Applied Surface Science 302 (2014) 231–235 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc Microstructuring of soft organic matter by temporally shaped femtosecond laser pulses Esther Rebollar a,∗ , Jutta Mildner b , Nadine Götte b , Dirk Otto b , Cristian Sarpe b , Jens Köhler b , Matthias Wollenhaupt b , Thomas Baumert b , Marta Castillejo a a Instituto de Química Física Rocasolano, IQFR-CSIC, Serrano 119, 28006 Madrid, Spain b Institut für Physik, Universität Kassel, Heinrich-Plett-Str. 40, D-34132 Kassel, Germany a r t i c l e i n f o Article history: Received 14 June 2013 Received in revised form 18 October 2013 Accepted 18 October 2013 Available online 29 October 2013 Keywords: Femtosecond laser processing Polymer films Temporally shaped femtosecond pulses a b s t r a c t Thin films of the biopolymers gelatine and chitosan were treated using femtosecond pulse shaping techniques combined with a microscope-based setup for material processing. The polymer films were irradiated with laser pulses of 35 fs and a central wavelength of 790 nm provided by an amplified Ti:Sapphire system. The effect of temporal pulse shaping, with quadratic and cubic spectral phases, on the induced morphology was analyzed by characterization of the created surface structures via scanning electron microscopy. We observed different material modification thresholds and different structure sizes for temporally asymmetric pulse shapes. The results indicate the possibility of control of the gen- erated microstructures and are discussed in relation to the formation of free electrons and the different contributions of multi-photon and avalanche ionization processes. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The generation of micro- and nanopatterns with complex mor- phologies remains an ambitious challenge in technology. Behind the fundamental interest in understanding the physical process, the motivation to study micro- and nanopatterning is also driven by the great potential of such structures for advanced applica- tions in e.g. nanofluidics, nanophotonics and biomedical devices [1–3]. Biopolymers constitute important sources of novel functional materials and advantageous alternatives to synthetic polymers in biomedical applications. For those applications, both the substrate topography and the substrate chemistry have an important effect [4,5]. In order to control these properties, a variety of processing techniques such as stamping, stereo lithography, two-photon poly- merization, electrospinning, polymer demixing and stencil [6–9] have been applied. However, common micro-fabrication tech- niques are prone to difficulties when dealing with biomaterials due to their typical non biocompatibility [7]. As an alternative, laser-based methods have been already used for porous scaffold fabrication on various materials [10–13], since they afford the sought versatility and reliability [14–17]. Recently, superficial laser foaming on films of biopolymers such as collagen and gelatine ∗ Corresponding author. Tel.: +34 915619400; fax: +34 915642431. E-mail address: e.rebollar@iqfr.csic.es (E. Rebollar). has been reported, a phenomenon that is induced by applying single pulses both in the nanosecond and femtosecond domains [14,16–21]. Lasers delivering ultrashort pulses have emerged as a unique tool for processing wide band gap materials for a variety of appli- cations [22–24]. Materials which are transparent to light in the visible and near infrared spectral region become highly absorb- ing when ultrashort laser pulses of sufficient high intensity are applied. This allows material processing, but effective control of laser induced effects is required. Related to this, a large number of experiments have been devoted to the study of laser induced damage in the case of dielectrics. These involve investigations on the effect of pulse duration [25,26] and the application of double pulses [27,28], pulse trains [29] and temporally shaped asymmetric pulses [30–32]. In this work, we present experimental studies with single 35 fs pulses with temporally asymmetric shapes obtained by introduc- ing a third-order dispersion (TOD) via spectral phase modulation, enabling the control of the effects induced on different biopolymer films. Also chirped pulses making use of group delay dispersion (GDD) (i.e. symmetric temporal pulse envelope but asymmetric instantaneous frequency) and combination of shaped pulses with GDD and TOD were tested. Our experiments reveal a systematic dependence of the surface damage threshold and of the size of the obtained structures on the phase mask applied in the case of TOD shaped pulses for the different biopolymers, while no significant differences are observed for those shaped via GDD. 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.111