Lanthanide-activated nanocrystals can convert long-wavelength photons into short-wavelength ones viatheir nonlinear response to incoming light, makingthem useful in biomedical, sensing, and photonic applications. The host matrix and the selection ofembedded lanthanide ions manipulate the luminescence characteristics of these nanocrystals. In particular, properties of the host, like phonon-cutoff frequency, affect the multiphonon relaxation rates and complex photophysical interactions between lanthanide ions. Subsequently, low-phonon-energy hosts are prioritized to boost light emission and reduce non-radiative losses.¹

We found that low-phonon-energy Nd³⁺-doped KPb₂Cl₅ nanocrystals exhibit steeply nonlinear photon avalanche response when excited with a 1064 nm pump laser.² Upon cooling, the emission intensity of these nanocrystals exhibits a nonlinearity exceeding200: in other words, increasing the pump power by around 1% at threshold yields an emission-intensity gain of more than two orders of magnitude.Furthermore, these nanocrystals exhibit optical bistability, in which two stable luminescence states canbe observed under identical excitation conditions.³ By employing a two-wavelength excitation scheme, we demonstrated transistor-like photoswitching in these nanocrystals. Currently, we are investigating lead-free heavy halide materials that can accommodate lanthanide ions and facilitate photon avalanche and optical bistability at room temperature. We believe these results will inspire further development of low-phonon-energy hosts, thereby fostering innovation in the architectures of nonlinear emitters and their applications in all-optical data handling and integrated photonics.

(1) Skripka, A. Heavy Halide Nanomaterials as Next-Generation Hosts for Lanthanide Upconversion. J. Am. Chem. Soc.2026, 148 (10), 10286–10307.

(2) Zhang, Z.; Skripka, A.; Dahl, J. C.; Dun, C.; Urban, J. J.; Jaque, D.; Schuck, P. J.; Cohen, B. E.; Chan, E. M. Tuning Phonon Energies in Lanthanide‐doped Potassium Lead Halide Nanocrystals for Enhanced Nonlinearity and Upconversion. Angew. Chem. Int. Ed.2023, 62 (1), e202212549.

(3) Skripka, A.; Zhang, Z.; Qi, X.; Ursprung, B.; Ercius, P.; Cohen, B. E.; Schuck, P. J.; Jaque, D.; Chan, E. M. Intrinsic Optical Bistability of Photon Avalanching Nanocrystals. Nat. Photonics2025, 19, 212–218.