Enhanced shock wave detection sensitivity for laser-produced plasmas in low pressure ambient gases using interferometry

We report results from the analysis of the shock wave formed following the creation of a laser-produced plasma in a gaseous atmosphere, using both interferometry and shadowgraphy. A Nomarski polarization interferometer and a focused-type shadowgraphy setup were utilized to track the evolution of the shock wave with high spatial and temporal resolution for a variety of incident laser energies and ambient gas pressures. It was found that the visibility of the shock wave was high for both techniques at high gas pressure (>= 100 mbar) and incident laser energies (>= 400 mJ). The velocity of the shock wave in these regimes was of the order of several km s(-1). At lower pressures (approximate to 1-10 mbar) and incident laser energies (approximate to 100-200 mJ), the visibility of the shock wave decreased dramatically and, in some cases, disappeared completely from the shadowgrams. In contrast, the shock wave remained visible in the interferograms, manifesting itself as a blurring of the fringes. The shock wave visibility was improved further by simply differentiating the interferograms to enhance the fringe boundaries. Shock velocities, exceeding 100 km s(-1), were detected at low background gas pressures where the enhanced shock wave visibility was provided by the interferometer.