Fabrication of a monolithic all-YAG crystalline microresonator through femtosecond laser nanolithography and thermal annealing

• Demonstrated straightforward fabrication of monolithic all-YAG crystalline microdisks. • Achieved surface tension reshaping of YAG microdisks through thermal annealing. • Thermal annealing enhances YAG microdisk Q factor by more than twofold. • YAG microdisks remain robust with no morphological changes up to 1180 °C. We report the first successful fabrication of monolithic all Yttrium Aluminum Garnet (YAG) crystalline microdisks directly in the bulk of a crystal, through a greatly simplified two-step process of three-dimensional femtosecond laser writing followed by chemical wet-etching, as compared to current multi-step approaches. The fabricated and optically characterized microdisk is 16.9 μm in diameter and 0.8 μm in thickness. We also present the first systematic study of surface tension reshaping in YAG by means of thermal annealing, identifying an optimal annealing temperature of 1475 °C for 5 h for smoothening surface irregularities. Optical characterization using tapered fiber loop evanescent coupling revealed a more than twofold improvement in the intrinsic quality factor of the Whispering Gallery Mode resonances, increasing from 3.6 × 10³ to 9.5 × 10³ after annealing. Furthermore, the YAG microdisks demonstrated outstanding thermal robustness, showing no observable morphological changes up to 1180 °C. This work establishes a robust and straightforward platform for fabricating monolithic inside-crystal YAG microresonators, enabling their application on chip-scale solid-state lasing and extreme environment sensing.