An illustration of the chain-reaction process that underlies the photon avalanching mechanism Columbia Engineering researchers have realized in their nanoparticles. In this process, the absorption of a single low-energy photon sets off a chain reaction of energy transfers and further absorption events that result in many highly excited ions within the nanoparticle, which then release their energy in the intense emission of many higher-energy photons. Credit: Mikolaj Lukaszewicz, Polish Academy of Sciences.
Images of a single thulium-doped avalanching nanoparticle.In an avalanche, a small event produces a disproportionally huge result. Now researchers have managed to achieve the same phenomena with photons using a laser-stimulated nanoparticle [Lee et al., Nature 589 (2021) 230–235, https://doi.org/10.1038/s41586-020-03092-9].
Photon avalanching in optoelectronics is vital for a range of activities such as high-resolution optical microscopy, accurate temperature and environmental sensing, infrared light detection, and quantum sensing where extremely small inputs need to be amplified to detectable or useful levels. The phenomenon is a special type of upconversion, in which multiple photons combine to create a single higher energy photon. Upconversion is well known in bulk materials and aggregates, particularly lanthanide-doped materials, where it is used in imaging technologies to produce images free from background signals.
“Nobody has seen avalanching behavior like this in nanomaterials before,” says P. James Schuck of Columbia University, who led the research with colleagues at Lawrence Berkeley National Laboratory (LBNL), Changchun Institute of Applied Chemistry in China, the Institute of Low Temperature and Structure Research in Poland, Korea Research Institute of Chemical Technology, and Sungkyunkwan University. “This exquisite sensitivity could be incredibly transformative.”
The researchers observed photon avalanching in single nanoparticles doped with ions of the lanthanide, thulium (Tm), at room temperature. The 20 nm core-shell sodium yttrium fluoride (NaYF4) particles are grown by colloidal synthesis with a proportion of the yttrium replaced by Tm ions in the core. The team used the Molecular Foundry’s nanocrystal-making robot WANDA (Workstation for Automated Nanomaterial Discovery and Analysis) at LBNL to create batches of different nanoparticles with varying amounts of Tm. Crucially, an insulating shell on the outside ensures that absorption and emission are confined to the core, improving efficiency.
Under weak illumination, the nanoparticles produce almost no emission at all but, in nanoparticles with an optimal amount of Tm ions (8%), laser stimulation triggers a photon avalanche when a threshold level is reached. At this point, absorption of a single photon within the nanoparticle, where it is confined to the core, leads to the emission of many more higher energy photons. The nanoscale confinement gives rise to enhanced upconversion and, ultimately, to an avalanche process that can exceed what would be possible with bulk material. Or, to put it another way, increasing the incident light by just 10% produces an increase in emitted light of 1000%.
The usefulness of this phenomenon is demonstrated with super-resolution imaging in the near-infrared region in a scanning confocal microscope. Without any additional computational analysis, the researchers achieve a spatial resolution of less than 70 nm. Moreover, applying the avalanching nanoparticles to imaging means that excitation intensities 100 times lower than other probes can be used, which is beneficial for delicate biological samples.
“The avalanching nanoparticles allow us to beat the resolution diffraction limit for optical microscopy by a significant margin,” explains Schuck. “We are very excited about our findings. We are only just starting to scratch the surface of what might be possible.”
Schuck believes that the nanoparticles could also be highly useful for sensing because small changes in the local environment – such as temperature, pressure, or humidity – could lead to a boost in light emission from the particles of 100–10,000 times, making signal detection easy.
“We expect [these nanoparticles] to lead to all kinds of revolutionary new applications in sensing, imaging, and light detection,” he adds. “They may also prove critical in future optical information processing chips, providing the amplifier-like response and small spatial footprint typical of a single transistor in an electronics circuit.”
The findings should also spur further exploration of other lanthanides that could produce avalanche emissions at different frequencies.
“This work is certainly inspiring with clear advances made in the understanding of the sophisticated photo physics of highly doped upconversion nanoparticles,” says Dayong Jin, Director of the Institute for Biomedical Materials & Devices at the University of Technology Sydney. “The paper provides clear evidence revealing truly remarkable photon avalanche generation from a single nanoparticle.”
The brightness of upconversion nanoparticles is usually limited by so-called “concentration quenching” where too many emitters lead to a reduction in the overall fluorescence, he points out. But the combination of high irradiance excitation and shell passivation alleviates the problem, while the high efficiency of the cross-relaxation process between Tm ions and their strong excited state absorption produces a very high upconversion quantum yield.
“The highly doped upconversion nanoparticles display high order nonlinear responses, setting a new benchmark,” adds Jin. “This is a great news for us to continue exploring the nonlinear responses of highly doped upconversion nanoparticles for super resolution microscopy.”
While the comprehensive characterization of single nanoparticle spectroscopy is impressive, says Jin, it does take a relatively long time (on the order of hundreds of milliseconds) to build the photon avalanche process. In the future, he suggests, implementing controlled epitaxial growth to produce uniform passivation shells, allowing systematic investigation of the Tm ion doping level, could further boost the nonlinear response and lower the threshold.
This article was originally published in Nano Today 37 (2021) 101111.