Multidiagnostic Characterization of Ultrashort and Short Pulse Laser Produced Plasma from Aluminium and Tungsten Targets

Abstract

The field of laser produced plasmas (LPP) has greatly attracted the research community because of its wide range of applications such as pulsed laser deposition, generation of light sources and ion beams, plasma-based acceleration etc. The present thesis reports space and time-resolved comparative studies of ultrashort and short laser pulse produced plasmas from Aluminium (Al) and Tungsten (W) targets at different ambient pressures. The thesis starts with a brief introduction to LPPs and their fundamental properties. Experimental techniques used and instruments employed are described in the second chapter. The chapters that follow discuss results obtained from optical time of flight (OTOF), optical emission spectroscopy (OES), time-resolved ICCD imaging and ion dynamics studies of plasmas generated in the targets, irradiated by ultrashort (100 femtoseconds) and short (7 nanoseconds) laser pulses, in air ambient with pressures ranging from 10−5 to 760 Torr. Electron temperature and number density of the plasma have been calculated from optical emission spectra. The intensities of the plasma plumes, as well as the corresponding neutrals to ions ratios, are found to be different between ultrashort laser ablation (ULA) and short laser ablation (SLA) because of the relative differences in laser-matter and laser-plasma interactions. OTOF measurements, time-resolved ICCD imaging and Ion emission measurements reveal the presence of both fast moving as well as slow-moving species in SLA, while this distinction is not so obvious in ULA. Linear, shock wave and drag models are used to model plume and ion dynamics in the low, intermediate and high-pressure regions respectively. In addition, the expansion dynamics of the ULA aluminum plasma is investigated as a function of the laser beam size on the target, using a combination of the above-mentioned diagnostic tools. Optical emission spectroscopic analysis shows that higher emission intensities and ion populations are obtained for smaller beam sizes. Time-resolved ICCD imaging of the expanding plasma shows a spherical morphology for plumes produced by smaller beam sizes, and a cylindrical morphology for those produced by larger beam sizes. A comprehensive comparison of X-ray emission from ULA and SLA plasmas also has been carried out for Al and W targets. The use of longer wavelengths with longer pulse durations leads to lower critical density LPP, which results in efficient laser-plasma heating. Enhanced X-ray emission can hence be achieved from laser-irradiated Ag and Au nanoparticle colloidal suspensions. A summary of the results and discussion, their significance and future scope are given in the final chapter.