High‐Quality CsPbBr3 Perovskite Films with Modal Gain above 10 000 Cm−1 at Room Temperature

Halide perovskite lasers based on CsPbBr3 micro‐ and nanoscale crystals have demonstrated fascinating performance owing to their low‐threshold lasing at room temperature and cost‐effective fabrication. However, chemically synthesized thin films of CsPbBr3 usually have rough polycrystalline morphology along with a large amount of crystal lattice defects and, thus, are mostly utilized for the engineering of light‐emitting devices. This obstacle prevents their usage in many photonic applications. Here, a protocol to deposit large‐grain and smooth CsPbBr3 thin films is developed. Their high quality and large scale allow to demonstrate a maximum optical gain up to 12 900 cm−1 in the spectral range of 530–540 nm, which is a record‐high value among all previously reported halide perovskites and bulk semiconductors (e.g., GaAs, GaN, etc.) at room temperature. Moreover, femtosecond laser ablation technique is employed to create high‐quality microdisc lasers on glass from these films to obtain excellent lasing characteristics. The revealed critical roles of thickness and grain size for the CsPbBr3 films with extremely high optical gain pave the way for development of low‐threshold lasers or ultimately small nanolasers, as well as to apply them for polaritonic logical elements and integrated photonic chips.