Solution-processed inorganic perovskite crystals as achromatic quarter-wave plates

Waveplates are widely used in photonics to control the polarization of light^1,2. Often, they are fabricated from birefringent crystals that have different refractive indices along and normal to the crystal axis. Similar optical components are found in the natural world, including the eyes of mantis shrimp^3,4 and the iridescence of giant clams⁵, fish⁶ and plants⁷. Optical retardation in biology relies on sophisticated self-assembly, whereas man-made systems comprise multiple-layered materials^8–11. Here we report a discovery that bridges these two design principles. We observe wideband achromatic retardation in the visible and near-infrared (532–800 nm) regions for Cs₄PbBr₆ perovskite crystals embedded with CsPbBr₃ nanocrystals. We explain our observations as matched dispersions of the refractive indices of the ordinary and extraordinary rays caused by the ordered embedding of the nanocrystals in the host. The wideband performance and ease of fabrication of these perovskite materials are attractive for future applications.