Birefringent crystal is an important photoelectric functional material, which is the key material to fabricate polarization devices such as polarization beam splitter, optical isolator, photoelectric modulator, etc. and widely used in the fields of laser polarization technology, optical communication, polarization information processing. With the continuous development of all-solid-state deep ultraviolet lasers (< 200 nm), it is urgent to develop suitable deep ultraviolet birefringent crystals. At present, commercial birefringent crystals mainly include YVO4, CaCO3, LiNbO3, rutile, α-BaB2O4 (α-BBO) and MgF2. Among them, α-BBO has large birefringence index, high laser damage threshold and wide transmittance range. It can be widely used in near infrared, visible and ultraviolet band high power laser systems. Notably, it is the only practical birefringent crystal material with large birefringence in ultraviolet region. However, α-BBO is difficult to apply in deep ultraviolet band due to the limitation of transmission range. MgF2 crystal is the only birefringent material used in deep ultraviolet region, whilethe birefringence of MgF2 is too small (~0.01)to beeffectively applied.
In order to explore high-performance deep-ultraviolet birefringent crystal materials, Shilie Pan research group at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science proposed[BO2] chain isthe best B-O functional unit to produce large birefringence and developed obtained the birefringent crystal Ca(BO2)2. High quality crystals with size of 50 × 41 × 22 mm3 have been grown by Czochralski method for the first time. Ca(BO2)2 has high birefringence(△n = 0.2471@193 nm) with short ultraviolet cut-off edge (169 nm), which is 20 nm shorter than that of the alpha-BBO cut-off edge (189 nm).A Gran polarizer for deep ultraviolet region based on Ca(BO2)2 birefringent crystal was fabricated for the first time,its validity was verified by 193 nm laser. The extinction ratio of the polarizer was up to 1.1×104, which can meet the practical application requirements. The study was published in the Journal of the American Chemical Society (J. Am. Chem. Soc. 2018, 140, 16311).
Recently, researchers continue to explore the borate system and design and synthesize a new deep ultraviolet birefringent crystal Li2Na2B2O5. Unlike traditional borate birefringent crystals with BO3 and B3O6 as functional units, the high birefringence of Li2Na2B2O5 is coming from parallel B2O5 unit, which provides a new idea for the design of deep ultraviolet birefringent crystals.Li2Na2B2O5 crystals with size of 35×15×5 mm3 were grown by top seed method, and their physical and chemical properties were systematically characterized. Li2Na2B2O5 has short ultraviolet cut-off edge (181 nm) and large birefringence index (0.095@532 nm), which promise it to be an exellent deep ultraviolet birefringent crystal. The study was published in the Journal of the American Chemical Society (J. Am. Chem. Soc., 2019, 141, 3258).
The research work was supported by the key projects of National Natural Science Foundation of China.
For details, please refer to:
Paper 1：https://pubs.acs.org.ccindex.cn/doi/abs/10.1021/jacs.8b10009
Paper 2：https://pubs.acs.org.ccindex.cn/doi/abs/10.1021/jacs.8b13402
(a)Photographs and optical properties of Ca(BO2)2birefringent crystal；(b) Structure and optical properties of Li2Na2B2O5birefringent crystal
