LAUSR

Controllable optical properties are crucial for the application of light‐emitting materials in optical devices. In this work, controllable photoluminescence in metal halide perovskite crystals is realized via photoactivation of their defects. It is found that under continuous excitation, the photoluminescence intensity of a CH3NH3PbBr3 crystal can be fully controlled by sub‐bandgap energy photon illumination. Such optically controllable emission behavior is rather general as it is observed also in CsPbBr3 and other perovskite materials. The switching mechanism is assigned to reversible light‐induced activation/deactivation of nonradiative recombination centers, the presence of which relates to an excess of Pb during perovskite synthesis. Given the success of perovskites in photovoltaics and optoelectronics, it is believed that the discovery of green luminescence controlled by red illumination will extend the application scope of perovskites toward optical devices and intelligent control.