Researchers have sought materials that can switch between dark and bright states when exposed to light, which could make switches for future photonic computers. Now scientists in California say they have made nanoparticles with this property for the first time.
This kind of switching behavior is a must for computing systems. To make digital logic, engineers need materials with bistability; that is, they can be switched between two different states that represent the “1” or “0” of digital logic. Today’s transistors use semiconducting materials, primarily silicon, that can be switched between conducting and insulating states. Berkeley Lab nanoparticle specialist Emory Chan says the new nanoparticles—which at tens of nanometers across are about the size of many features on modern microchips—could provide a similar switching behavior for optical systems.
The work grew out of Chan’s research on nanoparticles that are extremely responsive to laser light. His team came up with the first designs in 2014 after querying a computer model to find nanoparticles that would produce particular wavelengths of light. When the team made the suggested materials, they found a surprise. These nanoparticles had a property called “unintuitive photon avalanching,” he says. They emit light when illuminated by a laser, but when the power is increased slightly, there’s a disproportionate increase in their brightness. This behavior is called avalanching. In 2021, the team showed that doubling the laser power led to a 10,000-fold increase in the intensity of the nanoparticles’ glow.
Bistable Nanoparticles for Photonics
In their latest work, the team made nanoparticles that are bistable. The nanoparticles are made of potassium lead halides doped with neodymium; the latter element is commonly used in lasers. Chan says the neodymium ions are where the optical magic happens. The nanoparticles emit bright light when illuminated, and continue to shine even when the laser power is reduced. But when the laser is dimmed enough, the particles go dark. That provides two states, bright and dark. Scientists have seen switching behaviors in other optical nanomaterials before, but the behaviors were hard to control because they relied on temperature changes. The new bistable optical nanoparticles are highly sensitive to the power and frequency of laser light, allowing for finer control of their behavior.
The nanoparticles also seem to have a memory of their past states. The researchers could control the particles’ resistance to changing their response to illumination by altering the pulse frequency of the input light. At higher frequencies, the nanoparticles have more hysteresis—that is, they’re more reluctant to change states. With slow pulsing, the nanoparticles forget their previous state more rapidly. Chan compares it to turning off the heat under a boiling pot of water. It takes time for the water to stop boiling and come back to room temperature. And while it’s still hot, it takes less energy to get it back up to a boil.
He says it might be possible to take advantage of this feature in future memory cells. And it should be possible to write data to such devices very rapidly. “The only thing that’s changing is how the electrons are moving,” says Chan.
Right now, the optical-avalanche property works only at low temperatures. Chan’s group showed the effect at 160 kelvins (–113 °C). They’d like to find the behavior at room temperature. Their results were reported in the February issue of Nature Photonics.- Scientists Use Light to Control Nanobots - IEEE Spectrum ›
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Katherine Bourzac is a freelance journalist based in San Francisco, Calif. She writes about materials science, nanotechnology, energy, computing, and medicine—and about how all these fields overlap. Bourzac is a contributing editor at Technology Review and a contributor at Chemical & Engineering News; her work can also be found in Nature and Scientific American. She serves on the board of the Northern California chapter of the Society of Professional Journalists.
