Scattering effects and high-spatial-frequencynanostructures on ultrafast laser irradiatedsurfaces of zirconium metallic alloys with nanoscaledtopographies

International audienceThe origin of high-spatial-frequency laser-induced periodicsurface structures (HSFL) driven by incident ultrafast laser fields, with theirability to achieve structure resolutions below λ/2, is often obscured by theoverlap with regular ripples patterns at quasi-wavelength periodicities. Weexperimentally demonstrate here employing defined surface topographiesthat these structures are intrinsically related to surface roughness in thenano-scale domain. Using Zr-based bulk metallic glass (Zr-BMG) and itscrystalline alloy (Zr-CA) counterpart formed by thermal annealing from itsglassy precursor, we prepared surfaces showing either smooth appearanceson thermoplastic BMG or high-density nano-protuberances from randomlydistributed embedded nano-crystallites with average sizes below 200 nm onthe recrystallized alloy. Upon ultrashort pulse irradiation employing linearlypolarized 50 fs, 800 nm laser pulses, the surfaces show a range of nanoscaleorganized features. The change of topology was then followed undermultiple pulse irradiation at fluences around and below the single pulsethreshold. While the former material (Zr-BMG) shows a specific highquality arrangement of standard ripples around the laser wavelength, thelatter (Zr-CA) demonstrates strong predisposition to form high spatialfrequency rippled structures (HSFL). We discuss electromagnetic scenariosassisting their formation based on near-field interaction between particlesand field-enhancement leading to structure linear growth. Finite-differencetime-domain simulations outline individual and collective effects ofnanoparticles on electromagnetic energy modulation and the feedbackprocesses in the formation of HSFL structures with correlation to regularripples (LSFL)