Superimposed multiple shots of low-fluence femtosecond (fs) laser pulses form a periodic nanostructure on various kinds of solid surfaces through ablation. It has been proposed that the periodicity of the nanostructures is attributed to the excitation of surface plasmon polaritons (SPPs). However, the excitation of SPPs on non-metallic material surfaces was never been directly observed. We have observed anomalies appearing in the reflection of intense fs laser pulses at a Si grating surface with a grating structure. The results have exhibited an abrupt decrease to create a sharp dip at a specific incident angle, where the Si grating surface was deeply ablated along the edge of the grooves periodically. The experimental results and model calculation provide directly evidence that SPPs can resonantly be excited at the interface between air and the non-metallic material surface and that enhanced near fields can form periodic structures on the surface.
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.
Conference Committee Involvement (1)
Laser Applications in Microelectronic and Optoelectronic Manufacturing XV
25 January 2010 | San Francisco, California, United States