Scattering-controlled femtosecond-laser induced nanostructuring of TiO2 thin films
2011
The formation of laser induced periodic surface structures (LIPSS) is to a large extent of self-organizing nature and
in its early stages essentially influenced by optical scattering. The evolution of related mechanisms, however, has
still to be studied in detail and strongly depends on materials and laser parameters. Excitation with highly intense
ultrashort pulses leads to the creation of nanoripple structures with periods far below the fundamental wavelength
because of opening multiphoton excitation channels. Because of the drastically reduced spatial scale of such laser
induced periodic nanostructures (LIPNS), a particular influence of scattering is expected in this special case. Here
we report on first investigations of femtosecond-laser induced nanostructuring of sputtered titanium dioxide (TiO 2 )
layers in comparison to bulk material. The crucial role of the optical film quality for the morphology of the resulting
LIPNS was worked out. Typical periods of nanoripples were found to be within the range of 80-180 nm for an
excitation wavelength of 800 nm. Unlike our previously reported results on bulk TiO 2 , LIPNS in thin films appeared
preferentially at low pulse numbers (N=5-20). This observation was explained by a higher number of scattering
centers caused by the thin film structure and interfaces. The basic assumptions are further supported by
supplementary experiments with polished and unpolished surfaces of bulk TiO 2 single crystals.
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