Intrinsic optical bistability (IOB), an optical phenomenon that enables a material to use light to switch between two different states, could be the key to making smaller, faster components for next-generation computers. Researchers at Lawrence Berkeley National Laboratory (Berkeley Lab), Columbia University, and Universidad Autónoma de Madrid developed an optical material that demonstrates IOB. The material shows a disproportionately large increase in the light it emits when it is exposed to a small increase in laser power. The new material, which is made of photon-avalanching nanoparticles, could be used to fabricate small components like memory, transistors, and interconnects at high density, for optical computing on a scale comparable to the size of current microelectronics. At the Molecular Foundry at Berkeley Lab, the researchers fabricated 30-nm nanoparticles from a potassium-lead-halide (K-Pb-halide) material doped with neodymium (Nd), a rare-earth element commonly used in lasers. They excited the nanoparticles with light from an IR laser. When the nanoparticles were excited, they exhibited photon avalanching. Researcher Xiao Qi in the laser room at the Molecular Foundry. Qi used the setup to develop a new optical computing material from nanoparticles that exhibit a phenomenon known as photon avalanching, in which a small increase in laser power results in a giant, disproportionate increase in the light emitted by the nanoparticles. Courtesy of Marilyn Sargent/Berkeley Lab. The Nd-doped avalanching nanoparticles demonstrated more than 3 times the nonlinearity of the avalanching nanoparticles used in the team’s earlier work. According to researcher Emory Chan, the Nd-doped avalanching nanoparticles used in the current study exhibit the highest nonlinearities that have ever been observed in a material. The researchers found that the nanoparticles continued to emit brightly, even when the laser power was reduced below the designated threshold for laser power. Photon avalanching was only turned off completely when the laser power became extremely low. The nanoparticles switched, with high contrast, between luminescent and non-luminescent states, with hysteresis characteristic of bistability. There was a significant difference between the “on” and “off” threshold powers, indicating that if mid range-intensity lasers were used, the nanoparticles could be either bright or dark, depending on their excitation history. The researchers used computer models to investigate the source of the bistability. They found that the IOB in the nanoparticles came from the extreme nonlinearity of photon avalanching and from a structure that dampens vibrations in the particles. They uncovered a nonthermal mechanism in which IOB originates from suppressed, nonradiative relaxation in Nd ions and from the positive feedback of photon avalanching, resulting in greater than 200th-order optical nonlinearities. Until now, there has been a limited understanding of IOB, which has inhibited the development of nanoscale IOB materials for devices. IOB has been observed almost exclusively in bulk materials, which are too big for a microchip and challenging to mass-produce. In earlier reports of nanoscale IOB, the process was assumed to occur by heating the nanoparticles, which is inefficient and difficult to manage. Control over nanoscale IOB could enable the use of avalanching nanoparticles for photonic devices in which light is used to manipulate light. The ability of the photon-avalanching nanoparticles to switch optical properties, without any changes being made to the material, suggests that they could be used for optical memory at the nanoscale, particularly volatile random-access memory (RAM). “This is the first practical demonstration of intrinsic optical bistability in nanoscale materials,” Chan said. “The fact that we can reproducibly make these materials and understand their unintuitive properties is critical for making optical computers at scale a reality.” In the future, the researchers hope to study new applications for optically bistable nanomaterials and find new formulations for nanoparticles with greater environmental stability and optical bistability. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-024-01577-x).