Versatile femtosecond laser interference patterning applied to high-precision nanostructuring of silicon

Abstract

We present a high-precision nanostructuring technique based on beam interference from a commercial Ti:Sa femtosecond amplified laser (800 nm, 120 fs, 1 kHz, 1 mJ). Its potential and versatility is illustrated by applying the technique to the nanostructuring of silicon. Employing commercial and ad-hoc laser-fabricated diffractive optical elements together with standard optical components, periodic line gratings and spot arrays with tunable periods down to 650 nm can readily be fabricated. Moreover, fabrication of millimeter-size diffraction gratings via multipulse irradiation at processing speeds up to 0.5 mm/s is demonstrated. The variety of structures written in crystalline silicon feature amorphous fringes and dots with widths down to 300 nm. The corresponding complex topography profiles consist of surface elevations and depressions with sizes down to 120 nm that can be controlled by the laser fluence and fringe width, demonstrating the presence of several competing matter reorganization processes in the molten phase. The exceptional high-contrast ratio of the fringe intensity together with a near-Gaussian intensity envelope of the laser spot enable patterning with well-defined local fluences, paving the way for single-pulse fluence-dependent studies. The high-quality experimental data that can be obtained with this technique can prove critical for modelling attempts aimed at unravelling the underlying formation mechanisms of complex surface topographies in a wide range of materials. Furthermore, the technique presented here has outstanding potential for the rapid and versatile fabrication of high-precision metasurfaces.