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Laser_Induced Nonlinear Processes: New Insight into Convection Instabilities

Laser-induced nonlinear processes : New insight into convective instabilities S. Lugomer, “Rudjer Boskovic” Institute, Zagreb, Croatia Laser-matter interactions give rise to thermal gradients in the molten surface layer: (i) parallel to the surface, T‖, which is driving force for the Marangoni convection or the surface tension instability, and (ii) vertical to the surface, T⊥, which is driving force for the Rayleigh-Benard or the thermal convection instability. Surface tension instability: To generate T‖, a Gaussian power profile on the chromium nanolayer of 500 nm thickness has been used. The external control parameter was the laser energy E (at N = constant number of pulses), in the confined configuration of experiment. The surface tension gradient, ∂/∂T‖, caused the formation of traveling waves (TW) inclined under an angle with respect to T‖. The angle and the wavevector k change showing alternation with increasing E, thus establishing the cascade of the left-right inclinations. The left-inclined and the right-inclined TW can be simulated on the basis of CGLE, taking the critical wavevector kc, and the critical frequency c from the experiment. However, at E ~ 100 mJ, the wave inclination vanishes ( = 0), and the wavevector k decreases to some constant value (soft mode like behavior). This pretransitional effect is connected with initiation of the radial fluid flow and the new type of instability which is more efficient energy dissipation channel, like the RT and the RM instability. Thermal convection instability: To generate T⊥, a homogenized laser beam and flat power distribution on the SiON/Si interface layer of 350 – 400 nm thick, have been used. The external control parameter was the number of pulses N (at E = const.), in the open configuration of experiment. The Boussinesq conditions cause the formation of domains with the parallel roll organization, the inclined wavy-like, and the chaotic ones. The Fourier analysis of these domains gives the structure factor S(k), and the correlation length of the wavevectors. Numerical simulation based on 2D Swift-Hohenberg equation reproduces the roll organization together with the secondary instabilities that evolve in time, starting with the long-wavelength “ZIGZAG” instability, than both the “ZIGZAG” and the Eckhaus instability, and finally with the Eckhaus instability only. However, the micrographic analysis reveals that at N ≳ 20, the roll structures grow faster at some locations. This indicates the pretransitional behavior connected with initiation of the fluid flow from the center to the periphery of the spot and transition into the multiple absolute instability. Such cascade-like absolute instability (probably observed for the first time) under series of pulses, causes the wavy agglomeration of rolls into well separated bands. The roll structures become interconnected into the pattern of a high topological complexity that resembles the topology of neural networks.

marangoni instability ; inclined line solitons ; rayleigh-benard instability ; absolute instability ; Swift-Hohenberg equation

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