Plasmonic and hydrodynamic effects in ultrafast laser generation of periodic surface structures on metals
Résumé
Despite diffraction limit, in particular conditions, ultrafast laser excitation can structure well below the incident wavelength. In this context, ultrafast laser excitation triggers nanoscale self-arrangement of matter on irradiated surfaces, known as Laser-Induced Periodic Surface Structure (LIPSS). The understanding formation dynamics of regular self-organized nanoscale structures under laser exposure of materials still required rigorous concepts related to modulated energy deposition, laser-induced non-equilibrium phenomena and the related relaxation paths involving mass transport. A comprehension effort related to laser excitation mechanisms will be presented, able to assist and validate experiments. We will indicate some fundamental aspects deriving from the initial stage of plasmonic coupling, the electron-phonon nonequilibrium process, the subsequent sequence of phase transformation and finally the surface dynamics via surface tension gradients. In a first experiment, grating-coupled Surface Plasmon Polaritons (SPP) on metallic surface has been exploited to investigate the correlation between ripples formation under ultrashort laser exposure and SPP generation conditions. The main result was an experimental demonstration of the resonant coupling of the fs-laser pulse to the grating structure, based on SPP excitation, and its impact on the early stage of LIPSS formation [1]. In a second set of experiments, LIPSS amplitude has been measured 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 [2]. Finally, we will show that in the particular case of strongly absorbing materials which can undergo capillary action as metals, hydrodynamic modeling approaches should be involved to explain the observed patterns. Discussion will be made on how feedback mechanisms can cause the growth of grating periodicities by coupling to the diffractive laser light, involving plasmonic/hydrodynamic effects. [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).