The Role of Crystalline Orientation in the Formation of Surface Patterns on Solids Irradiated with Femtosecond Laser Double Pulses

Featured Application Laser processing appears to be a scalable method to fabricate highly ordered and complex biomimetic surface structures which can be used in an abundance of industrial applications. On the other hand, a multiscale theoretical approach represents a powerful tool that can provide significant information for designing materials. The approach is aimed to offer a cost-effective method towards minimising the use of trial and error approaches to reduce a large number of experiments towards fabricating application-based patterns. A theoretical investigation of the underlying ultrafast processes upon irradiation of rutile TiO2 of (001) and (100) surface orientation with femtosecond (fs) double pulsed lasers was performed in ablation conditions, for which, apart from mass removal, phase transformation and surface modification of the heated solid were induced. A parametric study was followed to correlate the transient carrier density and the produced lattice temperature with the laser fluence, pulse separation and the induced damage. The simulations showed that both temporal separation and crystal orientation influence the surface pattern, while both the carrier density and temperature drop gradually to a minimum value at temporal separation equal to twice the pulse separation that remain constant at long delays. Carrier dynamics, interference of the laser beam with the excited surface waves, thermal response and fluid transport at various pulse delays explained the formation of either subwavelength or suprawavelength structures. The significant role of the crystalline anisotropy is illustrated through the presentation of representative experimental results correlated with the theoretical predictions.