Femtosecond laser-writing of 3D crystal architecture in glass: Growth dynamics and morphological control

Abstract

A carefully controlled femtosecond laser irradiation offers the possibility of fabricating high quality, 3D functional single-crystal patterns designed for integrated optical devices deep inside glass. Notwithstanding, a general theory relating particular irradiation conditions to crystal growth dynamics and resulting morphologies remains to be established. To overcome this challenge, crystal lines written in glass under varying conditions are investigated in this work by optical microscopy, scanning electron microscopy, and electron backscatter diffraction in the congruently crystallizing LaBGeO5 model system. We show that despite a strongly preferential orientation of the optic axis along the direction of laser scanning, typical morphologies of fs laser-written crystals in this system are inhomogeneous due to low-angle grain boundaries and rotations or inversions of the preferential axis. Nevertheless, uniform single crystals can be produced under optimized irradiation conditions. The theoretical dynamics of crystal growth is developed within the unique environment of a moving laser-induced melt to explain these results, including the role of competitive growth in establishing the preferential lattice orientation; the effects of temperature gradient and focal scan rate in establishing crystal morphology and cross-section shape; and most notably, the specific design criteria that must be met for stable single-crystal growth.