Nano-dimple processing of silicon surfaces by femtosecond laser irradiation with dielectric particle templates in the Mie scattering domain

Dielectric particles sized comparable to the wavelength of light mounted on silicon substrates are irradiated with 800 nm femtosecond laser pulses. From this template of dielectric particles, a novel and interesting optical intensity distribution of the femtosecond laser irradiation is obtained. A result of this optical intensity distribution is a distinct pattern on the silicon substrate, which stems from the micro-lens and Mie scattering mechanism by the dielectric particles. In this paper, we investigated the dependence of the particle size, determined by the equation for size parameter α = 2πr/λ where r is the radius of the dielectric particle and λ is the incident laser wavelength, on the optical intensity distribution using the finite differential time domain method. A change in the size parameter induces a significant change in the optical intensity distribution in the vicinity of the particle. The distribution of the near-field intensity is analysed by its fingerprint on a substrate where the particle is deposited and irradiated by the femtosecond laser pulse. Using this method, we define the boundary between the lens effect and the contribution from Mie scattering. The experimental results indicate that the generated near-optical intensity, mediated by the dielectric particles, can produce a nano-hole with a size that overcomes the diffraction limit. Specifically, given certain boundary conditions, the processed nano-hole features have a characteristic shape governed by the incident light polarization, which has an ellipsoidal shape with the long axis perpendicular to the polarization of the incident light. In the case of using dielectric particles smaller than the incident wavelength, the contribution of the lens effect diminishes and the optical field intensity distribution is determined predominantly by the Mie scattering mechanism.