Mechanism and optimization of femtosecond laser welding fused silica and aluminum

•The pump–probe technique is firstly applied at optical connect interface. After 1 ns, the reflectance rose again due to the local non-equilibrium state.•For the first time, we captured plasma behaviors strongly converging at optical contact interface and diffusing along the non-optical contact interface.•Focal point pre-compensation was used to increase energy deposition at the interface and the maximum shear strength reached up to 25.75 MPa.