Nonlinear optics lie in the center of many optical technologies. As the superposition principle no longer holds, optical nonlinear response from materials frees many of the constraints in classical optics and has enabled a range of practical applications, from laser processing to quantum optics. Yet, irrespective of the advantages, the nonlinear response is typically observable at very high laser intensity. The photon avalanche (PA) effect is therefore exceptional, since it allows sufficiently weaker lasers for excitation. Unfortunately, PAs are mostly restricted to bulk materials and rely on cryogenic conditions,1 hindering their wide application.