Polarisation resolved spectroscopy of laser produced plasmas

The degree of polarisation of the light emitted from laser produced plasmas has been studied using time-resolved imaging along with time- and polarisation-resolved spectroscopy in the visible spectral range. A Q-switched Nd: YAG laser (wavelength = 1064 nm, pulse width = 14 ns) was used to ablate the target. Optical spectra were obtained using a Czerny-Turner spectrometer equipped with an intensified charge-coupled device (ICCD) readout. A Wollaston prism split the emitted radiation into orthogonally polarised states for simultaneous spectral acquisition. Polarisation anisotropy in line emission is the result of a non-statistical distribution of population amongst the available magnetic sublevels in the upper state of the transition of interest. The origin of the non-statistical distribution lies in an anisotropy in the electron velocity distribution function (EVDF) which is the most often described by a bi-Maxwellian function. For the continuum emission, it is found that the main contributor is recombination radiation and once again an EVDF anisotropy is the origin. In this case the partially preferred direction of motion of the recombining electrons is transferred into a partially preferred polarisation direction. The imaging results reveal stronger plasma emission polarisation in air than in vacuo. They also showed stronger polarisation of the continuum emission compared to line emission at higher pressure for Al (at 396.15 nm) and Al+ (466.3 nm) features. In vacuo, the degree of polarisation (P) of all charge states studied oscillates with time. It is suggested that this oscillation is a result of self-generated magnetic fields which cause the plasma itself to undergo compression and rarefraction cycles. In this case the EVDF will also oscillate and hence the degree of polarisation will do so. The magnetic field itself is thought to arise from a Rayleigh-Taylor (RT) instability. The effects of target geometry and incident laser polarisation on the emission were also studied. A greater effect was observed for continuum radiation compared to line emission. It is also observed that the laser polarisation has the greatest effect for oblique incidence on the target