high pulse energy interacts with a target, the entire laser pulse energy is not directly coupled with the target surface because of shielding effect. The effective coupling of the laser-target and laser-plasma depends strongly on the properties of incident laser (pulse duration, wavelength) 34,40 and also has a strong dependence on the ambient conditions (nature and pressure). The initially ejected material has low degree of ionization and continues to absorb energy (by the inverse Bremsstrahlung (IB) or multiphoton ionization) from the remaining laser pulse, resulting into shielding of the sample surface from the laser pulse.…”

Persistence of uranium emission in laser-produced plasmas

“…This rapid decrease in line emission intensity is due to plume confinement and reduction in plasma temperature with space. 22 The plasma/gas interface is also observed around 8 mm in the signal to background ratio for the U I line, where the signal intensity completely ceases. The S/B ratio is approximately constant from the target surface to 4 mm from the target and peaks between 4 and 5 mm from the target.…”

Nanosecond Laser Ablation: Mathematical Models, Computational Algorithms, Modeling

“…Despite extensive studies of fundamental properties of laser ablation performed earlier, a number of important physical phenomena still remain insufficiently well studied and understood. Previous studies have established that the nature of interaction of laser radiation depends both on the modes of action: wavelength [16,17], duration [18,19], and laser pulse intensity [20,21] and on thermophysical and optical properties of the target [22,23], presence of the surrounding gas [24] and its pressure [25,26]. The greatest differences in the physical mechanisms of laser ablation of metals are observed between short (ns) and ultrashort (ps, fs) pulsed modes [18,19].…”