Li2CeRbB18O30 and Li2Cs2SrB18O30: promising low-birefringence materials for UV and deep UV zero-order waveplates.

Zero-order waveplates, which are used to modulate the polarization of light, play a crucial role in scientific instrumentation, optical communications, and the laser industry. However, owing to the limitations imposed by birefringence and absorption edges, only few materials can produce desirable birefringence with suitable transparency in the ultraviolet (UV) and deep ultraviolet (DUV) regions. Borates represent promising materials with widely adjustable birefringence. In this study, we successfully synthesized two new borates, Li2CeRbB18O30 and Li2Cs2SrB18O30, using solid-state reactions in vacuum-sealed tubes; the fundamental building blocks (FBBs) of the two compounds are B3O7 and B9O19 units, respectively. Notably, Li2CeRbB18O30 and Li2Cs2SrB18O30 possess very low birefringence values of 0.0058 at 546 nm and 0.0068 at 546 nm, respectively, considerably lower than that of a commercial quartz waveplate (0.0092 at 532 nm). UV-Vis-NIR diffuse reflectance measurements show that the UV cutoff edges of Li2CeRbB18O30 and Li2Cs2SrB18O30 are lower than 320 and 200 nm, corresponding to the bandgaps of 3.20 and 6.20 eV, respectively. These two compounds can serve as potential zero-order waveplate materials in the UV and DUV regions.