“…Optical components such as optical circulator, − beam splitters, ,− electro-optics Q switch, optical isolators, , achromatic quarter waveplates ,, and phase compensators ,, are realized using birefringent crystals. Currently, noncentrosymmetric oxides have been extensively investigated as outstanding birefringent materials. ,, Oxide birefringent crystals such as TiO 2 , YVO 4 , LiNbO 3 , CaCO 3 , and α-BaB 2 O 4 , with birefringence ranging from 0.256 to 0.122 in the visible range are used commercially. , In spite of a few exceptions ,− birefringent materials have been almost exclusively limited to oxides to date. − Recently, halide perovskites emerged as potential energy materials owing to the impressive optical and electronic properties. − In comparison, the possibility of halide perovskites being birefringent materials has long been overlooked. Recently, birefringence was observed from the mixed halide or hybrid halide perovskite materials. ,,, Chen et al reported the fabrication of achromatic quarter-wave plates using inorganic Cs 4 PbBr 6 crystals embedded with perovskite CsPbBr 3 nanocrystals .…”