Femtosecond laser processing using subwavelength thin metal slit arrays

We have studied femtosecond laser processing using subwavelength thin metal slits patterned onto Si substrates. A thin Ta film (200nm thickness) with a periodic-structured slit array (200-1000nm width) was formed on the Si substrates. The ablation thresholds of the Ta and Si were investigated. Near-field enhancement effects were observed at the edge of the metal slit arrays. Ridge structures were created in the slits on the Si surfaces using a single laser shot. In addition, an interesting processing effect was observed that was dependent on the polarization of the beam; only a p-polarized beam could create grooves perpendicular to the slits on the Si substrate. The grooves were formed under the metal layer. Our experimental results concerning the enhancement of the electrical field at the edge of the slits were consistent with the results from finite-difference time-domain simulations.