Ultrafast thermal dynamics of nano-ripples formation via laser double pulses excitation Guangqing Du, Yanmin Wu, Noor Uddin, Qing Yang n , Feng Chen n , Yu Lu, Hao Bian, Xun Hou State Key Laboratory for Manufacturing System Engineering & Key Laboratory of Photonics Technology for Information of Shaanxi Province, School of Electronics & Information Engineering, Xi'an Jiaotong University, Xi'an 710049, PR China article info Article history: Received 4 February 2016 Received in revised form 11 April 2016 Accepted 28 April 2016 Available online 10 May 2016 Keywords: Nano-ripples Femtosecond laser Double pulses Thermal dynamics abstract The ultrafast thermal dynamics of nano-ripples formation on gold film via ultrafast laser double pulses excitation is theoretically investigated by numerical simulations. The non-equilibrium thermal mod- ulations with respect to the electron and phonon energy transfers within gold film is proposed for predicting the nano-ripples formation. It is revealed that the nano-ripples contrast on gold film surface can be well controlled via tuning the pulse energy ratio, pulse separation and pulse exchange of ultrafast laser double-pulse. It is attributed to the tunable energy transfer routes between the electron thermal diffusion and the electron–phonon coupling via tuning double pulses parameters. The study provides theoretical basis for producing high-contrast ripples for a wide range application in the fields such as high-absorptive solar cells, surface friction devices and super-hydrophobic surface. & 2016 Elsevier B.V. All rights reserved. 1. Introduction Laser-induced periodic surface structures (LIPSSs), also called ripples, have attracted much interest for preparing submicron- scale-structures for both scientific and practical purposes [1–6]. The ripples formation is widely accepted as the interference be- tween the incident laser and surface scattering wave, which may originate from the surface roughness, surface instability and sur- face plasmon excitation [7–11]. Based on the surface wave inter- ference theory, the ripples period can be predicted with respect to the laser parameters such as laser polarization, incident angle and laser wavelength. Recently, femtosecond laser double pulses with fs–ps temporal separations have found great potential in material patterning [12– 14]. It is experimentally observed that the patterned nano-ripples features can be well controlled via the femtosecond laser double- pulse train processing [15]. It rises a challenge for the conventional surface wave interference theory, which fails to predict the thermal modification of nano-ripples period because of the ignorance of material excitation. Bonse et.al. investigated the electron excitation role in affecting the ripples formation, revealing that the electron density can be an important role in regulating the ripples period [16]. Generally, it is accepted that the ultrafast non-equilibrium thermal dynamics can be mainly responsible for regulating the thermal ablation features [17–21]. For a metallic target, typical no- ble metal of gold, it is believed that the non-equilibrium thermal dynamics in the target material can be significantly modified by femtosecond laser double pulses excitation [22]. The thermal dy- namics for regulating nano-ripples features can be expected to play an important role in affecting the performances of the rippled surface devices. It raises great interest for exploring the thermal dynamics of the nano-ripples formation on a metal surface with respect to femtosecond laser double pulses excitation. Especially, it is important to understand the fundamental dynamics of nano- ripples formation with respect to the non-equilibrium thermal ex- citations by the complex parameters of femtosecond laser double pulses. However, the nano-ripples contrast modulation with respect to the non-equilibrium thermal excitation via tailoring femtosecond laser double pulses is less investigated so far. In this paper, the ultrafast thermal dynamics for patterning nano-ripples on gold film via femtosecond laser double pulses excitation is theoretically investigated. The periodical phonon temperature modulation for regulating the ripples profile was explored in details based on Finite Element simulations. The in- terplay of the electron thermal diffusion and the electron–phonon coupling are proposed for revealing the modulated ripples profile in the early stage of ripples formation. The theoretical study is helpful for understanding the basic thermal dynamics of nano- ripples formation thus providing the strategy for producing con- trollable nano-ripples structures via tuning the complex para- meters of femtosecond laser double pulses. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications http://dx.doi.org/10.1016/j.optcom.2016.04.064 0030-4018/& 2016 Elsevier B.V. All rights reserved. n Corresponding authors. E-mail addresses: yangqing@mail.xjtu.edu.cn (Q. Yang), chenfeng@mail.xjtu.edu.cn (F. Chen). Optics Communications 375 (2016) 54–57