_Applied_  
_sciences_  
_Article_  
**The Role of Crystalline Orientation in the Formation**  
**of Surface Patterns on Solids Irradiated with**  
**Femtosecond Laser Double Pulses**  
**[PERSON] 1,** **2**, **3**, **4**, **5**, **6**, **8**, **9**, **10**, **11**, **12**, **13**, **14**, **15**, **16**, **17**, **18**, **19**, **20**, **21  
Laser-induced periodic surface structures (LIPSS) on solids is one type of surface patterns that has been explored extensively, becausethe features of the produced structures provide impressive properties that can be used in many applications including microfluidics [1, 11], tribology [12; 13; 14], tissue engineering [11; 15] and advanced optics [16; 17]. There exists a wealth of reports related to the elucidation of the underlying physical processes for the formation of these structures in a variety of laser conditions (i.e., fluence, pulse duration, laser wavelength, pulse separation, polarisation state, energy dose) and in different materials [17; 18; 19; 20; 21]. Some of the most representative types of LIPSS are the coined low-spatial-frequency (LSFL) ripples [22; 23], high-spatial-frequency (HSFL) ripples [18; 24], grooves [20; 25] and spikes [25]. A series of multiphysical phenomena were analysed to explain the physical origins of the LIPSS formation [26; 27; 28; 29; 30]. More specifically, various theoretical models have been proposed to interpret the production of periodic structures: interference of the incident wave with an induced scattered wave [31; 32; 33], or with a surface plasmon wave (SP) [21; 22; 34; 35; 36; 37], or due to self-organisation mechanisms [38]. Although, the formation of LIPSS at long time scales is closely related to a phase transformation, one process that undoubtedly has to be investigated is the response of the material at short time scales. More specifically, as stated above, the impact of electromagnetic effects and plasma contribution [39], surface plasmon excitation [22; 34], wave interference effects [40] are some of the processes that need to be explored when the material is in a non-equilibrium state. Therefore, the ultrafast dynamics of the excited carriers is an important ingredient that greatly influences the LIPSS periodicities. To provide a comprehensive elucidation of the ultrafast dynamics of the charge carriers, it is important to elaborate on the influence of the laser parameters on the excited plasma characteristics and then, on the thermal response of a material. The existing multiscale models and experimental studies have successfully provided a detailed analysis of the physical mechanisms that lead to a plethora of structural effects [22; 41; 42; 43; 44; 45]. In principle, the laser beam parameters determine the first steps of the processes because energy absorption and relaxation processes influence significantly the material heating [43]. Nevertheless, although for many semiconducting materials, the physical mechanism that describes ultrafast dynamics is well-established and the theoretical model works effectively in various conditions, there is still a missing picture for some types of semiconductors such as TiO\({}_{2}\) and more specifically for materials with different crystal orientations. In particular, in a previous report, a parametric study of the laser conditions and the induced structures revealed that there is a difference in the response of the material if crystals with orientation (001) and (100) are irradiated [46; 47; 48].