Superimposed multiple shots of low-fluence femtosecond (fs) laser pulses form a periodic nanostructure on solid surfaces through ablation. We have demonstrated that the self-organization process of nanostructuring can be regulated to fabricate a homogeneous nanograting on the target surface in air. A simple two-step ablation process and an ablation technique using interfering fs laser beams were developed to control plasmonic near-fields generated by fs laser pulses. The results have shown the nature of a single spatial standing wave mode of surface plasmon polaritons of which periodically enhanced near-fields ablate the target surface, to fabricate the nanograting on gallium nitride (GaN) and metals such as stainless steel (SUS) and titanium (Ti).
This paper describes two topics of our recent studies on ultrafast strong-field interactions with atoms, molecules and
solid surfaces. One is concerned with the high-order harmonic generation (HHG) from molecules nonadiabatically
aligned with intense femtosecond (fs) laser pulses in a pump and probe experiment. The HHG is very sensitive to the
molecular orbital and its spatial orientation with respect to the laser polarization. Experimental and theoretical studies
demonstrate the characteristic properties of HHG from coherently rotating molecules. The other topic is the periodic
nanostructure formation observed in fs laser ablation of dielectric materials. The major interest is in the ultrafast
interaction process of nanostructuring on solid surfaces, for the purposes of potential applications of fs lasers to nanoprocessing.
The experimental results have shown that enhanced near-field initiates the ablation of surface area much
smaller than the laser wavelength and the origin of nanoscale periodicity can be attributed to the excitation of surface
plasmon polaritons in the surface layer.
We have studied periodic nanostructure formation processes on hard thin film surfaces in femtosecond laser ablation. Using diamond-like carbon films patterned with submicrometer-size stripes, we found that the nanoscale ablation is preferentially initiated by the enhancement of a local field on the stripe surface having high curvature. Based on the experimental results for the initial stage of nanostructuring, it is concluded that the nanoscale ablation is initiated with the enhanced local field, and the periodicity is developed with the excitation of surface plasmon polaritons.
Surface modification of thin diamond-like carbon (DLC) films deposited on stainless steel has been studied by irradiating linearly and circularly polarized femtosecond (fs), 800- and 267-nm Ti:sapphire laser pulses at laser fluence around the ablation threshold. The morphology of the DLC surfaces was observed and characterized with a field-emission scanning electron microscope and a scanning probe microscope. The linearly polarized light produced arrays of fine slender granular structure on the ablated surface, while the circularly polarized light formed a fine dot-like periodic structure. The mean spacing of these fine structures was 1/10 - 1/5 of the laser wavelength used. With an increase in the laser fluence, the size of structure was observed to increase, while the surface roughness decreased. We have analyzed the modified DLC surfaces by observing Raman spectra and found that the nanoscale modification of the DLC films is accompanied with a structural change into glassy carbon. This structural change depended on the fs laser fluence and little on the laser polarization and wavelength.
Linearly- and circularly-polarized femtosecond (fs) Ti:sapphire laser pulses at 800 and 267 nm were focused in air to ablate hard thin films of TiN and DLC deposited on stainless steel plates. The morphology of the thin film surfaces that were ablated by the fs laser pulses at an energy fluence slightly above the ablation threshold was observed and characterized with a field emission-scanning electron microscope. With the linearly-polarized light, arrays of fine slender granular structure were produced on the ablated surface, which were almost oriented to the direction perpendicular to the laser polarization. On the other hand, the circularly-polarized light is found to form fine dot structures on the film surface. The size of these surface structures was 1/10 ~ 1/5 of the laser wavelength used and was observed to decrease with a decrease in the laser wavelength. It should be noted that the size of surface structures observed is much smaller than that of the well-known surface ripple patterns produced by the laser-induced surface electromagnetic wave.