Engineers for the first time detect avalanches in nanoparticles

Researchers at Columbia Engineering report today that they have developed the first nanomaterial showing ‘photon avalanche’, a process unmatched in the combination of extreme nonlinear optical behavior and efficiency. The realization of photon avalanche in nanoparticle form offers a large number of sought-after applications, from optical micro-resolution with super-resolution, precise temperature and environmental observation and infrared light detection to optical analog-to-digital conversion and quantum observation. .

“No one has ever seen such avalanches in nanomaterials before,” said James Schuck, an associate professor of mechanical engineering, who led the study published today by Earth. “We studied these new nanoparticles at the single-nanoparticle level, which allows us to prove that avalanche behavior can occur in nanomaterials. This excellent sensitivity can be incredibly transformative. Imagine, for example, that we would sense changes in our chemical environment, such as variations in or the actual presence of molecular species. We can even detect coronavirus and other diseases. “

Avalanche processes – where a waterfall of events is caused by a series of small disturbances – occur in a wide range of phenomena outside of snow-sliding, including the inflation of champagne bubbles, nuclear explosions, burdens, neural networks and even financial crises . Avalanching is an extreme example of a non-linear process, in which a change in supply or excitation leads to an excessive – often excessively large – change in the output signal. Large amounts of material are usually required for the efficient generation of non-linear optical signals, and this has also been the case so far for photon avalanches.

In optics, photon avalanche is the process where the absorption within a crystal of a single photon results in the emission of many. Researchers have used photon avalanches in specialized lasers, where the photon absorption causes a chain reaction of optical events that ultimately leads to efficient lasing.

It is especially noteworthy for researchers that the absorption of just one photon not only leads to a large number of emitted photons, but also to a surprising property: the emitted photons are ‘upconverted’, each higher in energy (bluer in color) as the single absorbed photon. Scientists can use wavelengths in the infrared region of the optical spectrum to create large amounts of higher-energy photons that are much better at achieving desired chemical changes – such as killing cancer cells – at targeted locations deep in tissue, wherever the avalanche nanoparticles are positioned. .

The behavior of photon avalanche (PA) showed great interest more than forty years ago when researchers realized that its extreme nonlinearity can greatly affect many technologies, from efficient conversion lasers to photonics, optical sensors and night vision devices. PA behavior is similar to that of a transistor in electronics, where a small change in an input voltage results in a large change in the output current, which provides the amplification required for the operation of almost all electronic devices. . PA enables certain materials to function essentially as optical transistors.

PA has been studied almost exclusively in material based on lanthanide (Ln) due to their unique optical properties that enable them to store optical energy for a relatively long time. Achieving PA in Ln systems, however, was difficult – it requires cooperative interactions between many Ln ions, while also moderating the loss pathways, and is therefore limited to large materials and aggregates, often at low temperatures.

These limitations have transferred the fundamental study and use of PA to a niche role in photonic science, and have led researchers to focus almost exclusively on other conversion mechanisms in material development over the past decade, despite the unparalleled benefits that PA offers.

In this new study, Schuck and his international team of collaborators, including the groups Bruce Cohen and Emory Chan (The Molecular Foundry, Lawrence Berkeley National Lab), Artur Bednarkiewicz (Polish Academy of Sciences) and Yung Doug Suh (Korea Research Institute) Chemical Technology and Sungkyunkwan University), has shown that, by implementing some key innovations in the design of nanoparticles, such as selected lanthanide content and species, new 20 nm nanocrystals can successfully synthesize photon avalanches and the extreme non- shows linearity thereof.

The team observed that the non-linear optical response in these avalanche nanoparticles scales as the 26th force of the incident light intensity – a 10% change in the incident light causes more than a 1000% change in the emitted light. This nonlinearity exceeds the reactions previously reported in nanocrystals with lanthanide. This extraordinary response means that the avalanche nanoparticles (ANPs) show great promise as sensors, as a small change in the local environment can lead to the particles emitting 100-10 000 times brighter. The researchers also found that this giant non-linear response in ANPs enables optical imaging below sub-wavelength (with the ANPs used as luminescent probes, or contrast agents) using only simple scanning confocal microscopy.

“The ANPs allow us to beat the resolution diffraction limit for optical microscopy by a significant margin, and they do so essentially for free because of their steep non-linear behavior,” Schuck explains.

The lead author of the study Changhwan Lee, who holds a Ph.D. student in Schuck’s group, adds: “The extreme nonlinearity in a single ANP transforms a conventional confocal microscope into the latest super-resolution image resolution system.”

Schuck and his team are now working to use this unprecedented nonlinear behavior to detect changes in the environment, such as fluctuations in temperature, pressure, humidity, with a sensitivity that is not yet achievable.

“We are very excited about our findings,” Schuck says. “We expect them to lead to all sorts of revolutionary new applications for observation, imaging and light detection. It could also be critical in future optical information processing chips, with ANPs providing the amplifier-like response and a small spatial footprint typical of ‘ a single transistor in an electronic circuit. “

The study is entitled “Giant Nonlinear Optical Reactions of Photon-Avalanche Nanoparticles.”