LAUSR

Room‐temperature interaction between light–matter hybrid particles such as exciton–polaritons under extremely low‐pump plays a crucial role in future coherent quantum light sources. However, the practical and scalable realization of coherent quantum light sources operating under low‐pump remains a challenge because of the insufficient polariton interaction strength. Here, at room temperature, a very large polariton interaction strength is demonstrated, g ≈ 128 ± 21 µeV µm2 realized in a 2D nanolayered metal–organic framework (MOF). As a result, a polariton lasing at an extremely low pump fluence of P1 ≈ 0.01 ± 0.0015 µJ cm−2 (first threshold) is observed. Interestingly, as pump fluence increases to P2 ≈ 0.031 ± 0.003 µJ cm−2 (second threshold), a spontaneous transition to a polariton breakdown region occurs, which has not been reported before. Finally, an ordinary photon lasing occurs at P3 ≈ 0.11 ± 0.077 µJ cm−2 (third threshold), or above. These experiments and the theoretical model reveal new insights into the transition mechanisms characterized by three distinct optical regions. This work introduces MOF as a new type of quantum material, with naturally formed polariton cavities, that is a cost‐effective and scalable solution to build microscale coherent quantum light sources and polaritonic devices.