Recently, the authors of this article showed that thermal stress waves can be generated inside rock salt crystals following fs laser pulse irradiation. These waves then propagate away from the fs laser-excited region, and subsequently induce permanent dislocations in certain orientations of the crystal structure. Cleverly using their previous discoveries, the authors reveal in this Optics Express article how the fs laser-induced dislocations and thermal stress waves affect the formation of fs laser-induced cracks, and find a way to control the size and shape of these cracks in LiF by irradiating with three pulses, either sequentially or simultaneously. First, to understand the effects of dislocations on the formation of fs laser-induced cracks, two sequential pulses are applied inside the LiF crystal, producing dislocation bands in the crystal. A third pulse is then applied for which the dislocation bands can either affect or not the formation of cracks, showing that these bands, due to the higher mechanical strength, can prevent the formation of fs laser-induced cracks. Next, the authors use three simultaneous pulses instead, and control the interference of the fs laser-induced stress waves where the crack formation is expected. If compression waves are generated by the interference of the pulses and propagate from the tip to the center of a fs laser-induced crack in the time range of 2400-3000ps, the formation of fs laser-induced cracks is highly disturbed by the compression waves, and the length of the crack is shorter than the one without the waves. On the other hand, with a different distribution of the three simultaneous pulses, the authors generate tensile stress near the crack at 3500ps without strong compression waves, and find that this tensile stress helps the propagation of the crack, leading to the production of longer fs laser-induced crack.
In summary, the authors demonstrate sophisticated control of fs laser-induced cracks by using the permanent and transient change of mechanical properties in LiF: the dislocation bands and thermal stress waves, induced by fs laser pulse irradiation. The results reported in this article not only show control of fs laser-induced cracks but also potentially provide a new way of controlling many other types of fs laser-induced structures, possibly leading to the generation of new optical and physical properties in materials.
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