Designing an Excellent Deep-Ultraviolet Birefringent Material for Light Polarization

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.