Investigation of the effects of thermally-induced band gap modification on the size and shape of modification regions formed in ultrafast laser bonding

Experimental observation and numerical calculation of the nonlinear absorptivity of Corning^® EAGLE XG^® Glass substrates under ultrafast laser irradiation at high repetition rates are presented in this work. The temperature-dependent material band gap and absorption spectrum are obtained using a quantum mechanics-based computational methodology within density functional theory. The modeling predicts an increase of the multiphoton absorption coefficient and linear thermal absorption at high temperatures due to a reduction of the band gap. The impact of thermally-induced absorption at high substrate temperatures is investigated, which allows for a more accurate prediction of heat accumulation and welding geometry in ultrafast laser welding process.