Effects of specular reflectance in laser-induced breakdown of metals

We investigate the effects of specular reflection on the laser-induced breakdown (LIB) of copper, iron, and tungsten using fast photography and optical emission spectroscopy. The laser parameters include spot diameter ranging from 30.89 to 1589.33 μm, irradiance from 467.10 to 0.17 GW/cm2, with a single pulse of 6 ns duration and 21 mJ energy. As the laser spot defocuses, the plasma morphology changes from a single plasma near the target surface to a separated, independently evolving two-component plasma, and then to a single plasma suspended above. The defocusing distance for this transition is significantly influenced by specular reflectance. The separate plasma, comprising of a metallic component and an air component, occurs only under high specular reflectance conditions: ≥66.7% for copper, ≥51.4% for iron, and ≥44.9% for tungsten. The spectral emission of the metallic component initially increases and then decreases with reducing specular reflectance, due to a trade-off between enhanced surface absorption and reduced irradiance caused by surface roughening. LIB threshold irradiance increases with specular reflectance, rising from 0.31 to 1.22 GW/cm2 for copper, 0.24 to 0.70 GW/cm2 for iron, and 0.38 to 0.87 GW/cm2 for tungsten. These findings show the impact of sample pretreatment on LIB ignition and subsequent plasma evolution, offering insights into potential sources of inaccuracy in LIB applications.