Opposite and reversible optical switching in soft-lattice halide perovskites using light and heat.

Halide perovskites have soft lattices with strong interactions between electronic and structural degrees of freedom, making them promising for next-generation optoelectronics. Recent studies have uncovered their unique photo- and thermally induced optical behavior, yet the potential for dynamic, optical switching remains underexplored. Here, we demonstrate reversible and diametrically opposite optical switching in single-crystal cesium lead bromide (CsPbBr3), driven independently by either above-bandgap illumination or thermal excitation. We measure tunable changes in mid-infrared transmission from 3 to 20 micrometers, exceeding 10% at 20 micrometers, with opposite polarity under light and heat, and modulation times <1 millisecond. This antithetical behavior arises from a polaron-like lattice distortion under illumination (reducing transmission), and from lattice expansion from heating partnered with the material's negative thermo-optic coefficient (increasing transparency). Our findings establish inorganic halide perovskites as a viable material platform for reconfigurable photonics where transmission can be controlled independently through distinct mechanisms intrinsic to their soft lattices.