Lithium niobate crystals were grown in 3D through localized heating by femtosecond laser irradiation deep inside 35Li2O-35Nb2O5-30SiO2 glass. Laser scanning speed and power density were systematically varied to control the crystal growth process and determine the optimal conditions for the formation of single crystal lines. EBSD measurements showed that, in principle, single crystals can be grown to unlimited lengths using optimal parameters. We successfully tuned the parameters to a growth mode where nucleation and growth occur upon heating and ahead of the scanning laser focus. This growth mode eliminates the problem reported in previous works of non-uniform polycrystallinity because of a separate growth mode where crystallization occurs during cooling behind the scanning laser focus. To our knowledge, this is the first report of such a growth mode using a fs laser. The crystal cross-sections possessed a symmetric, smooth lattice misorientation with respect to the c-axis orientation in the center of the crystal. Calculations indicate the observed misorientation leads to a decrease in the refractive index of the crystal line from the center moving outwards, opening the possibility to produce within glass a graded refractive index single crystal (GRISC) optically active waveguide.

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Laser heating of chalcogenide glasses has successfully produced rotating lattice single crystals through a solid-solid transformation. To better understand the nature of complex, orientation-dependent lattice rotation, we designed heat profiles of the continuous wave laser by beam shaping, fabricated larger Sb2S3 crystal dots in Sb2S3 glass, and investigated the lattice rotation where the crystal could grow in all radial directions under a circular thermal gradient. The results show that the rate of lattice rotation is highly anisotropic and depends on crystallographic direction. The nature of this rotation is the same in crystals of different orientation relative to the surface. The growth directions that align with the slip planes show the highest rate of rotation and the rotation rate gradually decreases away from this direction. Additionally, the presence of multiple growth directions results in a complicated rotation system. We suggest that the growth front influences the density of dislocations introduced during growth under confinement and thus affects the lattice rotation rate in these crystals.

This study explores multiple fields as a mechanism of controlling the orientation of the nucleation process, which is generally believed to be a stochastic process, as observed in the equiaxed grain structure of glass ceramics. Specifically, as a model system, Sb2S3 single crystals are grown on the surface of Sb-S-I chalcogenide glass via laser crystallization. A spatial light modulator is used to configure the beam shape, thermal gradient, and/or polarization of the laser for controlling the orientation of Sb2S3 crystal seeds. We determine that the chemical and thermal gradient can affect the crystal morphology but do not significantly influence the crystal seed orientation. In contrast, the short crystal axis along [010] has a preferential alignment along the direction of polarization and perpendicular to the surface normal. Thus, we conclude that polarization can be used as an effective tool for controlling the nucleation and hence the orientation of Sb2S3 crystals on the surface of Sb-S-I chalcogenide glasses.

Practical applications of the recently demonstrated method of growing single-crystal architecture in glass by a femtosecond laser demand answers to two key questions: (1) How is the formation of a single crystal affected by the incongruent composition of glass? (2) Why are laser scanning speeds higher than a critical value necessary for obtaining single crystals in some cases, whereas smaller than a critical value are required in other systems? We have sought answers to both of these questions by investigating the effect of La 2 O 3 concentration on the crystallization of LaBGeO 5 in xLa 2 O 3 −B 2 O 3 −2GeO 2 glass as a model system that has been studied most extensively. The results show that for stoichiometric and La 2 O 3 -rich glass, a scanning speed above a critical value is required for fabricating LaBGeO 5 single crystal, while the opposite is true for the La 2 O 3 -deficient composition. The origin of these divergent behaviors is shown to be in the relative values of nucleation and growth rates and their temperature dependencies. This first systematic study illustrates the complex role of composition on the nucleation and growth mechanisms and provides a framework for determining the processing parameters needed for fabricating single-crystal architecture in a glass.

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