summary:

Laser-matter interactions give rise to thermal gradients in the molten surface layer: (i) parallel

to the surface, ΔT‖, which is driving force for the Marangoni convection or the surface tension

instability, and (ii) vertical to the surface, ΔT⊥, which is driving force for the Rayleigh-

Benard or the thermal convection instability.

Surface tension instability: To generate ΔT‖, 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, ∂σ/∂T‖, caused the formation of traveling waves (TW) inclined

under an angle φ with respect to ΔT‖. The angle φ 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 ωc from the experiment. However, at E ~

100 mJ, the wave inclination vanishes (φ = 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 ΔT⊥, 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

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.

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# Seminar

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## Laser-induced nonlinear processes : New insight into convective instabilities

Hold Date | 2012-02-10 15:30～2012-02-10 17:00 | |

Place | Seminar Room 3, Faculty of Mathematics building, Ito Campus | |

Object person | ||

Speaker | Stjepan Lugomer (Rudjer Boskovic Institute, Zagreb, Croatia) | |

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