Accumulation of thermal energies by highly repeated irradiation of femtosecond laser pulses inside a glass induces the heat-modification whose volume is much larger than that of the photoexcited region. It has been proposed that the heat-modification occurs in the region in which the temperature had overcome a threshold temperature during exposure of laser pulses. In order to understand the mechanism of the heat-modification, we investigated the temperature distribution during laser exposure and the threshold temperature by analyzing the volume of the modification based on a thermal diffusion model. We found that the threshold temperature becomes lower with increasing laser exposure time. The dependence of the threshold temperature on the laser exposure time was explained by the deformation mechanism based on the temperature-dependent viscosity and viscoelastic behavior of a glass under a stress loading by thermal expansion. The deformation mechanism also could simulate a tear-drop shape of a heat-modification by simultaneous double-beams’ irradiation and the distribution of birefringence in a heat-modification. The mechanism proposed in this study means that the temperature-dependence of the viscosity of a glass should be essential for predicting and controlling the heat-modification.
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