Scattering effects and high-spatial-frequency nanostructures on ultrafast laser irradiated surfaces of zirconium metallic alloys with nanoscaled topographies
Abstract
The origin of high-spatial-frequency laser-induced periodic
surface structures (HSFL) driven by incident ultrafast laser fields, with their
ability to achieve structure resolutions below λ/2, is often obscured by the
overlap with regular ripples patterns at quasi-wavelength periodicities. We
experimentally demonstrate here employing defined surface topographies
that these structures are intrinsically related to surface roughness in the
nano-scale domain. Using Zr-based bulk metallic glass (Zr-BMG) and its
crystalline alloy (Zr-CA) counterpart formed by thermal annealing from its
glassy precursor, we prepared surfaces showing either smooth appearances
on thermoplastic BMG or high-density nano-protuberances from randomly
distributed embedded nano-crystallites with average sizes below 200 nm on
the recrystallized alloy. Upon ultrashort pulse irradiation employing linearly
polarized 50 fs, 800 nm laser pulses, the surfaces show a range of nanoscale
organized features. The change of topology was then followed under
multiple pulse irradiation at fluences around and below the single pulse
threshold. While the former material (Zr-BMG) shows a specific high
quality arrangement of standard ripples around the laser wavelength, the
latter (Zr-CA) demonstrates strong predisposition to form high spatial
frequency rippled structures (HSFL). We discuss electromagnetic scenarios
assisting their formation based on near-field interaction between particles
and field-enhancement leading to structure linear growth. Finite-differencetime-
domain simulations outline individual and collective effects of
nanoparticles on electromagnetic energy modulation and the feedback
processes in the formation of HSFL structures with correlation to regular
ripples (LSFL).