Near field enhancement of TiO2 nanoparticle array on different substrates for femtosecond laser processing

Abstract

Optical near field enhancement on substrate can be achieved by localizing femtosecond laser energy with nanoparticles. The enhanced field is located in the region between nanoparticles and the substrate. The localized femtosecond optical field is of great significance for fabricating the micro/nano structure with characteristic size beyond the diffraction limit. Up to now, femtosecond processing nanohole assisted by particle array is only possible for metal particle (Au) and low-refractive-index dielectric polystyrene particle. However, previous research results show that it cannot be realized for metal particle arrays (Au) to form periodic nanohole arrays, and it is limited for polystyrene particle to choose the corresponding substrate. In this paper, a novel method is proposed, in which high refractive index TiO2 arrayed particles are placed on the substrate to achieve laser induced near field enhancement. This makes feasible the nanoscale processing beyond the diffraction limit. In this paper, near field distributions of TiO2 particle array on Si, Pt and SiO2 substrates are simulated by the finite-difference time-domain (FDTD) method. The results show that TiO2 particles concentrate the laser energy to a region with a diameter of 100 nm around the particle and the near field enhancement is 140 times higher than the incident laser intensity, which is beneficial to fabricating the nanostructure of super diffraction limit, such as sub-hundred nanometer nanohole ablation by femtosecond laser. For Si substrate, the near field enhancement is only about 30% lower for TiO2 particle array than that for single TiO2 particle. In order to explore the influence mechanism of the substrate material parameters on the near field enhancement of TiO2 nanoparticle array, we further simulate the enhancement factor for the substrates of different refractive indices. It is found that the near field is enhanced with the increase of substrate refractive index, and this is attributed to an increased interaction of the particle with the near field of substrate and the scattering effect in which the TiO2 particle supports forward near field intensity pattern. Moreover, the image charge model is introduced to analyze the effect of substrate optical parameters on local field enhancement. Results in this paper can be applied to most metals as well as dielectric substrate surfaces, and they open a new way for femtosecond laser near field nano-processing with characteristic size beyond the diffraction limit.