Tungsten microcone growth by laser irradiation

Nanosecond pulsed-Nd:YAG laser irradiation of a target material with second-harmonic waves (wavelength=532 nm) under various gas atmospheres resulted in the formation of tungsten microcone arrays with a high aspect ratio protruding from the initial surface of the target material. The peak laser fluence was changed from 3.0 to 19.1 J/cm2. A tungsten substrate was irradiated with between 1 and 48 000 laser shots. The number of laser shots strongly influenced the growth and morphology of the microcones: several tens or hundreds of pulses created only rough surfaces, and further pulses created the microcones. The microcones grew to heights of up to 20 μm and narrowed to tips of about 1.5 μm in diameter. Our observations reveal that tungsten microcone can be generated in various ambient gases. We experimentally evaluated the microcone morphology in order to determine the mechanism of microcone formation in ambient atmospheres of helium, SF6, and air. The microcone formation appears to be a function of the repeated melting and solidification of the tungsten tips of the structures by the pulsed laser irradiation on the top surface. The morphology of the microcone depends on the ambient gas atmosphere used.