Experimental and theoretical insights into the formation of femtosecond laser-induced periodic surface structures on metals

International audienceApplication potential for ultrashort laser pulses relies on their remarkable capacity to confine and localize energy on the smallest scales. In particular conditions, ultrafast laser excitation triggers nanoscale self-arrangement of matter on irradiated surfaces, known as Laser-Induced Periodic Surface Structure (LIPSS). A comprehension effort related to laser excitation mechanisms will be presented, able to assist and validate experiments. In this context, we investigate hypotheses assuming that low spatial frequency LIPSS, having a spatial period close to the irradiation wavelength, are formed due to Surface Plasmon (SP) excitation. If the most part of experiments are performed on a practically smooth surface, we will present irradiation results obtained on pre-structured surfaces. Grating-coupled surface plasmon resonance on metallic surface has been exploited to investigate the correlation between ripples formation under ultrashort laser exposure and SP generation conditions. For TM irradiation conditions we will show that a well-defined period of grating is a seed for low spatial frequency LIPSS formation [1]. To design matter transformation patterns with predictable properties for a wide range of materials, the contrast of the nanostructuring process is another key parameter. Following localized photoexcitation, the sub-surface region undergoes transient melting and ultra-rapid quenching at the nanoscale. A thorough understanding of the involved physical mechanisms during the matter transformation will therefore help to control LIPSS morphology. By taking advantage of different metals behavior under nonequilibrium conditions, we propose experiments dedicated to evaluate the effects of energy coupling and relaxation strength on LIPSS formation [2]. We generated LIPSS on various materials with different electronic configuration in order to investigate the influence of the electron-phonon coupling strength and thermal diffusion efficiency on the specific contrast of LIPSS. For metals, the initial energy redistribution and the dimensional size of the region undergoing transient melting and resolidification appears to be correlated to the ripples height. The involved mutual influence of phase transformation and capillarity phenomenon can be effectively understood by applying theoretical methods accounting for the particular effects. Hydrodynamic simulations of laser-matter interaction will be presented to estimate laser energy deposition, electron-ion nonequilibrium stage, material heating and growth [3]. Time history of LIPSS formation will be discussed in this context of matter transformation phases using a capillarity approach. [1] F. Garrelie, J.P. Colombier, F. Pigeon, S. Tonchev, N. Faure, M. Bounhalli, S. Reynaud and O. Parriaux, Evidence of surface plasmon resonance in ultrafast laser-induced ripples, Opt. Express 19, p. 19150 (2011). [2] J.P. Colombier, F. Garrelie, N. Faure, S. Reynaud, M. Bounhalli, E. Audouard, R. Stoian, and F. Pigeon, Effects of electron-phonon coupling and electron diffusion on ripples growth on ultrafast-laser-irradiated metals, J. Appl. Phys. 111, p. 024902 (2012). [3] J.P. Colombier, P. Combis, F. Bonneau, R. Le Harzic and E. Audouard, Hydrodynamic simulations of metal ablation by fs laser irradiation, Phys. Rev. B 71, p.165405 (2005)