Internal modification morphologies in glasses irradiated by nanosecond laser pulses

The absorption and heat accumulation of successive laser pulses with nanosecond pulsed width and small numerical aperture focused in the glasses results in the melting of the glass materials. The features of the inscribed internal modification marks within the trace along the focus axis are analyzed under different operational parameters. A dynamic absorptivity evaluation measured the ratio of the transmitted energy to the incident energy is conducted. It is found that the feature of mark extends from a thread to a baseball bat, which includes three individual features of a thread, a cascade, and a molten region appeared clearly with the increase of the laser input energy. The mark length increases with the increases of both the pulsed energy at the fixed frequency and pulsed frequency at a specific energy. The mark length increases significantly with the number of pulses below the 20 shots, but approaches the plateau when the number of pulses are over 40 shots. The mark width increases with the number of pulse increase, but decreases when a black spot appears at near top of the modification mark. The intensity of peaks measured at 580 and 1118 cm −1 of the irradiated modification marks is smaller than that of unprocessed area revealed by the Raman spectrum. The peak position is found to slightly shift to low frequency for the irradiated mark. The dynamic absorptivity increases abruptly with increase of the pulsed energy and reaches a plateau. The maximum allowable pulsed energy to evaluate the dynamic absorptivity decreases with the increase of the pulsed frequency. The thermal diffusion model is provided to interpret qualitatively the observed scenery of the variation of mark width with the number of pulses in the experiment for a pulsed energy and a frequency. A mechanism is proposed to interpret the collaboration of the multi-pulses into a modification mark and the increase of interspacing distance between the marks with the scanning velocity. Finally, a relationship between the fracture shear strength and the focused depth beneath the interface of the glass substrates in the spot welding is experimentally reported.