Polarization‐Dependent Large Photorefractive Effect In A Wide Bandgap 2D Metal Halide Ferroelectric

Abstract Photorefractive effect of ferroelectrics refers to the light‐induced change of refractive index, which is an optical controlling avenue in holographic storage and image processing. For most ferroelectrics, however, the small photorefractive effect (10 −5 –10 −4 ) hinders their practical application and it is urgent to exploit new photorefractive system. Here, for the first time, strong photorefractive effects are achieved in a 2D metal‐halide ferroelectric, [CH 3 (CH 2 ) 3 NH 3 ] 2 (CH 3 NH 3 )Pb 2 Cl 7 (1) , showing large spontaneous polarization (≈4.1 µC cm −2 ) and wide optical bandgap (≈3.20 eV). Notably, under light irradiation, 1 enables a large variation of refractive indices up to ≈ 1× 10 −3 , being one order higher than the existing materials and comparable to the state‐of‐the‐art inorganic ferroelectrics. This intriguing photorefractive behavior involves with the sharp variation of polarization caused by photo‐pyroelectricity. As the first report of 2D metal‐halide photorefractive ferroelectric, this work sheds light on optical controlling of physical properties in electric‐ordered materials.