Direct laser writing in YAG single crystal: Evolution from amorphization to nanograting formation and phase transformation

Femtosecond laser writing is a versatile and effective technique for high-precision 3D micromachining of crystals for applications in photonics. Here, we demonstrate the femtosecond laser-assisted control over the structure and phase composition in yttrium aluminum garnet (YAG) single crystal. Increasing the energy of the laser pulse enables achieving extremely high temperature and pressure in the laser beam waist region, which leads to pronounced plastic deformation of the crystal structure and even to its complete amorphization inside forming nanogratings or a phase transition from the garnet phase to the perovskite phase. The main types of laser-induced modifications in YAG single crystal are described using quantitative phase microscopy, quantitative birefringence imaging and transmission high-resolution electron microscopy. For the first time, the mechanism of plastic deformation responsible for the formation of dislocations and amorphous phase is experimentally confirmed. The results obtained contribute to the deeper understanding of fundamentals of the direct laser writing in crystals which are essential for the optimization of selective etching processes and fabrication of photonic crystal waveguides.