Heat‐Assisted Reversal of Femtosecond Laser‐Induced Birefringence

Precisely embedding designed nanostructures in homogeneous materials is essential for the development of high‐performance photonic devices. However, existing techniques typically struggle to achieve simultaneous control over location and yield, limiting their use in optical data storage and integrated photonics. Here, we report an observation of a heat‐assisted birefringence orientation reversal (HABOR) phenomenon in lithium disilicate glass. Based on this phenomenon, we propose a method that enables precise spatial control of crystallization while maintaining high yield. The method employs femtosecond laser direct writing combined with heat treatment, facilitating oriented nanocrystal growth with an intriguing reversal of the birefringence slow‐axis orientation. In particular, the oriented nanocrystals produce birefringence retardance up to 35.11 ± 2.34 nm, which is 6.80 times higher than that achieved by femtosecond laser direct writing. Their birefringence shows excellent thermal stability, with retardance remaining relatively stable over 24 h at 700°C. These superior properties enable applications in 5D optical data storage, fabrication of arbitrary retarders and varying optical elements.