Nanohole processing on silicon substrate by femtosecond laser pulse with localized surface plasmon polariton

We demonstrate nanohole fabrication on silicon surface by femtosecond laser pulse irradiation mediated by gold nanoparticles. Gold spheres with diameters of 40, 80 or 200 nm are placed on the silicon substrate surface by a spin-coating method. The laser pulse with duration of 150 fs and wavelength of 820 nm is used to irradiate the Si substrate. Laser fluences applied are in the range of 140–300 mJ/cm2, i.e. below or near the ablation threshold fluence of the bulk silicon substrate without gold particles. The morphological changes of the laser-irradiated areas are investigated by scanning electron microscope (SEM) and atomic force microscope (AFM). Their dependence on the particle diameter, shape and laser fluence is investigated. The ablated surface morphologies are found to strongly depend on the polarization and the energy of the laser pulse. Nanoholes with diameters of about 150 nm and depths in the range of 30 nm are produced in the case of 200 nm diameter particles at fluences below the threshold for Si without Au particles. At fixed laser fluence the diameter and depth of the holes increase with the particle sizes. The optical field enhancement factor on the Si surface is calculated using an FDTD simulation code. A maximal value of about 26 is obtained for 200 nm Au particles. The comparison between the theoretical results for the electromagnetic field enhancement factor achieved and the experimental results is made in order to explain the physics of the nanomachining process.