Surface microcavities at nanoscale depths produced by ultrafast laser pulses

Abstract

The evolution of silicon surface (111) induced by a single femtosecond laser pulse (350 fs, 1028 nm) at near-threshold fluences in viscous liquid is investigated by SEM and AFM. Depending on the deposited energy, the near-surface layers undergo several transformations that generally resulted in controlled formation of simple and complex bowl-shaped surface microcavities at nanoscale depths, surrounded by characteristic rims. At fluences below the ablation threshold, a 2-nm-depth microcavity is observed. The embedded microcavity has the form of an ultra-smooth paraboloid with a roughness ∼0.1 nm rms. Its formation is associated with a flow of melt from the center to the edges due to surface tension variations induced by a single Gaussian-shaped femtosecond laser pulse above the melting threshold. Similar microcavities with depths up to 4 nm are also observed with an increasing fluence. However, at higher fluences complex microcavities, characterized by a central sub-nanostructured region (with a roughness ∼0.5 nm rms) encircled with a crown-like rim of ∼5 nm height and ∼50 nm width (at FWHM), are found. The carried out experiments clearly demonstrate the surrounding medium to play a crucial role in producing of smooth and nanostructured surface microcavities at nanoscale depths with single femtosecond laser pulses.