Owing to their vital role in creating and controlling polarized light, birefringent materials are used extensively in various advanced optical systems which in turn impact a large, rapidly increasing range of applications in science and technology. Currently, the fairly small birefringence of MgF₂ and the low transmittance of α-BaB₂O₄ (α-BBO) hinder their efficient application for wavelength below 200 nm. For example, deep-ultraviolet (DUV) birefringent materials for light polarization are urgently needed for DUV lithography. Here we demonstrate based on computational and experimental results that parallel chains of corner-connected planar sp²-hybridized BO₃ groups found in Ca(BO₂)₂ effectively produce large birefringence. Ca(BO₂)₂ achieves three vital "best" properties including the shortest UV cutoff edge, the largest birefringence, and the highest laser-induced damage threshold (LIDT) compared to all the reported borate birefringent materials. On the basis of a Ca(BO₂)₂ single crystal, a DUV Glan polarizer has been realized and is more efficient than one constructed with commercially available MgF₂.
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Chen et al. (2018) studied this question.
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