Self-organized pattern formation upon femtosecond laser ablation by circularly polarized light Olga Varlamova a , Florenta Costache a , Ju ¨rgen Reif a, * , Michael Bestehorn b a LS Experimentalphysik II and IHP/BTU JointLab, Konrad-Wachsmann-Allee 1, 03046 Cottbus, Germany b LS Theoretische Physik II, Brandenburgische Technische Universita ¨t Cottbus, Konrad-Wachsmann-Allee 1, 03046 Cottbus, Germany Received 3 May 2005; accepted 3 August 2005 Available online 23 November 2005 Abstract Surface ripples generation upon femtosecond laser ablation is attributed to self-organized structure formation from instability. We report that linear arrangements are observed not only for linearly polarized light but also for ablation with circularly polarized light. Long ordered chains of spherical nanoparticles, reminding of bead-strings are almost parallel but exhibit typical non-linear dynamics features such as bifurcations. In a first attempt to understand the self-assembly, we rely on models recently developed for the description of similar structures upon ion beam erosion and for the simulation of instabilities in thin liquid films. Our picture describes an unstable surface layer, non-uniformly eroded through Coulomb repulsion between individual positive charges. # 2005 Published by Elsevier B.V. Keywords: Femtosecond laser ablation; Ripples; Circular polarization; Self-organization 1. Introduction Laser induced periodic surface structures (LIPSS) or ripples have been known from the advent of lasers and of laser–matter interaction [1,2]. Investigations performed on various target materials and under different laser irradiation conditions [1–17] generally revealed ripples of periods at the order of the laser wavelength for normal incidence, of orientations depending on the polarization of the incident laser light. The early models based the origin of ripples on an inhomogeneous energy input, due to interference between the incident laser and a surface scattered wave or surface corrugation periodicities [2,5–8]. The research on LIPSS became attractive again since the avail- ability of high intensities delivered by ultrashort laser pulses. Despite the fact that the femtosecond laser pulse stops long before the surface relaxes, surface structures were observed [9– 17]. Born upon surface relaxation, which keeps the memory of the laser beam polarization, these ripples exhibit peculiar features such as periods down to a fraction of laser wavelength for normal incidence and bifurcations [9–18]. In the ripples development, a positive feedback is attained when accumulat- ing multiple pulses on the surface, at low intensities below the damage threshold/plasma onset. The previous interference picture has been shown to have limited applicability on the new structures [9–18] although several papers continued this theoretical work by invoking the formation of higher harmonics of the ripples wavelength [12,15]. Several years ago, self-organization of an unstable surface was proposed as a responsible mechanism [9,10]. Indeed, for strong electron–phonon coupling materials like dielectrics or semi- conductors, the absorption of ultrashort laser pulses, i.e. the electronic excitation, leads to a destabilization of the crystal lattice within less than a picosecond [19]. Also the fundamental ablation mechanisms for such materials, Coulomb explosion [20,21] or phase explosion, are closely related to the generation of electrostatic or thermodynamic instabilities in the target. Self-organization as the origin of surface patterning has been intensely studied for more than three decades and has been successfully applied to explain surface ripples induced by off- axis ion beam sputtering [22], structures similar to LIPSS. The model is based on the assumption that incident ions penetrating the material determine a collisional atomic cascade and random slowing down of target atoms [23]. The resulting perturbation leads to different sputtering rates, hence a modulated surface www.elsevier.com/locate/apsusc Applied Surface Science 252 (2006) 4702–4706 * Corresponding author. Tel.: +49 355 69 3986; fax: +49 355 69 3985. E-mail address: reif@tu-cottbus.de (J. Reif). 0169-4332/$ – see front matter # 2005 Published by Elsevier B.V. doi:10.1016/j.apsusc.2005.08.120