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.