“…Positive type I and negative type II index modifications are associated, respectively, to local densification and expansion. While laser‐induced local expansion can have a thermodynamic origin, as the local temperature increases sufficiently to melt the glass or drive a thermal expansion in solid phase, this is presumably not the case of ultrafast laser‐induced local densification, which is mainly a purely photo‐induced effect. It has been reported in the literature that the capability of a ChG glass structure to evolve toward strongly or weakly densified regions, when irradiated with ultrafast laser pulses, depends on its initial structural flexibility (or more specifically, constraints imposed to the structural flexibility), where a high or low degree of flexibility is associated, respectively, to a high or low initial volume (or enthalpy), and it is strongly linked to the connectivity of the microscopic glass structure .…”