Distinct Tunable In‐Plane Birefringence via Domain Manipulation in a 2D Metal Halide Ferroelectric

Single crystals with tunable birefringence hold an exciting position in optical and photonic devices due to their exceptional ability of light manipulation. Although ferroelectric domains have been utilized to regulate physical properties, studies on tuning their birefringence by virtue of domain structure remain largely scarce. Herein, we have demonstrated the modulation of in‐plane birefringence through manipulating domain structures in a 2D metal halide ferroelectric, (2‐MBA) 2 PbCl 4 ( 1 , 2‐MBA = 2‐methylbutylamine), which exhibits a phase transition at 314 K with spontaneous polarization of 1.6 µC/cm 2 . Crystal 1 exhibits controllable birefringence that can be modified by thermal and optical stimuli; this behavior directly involves with its ferroelectric properties. Notably, we have achieved unusual in‐plane birefringence tuning via domain structure, which is further modulated by coupling domain manipulation with heat, light, and mechanical pressure. This work provides a new strategy for controlling birefringence, and advances the development of ferroelectric‐based photonic devices for data storage and integrated optoelectronics.