Engineers observe nanoparticle avalanches for the first time

Columbia Engineering researchers report today that they have developed the first nanomaterial that demonstrates “photon avalanche,” a process that is unmatched in its combination of extreme nonlinear optical behavior and efficiency. The realization of the avalanche of photons in the form of nanoparticles opens up a multitude of sought-after applications, from real-time super-resolution optical microscopy, to the precise detection of temperature and the environment, to the detection of infrared light, optical analog-to-digital conversion and quantum detection.

“No one has seen such avalanche behavior in nanomaterials yet,” said James Schuck, associate professor of mechanical engineering, who led the study published today by Nature. “We studied these new nanoparticles at the level of a single nanoparticle, which enabled us to prove that avalanche behavior can occur in nanomaterials. This exquisite sensitivity could be incredibly transformative. For example, imagine if we could sense changes in our chemical environment, such as variations in or the actual presence of molecular species. We might even be able to detect the coronavirus and other diseases. “

Avalanche processes – where a cascade of events is triggered by a series of small disturbances – are found in a wide range of phenomena beyond snow slides, including bursting champagne bubbles, nuclear explosions , lasers, neural networks and even financial crises. Avalanche is an extreme example of a nonlinear process, in which a change in input or excitation leads to a disproportionate – often disproportionate – change in the output signal. Large volumes of material are usually required for the efficient generation of nonlinear optical signals, and this had also been the case with the avalanche of photons, heretofore.

In optics, the photon avalanche is the process by which the absorption inside a crystal of a single photon results in the emission of several. The researchers used the avalanche of photons in specialized lasers, where the absorption of photons sets off a chain reaction of optical events that ultimately lead to an efficient laser.

It should be noted in particular for the researchers that the absorption of a single photon leads not only to a large number of emitted photons, but also to a surprising property: the emitted photons are “up converted”, each higher. in energy (bluer color) than 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 inducing desired chemical changes – such as killing cancer cells – at targeted locations in the region. deeper into the tissues, where the avalanched nanoparticles are positioned.

The behavior of the photon avalanche (PA) aroused considerable interest over 40 years ago, when researchers recognized that its extreme non-linearity could have a significant impact on many technologies, from up-conversion lasers to efficient with photonics, optical sensors and night vision devices. The behavior of the PA is similar to that of a transistor in electronics, where a small change in input voltage results in a large change in output current, providing the amplification necessary for the operation of almost all electronic devices. PA allows certain materials to function essentially as optical transistors.

PA has been almost exclusively studied in lanthanide (Ln) based materials due to their unique optical properties that allow them to store optical energy for relatively long periods of time. However, achieving PA in Ln systems has been difficult – it requires cooperative interactions between many Ln ions while also moderating the loss pathways, and has therefore been limited to bulk materials and aggregates, often at low temperatures. .

These limitations have relegated the basic study and use of PA to a niche role in photonics science, and has led researchers to focus almost exclusively over the past decade on other upconversion mechanisms. in material development, despite the unparalleled advantages offered by AP.

In this new study, Schuck and his international team of collaborators, including the groups of 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 of Chemical Technology and Sungkyunkwan University), showed that by implementing some key innovations in nanoparticle design such as selected lanthanide contents and species, they could successfully synthesize new 20nm nanocrystals that demonstrate the avalanche of photons and its extreme non-linearity.

The team observed that the nonlinear optical response in these avalanched nanoparticles scales as the 26th power of the intensity of the incident light – a 10% change in the incident light causes a change of more than 1000% in the light emitted. This non-linearity far exceeds the responses reported previously in lanthanide nanocrystals. This extraordinary response means that Avalanch Nanoparticles (ANP) hold great promise as sensors, as a small change in the local environment can cause the particles to emit 100 to 10,000 times brighter. The researchers also found that this giant nonlinear response in ANPs enables deeply subwavelength optical imaging (with ANPs used as luminescent probes or contrast agents), using only simple confocal scanning microscopy.

“ANPs allow us to beat the diffraction limit of resolution for light microscopy by a significant margin, and they do so essentially for free, due to their strongly nonlinear behavior,” explains Schuck.

Lead author of the study, Changhwan Lee, who holds a PhD. a student in Schuck’s group, adds: “The extreme non-linearity in a single ANP turns a conventional confocal microscope into the newest super-resolution imaging system.”

Schuck and his team are now working on how to use this unprecedented nonlinear behavior to detect changes in the environment, such as fluctuations in temperature, pressure, humidity, with a sensitivity that has yet to been achievable.

“We are very excited about our findings,” says Schuck. “We expect them to lead to all kinds of revolutionary new applications in sensing, imaging and light sensing. They may also prove essential in future optical information processing chips,” with PDAs providing the amplifier-like response and a small spatial footprint typical of a single transistor in an electronic circuit. “

The study is titled “Giant Nonlinear Optical Responses of Photon Avalanche Nanoparticles”.