Pathways control in modification of solid surfaces induced by temporarily separated femtosecond laser pulses

Reaction control by laser light is a unique method of the reaction dynamics mastery in the molecular chemistry. We provide evidence of phase control processing with femtosecond lasers in macroscopic solids. Rutile TiO2 monocrystals with (001) and (100) surface orientations were irradiated with repetitive pulses of femtosecond KrF laser of variable fluences and a temporal delay between two superimposed linearly polarized beams. The appearance of three types of surface morphology was thoroughly analyzed: low-spatial frequency laser-induced periodic surface structures (LSFL), grooves and unusual featureless flat area (FFA). The interaction of light with the excited surface led to the onset or suppression of the subwavelength LSFL, depending on whether the temporal delay between laser beams is larger or smaller than the critical value of -6 ps. By contrast, the suprawavelength grooves and FFA appeared at longer temporal delays. A strong decrease of the grooves onset energy was observed on the (001) oriented crystal after the delay of -8 ps; the decrease of onset energy was also observed on the (100) oriented crystal, where FFA appeared instead of grooves. The critical delay is discussed in framework of a phenomenological model describing the energy evolution of excited transient states along the "reaction" coordinate.