Using the well-known LiB 3 O 5 as the template, by improving the angle between B 2 O 5 units in its structure to reduce birefringence, two novel birefringent crystals Li 2 MLaB 18 O 30 (M = Rb, Cs) for preparing zero-order wave-plates are obtained by a high-temperature solution method in a closed system. Interestingly, the neighboring B 2 O 5 units are aligned almost perpendicularly, making them exhibit tiny birefringence, which is much smaller (0.0032@532 nm and 0.0027@532 nm, respectively) than that of the commercial quartz crystal (Δ n ~0.0092@visible region). Moreover, the ultraviolet cutoff edges of Li 2 MLaB 18 O 30 (M = Rb, Cs) crystals are below 200 nm, demonstrating that they can be applied as zero-order waveplate materials in the deep-ultraviolet region.
Qian Wang,Fei Yang,Xing Wang,Jing Zhou,Jia Ju,Ling Huang,Daojiang Gao,Jian Bi,Guohong Zou
By introduction of K+, Rb+, and Cs+ cations into the classical commercial nonlinear optical crystal LiB3O5 (LBO), the series of novel mixed-alkali-metal borates Li2.6K0.4[B5O8(OH)2] (K-LBO), Li2.85Rb0.15[B5O8(OH)2] (Rb-LBO), and Li2.9Cs0.1[B5O8(OH)2] (Cs-LBO) have been obtained under hydrothermal conditions. The steric hindrance effect generated by the introduction of large alkali-metal cations and partial substitution of small Li+ cations broke the three-dimensional (3-D) framework of [B3O7]5– borate–oxygen clusters in LBO and resulted in a structure rearrangement to produce infrequent [B10O26]22– 2-D layers. The unique layered structure induced an increase in birefringence in A-LBOs (A = K, Rb, Cs), which is favorable for phase matching during second-harmonic generation. All three compounds are potential deep-ultraviolet nonlinear optical materials, which was proved by UV–vis–NIR diffuse reflectance spectroscopy and second-harmonic-generation measurements.
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