The Nanostructures Produced by Laser Ablation of Metals in Superfluid Helium

Abstract

The coagulation of impurities in superfluid helium, in contrast to that in all other liquids where spherical colloid particles are usually produced, led to producing thin and long nanowires with regular internal structure. This is due to the presence in HeII of quasi one-dimensional quantized vortices serving as condensation nuclei and providing a catalyzing effect on the process of any impurities coagulation. The metal was introduced into superfluid helium by laser ablation of targets made of gold, copper, nickel, permalloy, indium, lead, tin and bismuth immersed in liquid HeII. For all of these metals, the formation of thin (about 8 nm in diameter), long high-quality nanowires was observed after laser ablation. The structure of nanowires as well as of micron-sized metallic spheres, appeared as products at high laser pulse energy, providing evidence that they were formed via molten state. The spheres are metastable, and under damage of their surface, thousands of nanoballs emerge from their interior. The hollow shells left after this event are similar to those found as the products of laser ablation in normal fluids. The metal ablation into HeII bulk from thin film was found much less effective then that from thick foils.