Polygon-facilitated generation of colloidal gold nanoparticles by multi-MHz ultrashort-pulse laser trains: key optical factors

Abstract

Abstract Generation of colloidal gold nanoparticles was for the first time studied at ultra-high, multi-MHz repetition rates of 10 ps, 1030 nm laser pulses, ablating 70 nm gold film in water at the constant average power of 270 W and focusing conditions. Ultrafast (550 m s−1) laser scanning of gold films was facilitated by a polygon reflector, provided single-shot per spot through ablation of the films. According to our hypothetical model, the rate-dependent exposure of the films at the constant average power resulted in four-fold increase of the optical density of the colloidal solutions in the interband-transition spectral range (about 400 nm) due to the increasing number of incident pulses over the fixed area of 1 cm × 1 cm square, despite the decreasing ablation rate at lower, inversely-proportional fluences and dynamic overlap of plume-induced bubbles.