Influence of silicon particle morphology on laser-induced plasma properties

Abstract

In this work, Si particles of varying sizes and a bulk Si wafer are exposed to high-energy nanosecond-pulsed laser excitation. The subsequent laser-induced plasma is characterized using a suite of diagnostic techniques, including spatiotemporal species imaging via high-speed videography and high-resolution gated spectroscopy. The intensity and location of various species in the plasma (e.g., Si I and Si II) are tracked as a function of time over two time regimes, 2–10 μs and 20–80 μs, and show differences in impurities and peak intensities between the various particle sizes and compared to the Si wafer. Electron densities under these conditions decrease from ≈1.8 to 0.8 × 1017 cm−3 from 2 to 8 μs, while plasma excitation temperatures determined from the Boltzmann method range from 9200 to 8000 K over the same time window. The powdered Si samples have characteristically larger and more intense plasmas than the wafer sample. The laser-induced air-shock from energetic materials (LASEM) method was employed on these materials, in concert with their acoustic response, to establish their relative microsecond energy releases. While the powders have similar laser-induced shock velocities, they appear to have differences in their acoustic response which roughly scales as a function of plasma carbon content. The proposed reasons for these shock and acoustic trends are discussed, which include particle size and morphology, surface impurities, and dispersion and flow within the plasma.