High aspect ratio of near-field nano-lens for deep nano-crater patterning

Enhanced optical field close to nano-dielectric spheres excited by a femtosecond laser enables high-throughput nano-crater patterning. With spheres larger than the incident wavelength, the focused far field is well known in optics to be governed by micro-lens, while the enhanced near field with spheres smaller than or equivalent to the incident wavelength is dominated by the resonant Mie-scattering. The crater fabricated by the near-field nano-lens is much shallower than by the micro-lens. Revealing the largest crater depth relative to the diameter will advance the smart applications for nanotribology, nano-sensors and nano-biomedicine. Here, we study the aspect ratio (the depth profile in the substrate relative to the diameter of the intensity profile on the surface of the near-field intensity distribution in the substrate). It is because the fabricated nano-crater depth is empirically determined by the near-field intensity distribution. A maximal vertical intensity profile is found as a function of refractive index and sphere diameter. The dielectric spheres ranging from 400 to 1000 nm diameter on the Si substrate are studied at 800 nm wavelength. Using a sphere with the smaller refractive indices, the larger aspect ratio is achieved. However, a maximal optical intensity is sacrificed for the high aspect ratio. Largest aspect ratios for the near-field nano-patterning range from 3.0 through 4.2 using available spheres with refractive indices of from 1.4 to 3.0. We also consider the difference of the enhanced optical intensity distribution between the systems consists of a single isolated dielectric sphere on a silicon substrate and that consisting of mono-layered hexagonal dielectric sphere array on a silicon substrate.