Formation of disruptions in molten fused silica induced by heat accumulation of ultrashort laser pulses at high repetition rates

The absorption of ultrashort laser pulses at high repetition rates leads to heat accumulation within the irradiated material. Thereby, the focal region as well as a well-defined vicinity of the surrounding material is molten. This method can be used for local bonding of transparent materials. Figure 1 a) shows a top view of laser molten material within fused silica. Within the traces of the molten material, large disruptions can be found as darks spots. The shape of the molten material narrows after the formation of a disruption, which indicates an interruption of the laser heating. Thus, these disruptions induce tensions and reduce the strength of the bonded material. The appearance and periodicity of these micro-bubbles depend on the processing parameters. In addition, disruptions are always generated at the end of a molten line when the laser irradiation stops. So far, the origin and inner structure of these defects were unknown. We investigated the structure and periodicity of these disruptions in fused silica. In addition, we propose a model, which explains their formation.