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Spotlight on Optics


  • November 2013

Optics InfoBase > Spotlight on Optics > Modulation of laser induced-cracks inside a LiF single crystal by fs laser irradiation at multiple points

Modulation of laser induced-cracks inside a LiF single crystal by fs laser irradiation at multiple points

Published in Optics Express, Vol. 21 Issue 22, pp.26921-26928 (2013)
by Masaaki Sakakura, Yuki Ishiguro, Naoaki Fukuda, Yasuhiko Shimotsuma, and Kiyotaka Miura

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Spotlight summary: One of the most attractive and unique capabilities in laser material processing is that of selectively modifying the optical and physical properties of materials through structural changes constrained to within the focal volume. Femtosecond (fs) laser processing in transparent materials has received special attention since the laser beam can be tightly focused, and therefore induce various nano and microscale structures such as cracks, dislocations, and voids inside the material without compromising the properties of other regions not so highly excited by the laser, including the material’s surface. Such structural changes inside the material are extremely difficult to achieve through other processing technologies. To make use of this unique benefit of fs laser processing with a higher precision, the size and shape of the structures need to be well controlled, since they critically affect the volume and properties of the laser-processed region. Therefore, finding new ways for accurately controlling these structures and achieving a deeper understanding of the mechanisms that lead to structural changes following fs laser pulse irradiation is of great importance.

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.

--Taek Yong Hwang

Technical Division: Optoelectronics
ToC Category: Lasers and Laser Optics
OCIS Codes: (140.3390) Lasers and laser optics : Laser materials processing
(140.7090) Lasers and laser optics : Ultrafast lasers
(160.3220) Materials : Ionic crystals
(320.5390) Ultrafast optics : Picosecond phenomena
(320.7130) Ultrafast optics : Ultrafast processes in condensed matter, including semiconductors
(070.6120) Fourier optics and signal processing : Spatial light modulators

Posted on November 15, 2013

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