Cross-coupled photon avalanche drives room-temperature intrinsic optical bistability
Optically bistable materials promise technological advances in optical information processing, memristive devices, and multiparametric sensing. However, optical bistability is predominantly observed under cryogenic conditions or is governed by subtle thermo-optical nonlinearities. Here, we report on Ho3+-activated nanocrystals that exhibit cavity-free intrinsic optical bistability (IOB) at ambient conditions. Non-thermal IOB stems from a cross-coupled photon avalanche (XPA) mechanism enabled by a low-phonon-energy double-perovskite host, which reduces nonradiative losses and facilitates excited-state population via XPA, even when the pump intensity is below threshold. Time-resolved luminescence measurements support the proposed XPA, as evidenced by a double-rise profile consistent with two reservoir states interlocked in energy looping. IOB with high-contrast photoswitching (up to 46 dB) is observed at room temperature across multiple visible emission lines, enabling wavelength-division multiplexing and compatibility with common detectors. IOB is preserved across nano- to microscale crystals and different double-perovskite compositions, broadly enabling information photonics and advanced sensing.