Fabrication of porous metal nanoparticles and microbumps by means of nanosecond laser pulses focused through a fibre microaxicon

We present a novel optical element: a fibre microaxicon (FMA) for laser radiation focussing into a diffraction-limited spot with a Bessel-like profile as well as for precision laser nanostructuring of metal film surfaces. Using the developed FMA for single-pulse irradiation of Au/Pd metal films on a quartz substrate, we demonstrated the formation of submicron hollow microbumps, each with a small spike atop it as well as hollow spherical nanoparticles. The experimental conditions for the controllable and reproducible formation of ordered arrays of such microstructures were defined. The internal structure of the fabricated nanoparticles and nanobumps was experimentally studied using both argon-ion polishing and scanning electron microscopy. These methods revealed a porous inner structure of laser-induced nanoparticles and nanobumps, which presumably indicates that a subsurface boiling of the molten metal film is a key mechanism determining the formation process of such structures.