Ba₂BeB₂O₆: A True Zero-Order Waveplate Material with Minimal Birefringence Achieved via Structural Regulation

Waveplates are commonly used as phase retarders in the field of optics, capitalizing on their inherent birefringence to manipulate the polarization state of light. True zero-order waveplates exhibit superior retardation stability against the wavelength shift, ambient temperature change, and incident angle deviation compared to other waveplates. However, the fabrication of true zero-order waveplates is challenging, often necessitating materials with minimal birefringence. In this study, we proposed a novel strategy to achieve birefringence regulation by adjusting the angle between dangling [BO₃] groups and intralayer [BO₃] groups in Ba₂ZnB₂O₆. We have successfully synthesized a new compound, Ba₂BeB₂O₆, which demonstrates an extremely small birefringence (n - n = 0.0002 at 532 nm). This material is suitable for fabricating true zero-order half-waveplates (λ/2 plates) with a thickness of 1.33 mm at a wavelength of 532 nm. Additionally, we synthesized the compound Ba₂BeZn_1-B₂O₆ (x = 0.5), enabling continuous modulation of birefringence. This work provides a promising pathway for the development of advanced waveplate materials with tailored optical properties.