Challenges of laser-induced single crystal growth in glass: nonstoichiometric matrix composition and laser scanning rate

Practical applications of the recently demonstrated method of growing single crystal architecture in glass by femtosecond laser demand answers to two key questions: 1. How is the formation of a single crystal affected by the non-congruent 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 La2O3 concentration on the crystallization of LaBGeO5 in xLa2O3-B2O3-2GeO2 glass as a model system that has been studied most extensively. The results show that for stoichiometric and La2O3-rich glass, a scanning speed above a critical value is required for fabricating LaBGeO5 single crystal, while the opposite is true for the La2O3-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.