On the Origin of Room-Temperature Amplified Spontaneous Emission in CsPbBr₃Single Crystals

CsPbBr₃is an all-inorganic halide perovskite with excellent photoluminescence (PL) properties for laser applications. Amplified spontaneous emission (ASE) is a prerequisite for lasing and typically observed from low-dimensional CsPbBr₃nanostructures, where quantum confinement enhances ASE. However, a gain medium for lasing should be prepared into a robust bulk form that works under intense light illumination. Here, we demonstrate that bulk CsPbBr₃single crystals exhibit highly efficient ASE with a threshold of 46 MWcm^-2at 520 nm, if PL reabsorption via the indirect Rashba gap is properly suppressed by thickness control. Based on a series of spectroscopic and microscopic measurements, we show that this below-the-gap absorption can significantly alter the PL feature and even the apparent color of the crystal depending on the crystal size. Our results show that a thin CsPbBr₃single crystal can be utilized for lasing applications when engineered into a submillimeter thickness for effective light-matter interaction.