Directionally controlled plasmon excitation in gold nanoparticles for near-field nanopatterning by femtosecond laser

We present theoretical and experimental results of nanohole fabrication on a silicon substrate by plasmonic near field around multiple gold nanoparticles excited by oblique incidence of femtosecond laser. Using the enhanced near field around a gold nanoparticle, nanohole can be fabricated on the substrate surface even at the near-infrared laser excitation. The formation of nanoholes with the near-infrared incident wavelength will open up smart applications for new optical device fabrication in air. However, the plasmons inside gold nanoparticles are affected by the plasmons of neighboring gold particles, resulting in an alteration of near-field distribution and the shift of resonant wavelength of plasmons inside the gold particles. These cause inhomogeneous shape of nanoholes and decrease the near-field intensity on silicon substrate. Therefore it is necessary to control the direction of plasmon for precise nanohole fabrication. We propose a new method to reduce plasmon interaction between particles by using p-polarized irradiation at oblique incidence to the substrate surface. Experimental and theoretical study demonstrated that uniformed nanoholes can be achieved by restraining effects from neighboring plasmon when p-polarized beam is irradiated to gold nanoparticle arrays with optimal oblique incidence.