Interaction of ultraviolet femtosecond laser pulses with silicon and generation of surface structures has been studied at two energy densities, E

_{1}~ E

_{thr}~ 0.17 J/cm

^{2}, (near the ablation threshold) and E

_{2}~ 0.42 J/cm

^{2}>> E

_{thr}, much above the ablation threshold. In both cases the number of pulses, N, is the external control parameter that controls the formation and transformation of structures.

E_{1} ~ E_{thr}: For N < 200, the «carpet-like» pattern of nano-, and micro-spikes is generated by the bubble explosion in a silicon foam-layer*.* The accumulation of nanobubbles due to repetition of laser pulses and their explosion cause damped membrane-like oscillation of the foam-layer. For N ≥ 200, bifurcation of surface morphology takes place: (i) In the peripheral region of the spot, the surface tension waves with L_{1} ~ 180 mm, have been formed. The change of their wavelength into L_{2} ~ 40 mm with propagation distance indicates the Eckhaus instability caused by the phase modulation with increasing N. The simulation of the left and the right propagating waves can be based on the complex Landau-Ginzburg equation.(ii) In the central region of the spot, a rough irregular morphology appears caused by the fast spinodal decomposition and fragmentation of superheated silicon layer.

E_{2} >> E_{thr}: The train of 120 ≤ N ≤ 190 pulses establishes the 2D unidirectional cnoidal-like waves as well as the Y- and X-type configurations. In the region of high laser intensity, the interaction of stable line solitary-like waves give rise to the complex network structure. The network of line-solitons was shown to be decomposed into segments with more simple configurations which can be simulated by the Kadomtsev-Petviashvili (KP-II) equation with negative dispersion as 2-soliton solutions. For the sequence of pulses 200 ≤ N < 220, the transition from stable into unstable waves takes place. The unstable wiggling cnoidal and decaying line-soliton waves were shown to be reproduced by using the KP-I equation with positive dispersion. The sequence of pulses approaching the critical value, N_{critical} ( ≥ 230), destruction of unstable cnoidal and solitary waves into localized lump solitons, takes place.