Cross-coupled photon avalanche drives room-temperature intrinsic optical bistability

Abstract: 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.