Illumination-Induced Transparency in 2D Layered Perovskites via Lattice Energy Reservoirs

We report the first observation of illumination-induced transparency (IIT) in two-dimensional Ruddlesden-Popper halide perovskite single crystals. Under continuous excitation, the crystal becomes progressively transparent to its own photoluminescence (PL), particularly near the absorption edge where reabsorption is initially strong. The effect evolves over seconds, shows an excitation fluence threshold and persistence. We attribute IIT to the activation of lattice energy reservoirs (LERs), which are spatially non-uniform nanodomains that store phonon energy as elastic potential under optical excitation. These LERs enable subgap carrier upconversion thus generate photon gain that compensates the intrinsic absorption. The effect reveals a previously unrecognized optoelectronic behavior in 2D perovskites, governed by dynamic lattice–carrier interactions rather than electronic band filling. This phenomenon demonstrates a new light–matter interaction regime in perovskites and highlights LERs as optically accessible energy gateways. Our findings suggest new routes toward photon-recycling, upconversion, and reconfigurable photonic materials based on soft-lattice semiconductors.