Study of nanostructure formation processes on thin film surfaces irradiated with femtosecond laser pulses and development of its physical model

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For the purposes of developing a new nanotechnology using femtosecond (fs) lasers, this research project was aiming at making clear experimental conditions and constructing a physical model for the periodic nanostructure formation and the selective change in bonding structure that we have recently found on hard thin films irradiated with fs laser pulses. The main results obtained are as follows.・The nanostructure size observed was λ/10〜λ/5 of the laser wavelength λ when fs laser pulses were irradiated on the thin films such as TiN and diamond-like carbon (DLC) at a low fluence near ablation threshold. We have shown that the nanostructure and its size could be controlled with the laser polarization, wavelength and fluence.・In addition, we have found that the DLC consisting of sp^3 and sp^2 can be modified into the conductive glassy carbon (GC) through the selective transition of sp^3 into sp^2 under almost the same experimental condition as for the nanostructure formation.・Reflectivity of DLC film surface was measured as a function of fluence and laser pulse shots. The results have shown that the reflectivity change is closely associated with the nanostructure formation, and that the superimposed laser shots and resulting incubation effect mainly contribute to the nanostructure formation.・The reflectivity of DLC was also measured using a pump and probe technique. The results have shown a characteristic reflectivity change due to the formation of a grating on the surface, where the selective change in the bonding structure from sp^3 to sp^2 induces a swelling of the film surface.・The measurements have demonstrated that the nanostructure formation on the DLC surface is certainly preceded by the change in the bonding structure.・On the basis of the experimental results obtained we have been developing a physical model for the nanostructure formation that relies on a localized field created in the interaction.