![]() On the contrary, spatially selective crystallization of glasses by continuous-wave and nanosecond lasers requires a certain level of absorption at the fundamental wavelength and is basically suitable for growing crystalline patterns at the glass surface though the growth of crystalline line from the surface to the inner parts of the glass volume was also demonstrated. ![]() ![]() In particular, these architectures can be represented by continuous or, in some cases, nanoperiodical crystalline lines with a few μm sized cross-section and oriented crystalline structure formed by nonlinear optical phases such as, in particular, LaBGeO 5, LiNbO 3, Ba 2TiSi 2O 8, Sr 2TiSi 2O 8, Pb 5Ge 3O 11, Li 2Ge 7O 15. Due to the nonlinear character of absorption of focused ultrashort laser pulses confined near the beam waist, femtosecond direct laser writing (DLW) is favorable for growing crystalline architectures inside transparent glasses. This phenomenon is widely studied due to the possibility of precipitating non-centrosymmetric phases, including nonlinear optical crystals, which could be a base for functional photonic components integrated in glass. Among other modification types, the ability to locally crystallize glasses by the focused beams of various lasers drew considerable attention. The unique ability of high-precision spatially selective processing of the inside of transparent materials has made ultrafast lasers an efficient tool for the fabrication of integrated optical and photonic components and devices. ![]() The proposed technique suggests a new way to control the morphology of laser-written crystal-in-glass waveguides by tailoring their cross-section, which is expected to improve the mode structure of the guided light. Thermal-induced stress emerging during the tailoring procedure was efficiently eliminated by fine post-annealing. At optimized laser processing parameters, most of the track was vitrified, and the residual part of the crystalline cross-section had an aspect ratio of about 1:1. The sinusoidal path was shown to be favorable for tailoring the improved cross-section of the crystalline lines by partial remelting. To form a smoother temperature field, the beam waist was moved along the helical or flat sinusoidal path along the track. Exposure to femtosecond laser pulses at 200 kHz repetition rate provided cumulative heating of the sample in the vicinity of the beam waist sufficient to provide space-selective melting of crystalline LaBGeO 5. Here, we investigated the conditions of partial remelting of laser-written LaBGeO 5 crystalline tracks in lanthanum borogermanate glass by the same femtosecond laser beam which had been used for their writing. However, femtosecond-laser-written continuous crystalline tracks typically have an asymmetric and strongly elongated cross-section, which causes a multimode character of light guiding and substantial coupling losses. ![]() They are regarded as promising components for novel integrated optical circuits. Space-selective laser-induced crystallization of glass enables direct femtosecond laser writing of crystal-in-glass channel waveguides having nearly single-crystal structure and consisting of functional phases with favorable nonlinear optical or electrooptical properties. ![]()
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