Ultrashort pulse laser-induced submicron bubbles generation due to the near-surface material modification of soda-lime glass

Abstract

Experimental evidence for near-surface micro- and nanoscopic processes at ultrashort pulsed laser irradiation(wavelength 775 nm, pulse duration 150 fs) of soda-lime glass near the laser ablation threshold is acquired by transmission electron and secondary electron microscopy (SEM, TEM) as well as by energy disperse x-ray spectroscopy (EDX) investigations of laser-induced subsurface bubbles. Well separated laser irradiations result in individual micrometre bubbles while submicron bubble groups or asymmetrically damaged bubbles appear for overlapping pulses. The overlapping laser irradiations result in a direction dependent damage for the successive bubbles when the inter-pulse distance is small. TEM/SEM studies of cross sectioned bubble prove that precipitations and jets are formed in the centre of the micron bubbles’ bottom. This nano-jets inside the bubble can be related to the self-focusing of the femtosecond laser. A modified layer with a composition deviation from the bulk glass can be a key factor for the formation of the bubble with a thin shell about 100 nm. Moreover, next to the central main bubble, nanometre sized bubbles appear below the surface. Thermal processes inside the bubble cause a smooth inner surface as well as redeposited material mainly opposite to the jet formed at the bottom. The different glass composition near the surface strongly suggests that the altered glass surface composition contributes to the forming mechanism of these near-surface features. Implications of these sub-surface bubbles and jet formation for laser ablation related applications comprise incubation as well as roughness development that limits the achievable precision.