Abstract
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.
Highlights
The impact of the employment of ultra-short pulsed laser sources in material processing has received considerable attention due to its important applications, in particular in industry and medicine [1,2,3,4,5,6,7,8,9,10]
We present a detailed theoretical approach that describes the ultrafast dynamics in rutile TiO2 [22,26,30,56,57,58], to account firstly, for excitation and electron-phonon relaxation upon irradiation of TiO2 in two crystal orientations with periodic ultrashort laser double-pulses separated by a temporal delay from zero to tens of picoseconds (Section 2)
Results manifest that the crystal orientations and interpulse delays play an important role in the onset of surface pattern formation because they influence both the carrier dynamics and thermal response of the irradiated structure
Summary
The impact of the employment of ultra-short pulsed laser sources in material processing has received considerable attention due to its important applications, in particular in industry and medicine [1,2,3,4,5,6,7,8,9,10]. 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]. 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]
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