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Linking energy density with Morphology in laser grooving of sapphire

: Sakurai, H.; He, C.; Konishi, K.; Tamaru, H.; Yumoto, J.; Kuwata-Gonokami, M.; Gillner, A.


Institute of Electrical and Electronics Engineers -IEEE-:
Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference, CLEO/Europe-EQEC 2017 : 25-29 June 2017, Munich, Germany
Piscataway, NJ: IEEE, 2017
ISBN: 978-1-5090-6736-7
ISBN: 978-1-5090-6737-4
Conference on Lasers and Electro-Optics Europe (CLEO) <2017, Munich>
European Quantum Electronics Conference (EQEC) <2017, Munich>
Fraunhofer ILT ()

Summary form only given. With the maturing of laser technologies, laser processing is presenting itself as an attractive alternative to traditional machining techniques due to its free-form, non-contact nature. It is believed to be especially relevant for hard and brittle materials, such as sapphire. However, due to the complex nature of the physical processes involved, especially for transparent materials, optimization of laser processing procedures often requires tedious amounts of trial-and-error work. In such regards, scaling relationships prove to be an invaluable resource from both a basic science and application standpoint, as they not only reveal physical relationships which aid in the development of theoretical models, but also reduce the experimental trial space for optimization. In this study, we introduce a new scaling relationship that we have observed during systematic studies on the morphology dependence of grooves cut into crystalline sapphire plates. In the experiment, 7 ps, 515 nm ultrashort laser pulses at 50 kHz repetition rate were introduced into a rotating dove prism based helical drilling optic setup [1,2], and focused onto the sample surface. The helical drilling optic allowed us to create highly compact and uniform circular grooves with various interpulse overlaps. Incident laser fluence was varied, with all conditions above the single pulse damage threshold for sapphire. To simplify analysis, the laser was switched so that only one pass was made in the circular trajectory, and all groove depths were an order of a magnitude smaller than the Rayleigh length of the pulse, allowing us to ignore defocusing effects. Morphology of these grooves were characterized by a laser scanning microscope, which enabled us to retrieve three-dimensional information of the machined surface. Fig. 1(a) and (b) show microscope images of typical results. They are believed to correspond respectively to the well-known “gentle” and “strong” ablation phases of sapphire [3].