Simulation of Possible Scenarios of the development of Laser Induce Periodic Surface Structures (LIPPS) with COMSOL Multiphysics® and the Molecular Dynamic Code LAMMPS
Irradiation of various surface materials by ultra-short laser radiation, typically with pulse widths of 10-300 fs, results in the development of Laser-induced self-organized periodic surface structures (ripples). The periodicity of these structures ranges from the wavelength λ of the laser to well below λ. Structures down to a few tens of nm can be observed. However, the existence and form of the LIPSS critically depends on the intensity, fluence, wavelength and pulse width of the laser light as well as on the number of laser pulses and, not to forget, the material properties [3].
The development of the LIPSS structure can be regarded as a complex interplay between the electric field on the surfaces (including surface plasmon-polaritons)[2] and the ultra-short-laser-ablation dynamics [1]. Since the formation of LIPSS in most cases requires multiple laser pulses, the surface topography and hence the electric field distribution will change from shot to shot. Only the resulting feedback ensures the development of the LIPSS. The simulation realized her follows a procedure proposed by [5].
In order to manage a physically reasonable sample size of 500 nm to 1 µm (x and y -dimension) and 200 nm in the z direction, with a resolution of below 1 0nm (which is required for simulating experimentally observed LIPSS with 10 nm and larger frequencies), the simulation was implemented on a HPC Cluster (VSC3-Vienna Scientific Cluster) an consists of several steps, which are executed in a loop for a certain number of laser pulses: 1.) Creating a sample with rough surface in LAMMPS [4]; 2.) Extracting the surface for input into COMSOL Multiphysics®; 3.) Simulation of the electric field distribution on the surface and in the sample with COMSOL Multiphysics® (Wave Optics Module, Data Input and Output handled via Application Builder in the batch script). 4.) Conversion of the calculated field distribution into possible energy scenarios leading to ablation (particle ejection) 5.) MD simulation on the atomic scale taking into account the results from step 4 to obtain the new sample (surface) morphology. 6.) Repeat from step 2.
[1] Dachraoui, H. and W. Husinsky (2006). "Fast electronic and thermal processes in femtosecond laser ablation of Au." Applied Physics Letters89(10): 4102-4102.
[2] Kudryashov, S. I., S. V. Makarov, A. A. Ionin, C. S. R. Nathala, A. Ajami, T. Ganz, A. Assion and W. Husinsky (2015). "Dynamic polarization flip in nanoripples on photoexcited Ti surface near its surface plasmon resonance." Optics Letters40(21): 4967-4970.
[3] Nathala, C. S. R., A. Ajami, A. A. Ionin, S. I. Kudryashov, S. V. Makarov, T. Ganz, A. Assion and W. Husinsky (2015). "Experimental study of fs-laser induced sub-100-nm periodic surface structures on titanium." Optics Express23(5): 5915-5929.
[4] Plimpton, S. (1995). "Fast parallel algorithms for short-range molecular dynamics." Journal of Computational Physics117(1): 1-19.
[5] Skolski, J. Z. P., G. R. B. E. Römer, J. Vincenc Obona and A. J. Huis In 'T Veld (2014). "Modeling laser-induced periodic surface structures: Finite-difference time-domain feedback simulations." Journal of Applied Physics115(10).
