Submicron bubbles/voids formation in the subsurface region of soda-lime glass by single pulse fs laser-induced spallation

The formation of bubbles near the surface of soda-lime glass induced by single pulse irradiation with Ti: Sapphire laser of a wavelength of 775 nm and 150 fs pulse duration below the laser ablation threshold is reported. The bubbles with a diameter of 0.3-3 mu m are much smaller than the spot size of similar to 12 mu m and exhibit a semi-spherical surface, an ellipsoidal void and an upper shell thickness of approx. 100 nm. The bubbles are formed in a narrow energy range between bulk glass modification and glass ablation. The bubbles formation is accomplished with some changes of the glass morphology around the bubbles. Damaged bubbles occur near the upper laser energy limit for bubbles formation that show nanoholes or slits in the bubbles shell and opened shells exposing the bottom of the void. It is suggested, that the bubble forming mechanism is related to nonlinear processes of laser photon absorption, laser-induced spallation processes, materials heating after electron relaxation, and thermal driven, viscoplastic-mechanical processes. Mainly the material composition gradient of the soda-lime glass near the surface that affects photon absorption, melting, ablation and material expansion processes enable the formation of these bubbles. The void inside the bubble is formed first by pre-spallation resulting in a detached layer that is lifted further forming the bubbles shell by subsurface laser ablation/evaporation processes of the bulk glass. The upper shell of the bubble is formed by a thermally stable crust of corroded soda-lime glass. The results may have valuable impact to laser ablation processes for high precision fabrication. Further, the fast fabrication of such 3D submicron structures due to the single pulse interaction with the glass enables new technical processes for potential applications in sensing, photonic or microfluidic.