Imaging and modeling the multipulse dynamics of ultrafast laser-induced self-organization

International audienceTo form regular periodic patterns on surface irradiated by ultrafast laser with a nanoscale control, the understanding of the light coupling process is the most promising strategy. Ultrafast optical scattering by surface is governed by the local nano-roughness and the mutual response of the nanostructures through both radiative and non-radiative fields interaction. The spontaneous ordering occurs as a fingerprint of coherent light, potentially involving surface plasmonic effects and optical resonances but also modulated heat confinement and hydrodynamic movements [1-2]. Upon multi-pulse laser irradiation, feedback mechanisms regulate the surface self-patterning, feeding the debate regarding the exact nature of the involved surface waves. In particular, a periodicity shift is reported with the increase of the applied pulses, interrogating on the light distribution evolution with the transiently growing surface relief. The progressively formed nano-topography influences strongly the local energy deposition up to achieve spatial pattern stability in terms of periodicity and depth for all metals. To identify the nature of this intriguing self-regulation process, an imaging approach with a spatial nm resolution coupled with a self-consistent theoretical study are proposed.Laser-induced periodic surface structures were realized on Ti samples and photoemission electron microscopy was employed for imaging the ultrafast laser light distribution on the pre-structured surface. Periodical distributions of light were progressively observed with a frequency shift as the number of applied pulses increases. Concentrated hot-spot patterns were revealed to be superposed to this expected periodic absorption, with the demonstration that nanostructures periodicity reducing is driven by the concentration of the local-field enhancement centers. The roles of collective effects and inter-pulse feedback on the resulting surface topographies are then elucidated by a 3D numerical approach combining electromagnetism and hydrodynamics to simulate the multipulse dynamics. Complex surface topography is driven by the regulation that originates from the constructive and destructive roles of the electromagnetic and hydrodynamic processes. Also, the role of dipole-dipole nano-hole intercoupling is originally proposed to interpret the period decrease [3]. This nanoscale imaging coupled with dynamics calculations unravelling the intricate role of light absorption and nanostructure growth, opening new ways towards efficient fabrication of surface periodic nanostructures.References [1] A. Rudenko, J.P. Colombier, S.H ̈ohm, A. Rosenfeld, J Kr ̈uger, J. Bonse, andT. Itina, Scientific Reports 7, 12306 (2017).[2] X. Sedao, A. Abou Saleh, A. Rudenko, T. Douillard, C. Esnouf, S. Reynaud,C. Maurice, F. Pigeon, F. Garrelie, and J.P. Colombier, ACS photonics 5, pp.1418 (2018).[3] A. Rudenko, C. Mauclair, F. Garrelie, R. Stoian, and J.P. Colombier, Nanopho-tonics 8, pp. 459, (2019)