Photon Avalanche emission

Among different anti-Stokes emissions, where emitted photons have larger energy than the absorbed photons, photon avalanche (PA) phenomenon is unique. It originates from a very non-linear increase of luminescence intensity in response to minute rise of excitation intensity above some threshold pump power density. PA was first observed in the year 1979 in Pr³⁺ doped LaCl₃ quantum counters. Since then, PA became an interesting topic and was investigated in various bulk materials doped with lanthanide ions such as Tm³⁺, Pr³⁺, Ho³⁺ or Er³⁺ . It became a challenge to demonstrate the PA at a smaller scale materials, and only recently PA at nanoscale was achieved for Tm³⁺ doped NaYF₄ and LiYF_­4 nanocrystals. 

Understanding photon avalanche process 

In order to comprehend the origin of unusual PA luminescent properties, it is necessary to describe the underlying energy transfer processes:
1. Ground state absorption (GSA) – Weak sideband absorption is necessary as the first step to promote at least single lanthanide ions to the first excited level. The population of these intermediate level is required to enable the energy looping and ultimately significantly increase luminescence intensity.
2. Excited state absorption (ESA) – The excited electrons can further resonantly absorb the light and futher advance to higher energy levels. Due to energy mismatch, GSA is much less probable than the resonant ESA (typically the absorption cross-section ESA to GSA ratio should go above 10⁴). Due to ESA, higher energy levels are reached.
3. Cross relaxation (CR) – CR is non-radiative energy transfer process, in which excited electron from higher excited state non-radiatively transfers part of it’s energy to neighboring lanthanide ions, whose electron stay in ground state. The yield of such energy transfer are two electrons that are both in some intermediate excited state. Consequently, the energy of photons being absorbed by PA material are not re-emitted but cumulate in the excited levels of rising number of lanthanides.
4. Energy looping (EL) – It is basically a cycle of ESA and CR processes, that lasts as long as there are electrons that still can be excited. It’s final step that leads to saturation of the system with excited electrons. If there are no more electrons to excite the PA threshold has been reached and strong emission occurs which is accompanied by electrons returning to the ground state. These electrons can de novo participate in energy looping under continuous wave excitation. 

The photon avalanche is a positive looping system with high gain, that lead to highly non-linear relationship between input (pump photon flux) and output (photon avalanche emission). Originally only two possible applications gained attention, namely medium infrared photon counters and upconversion lasers. Currently, having nanoscale photon avalanche materials available, some new types of possible applications can be predicated or have been already demonstrated.

Applications:

PA for superresolution imaging In order to overcome limit of diffraction, multiple techniques were designed, that allows to visualize nanoscale objects beyond limit of light diffraction. However many of those techniques are often riddled with various issues, that hinder widespread application, such as photobleaching of organic dye labels or necessity of employment of complex & quite often particularly expensive equipment. To address those issues it was proposed to use PA lanthanide-doped nanoparticle labels, that are resistant to photobleaching and can significantly simplify necessary optical setup (e. g. eliminating the need for second “depletion” beam spatial overlap or temporal synchronization in STED).

Because PA is a looping system, the utilization of chemical (acceptor molecules) or physical  (temperature or pressure) factors that disrupts PA gain, provides efficient way to detect biological / physical processes at the molecular scale. For example, Förster Resonance Energy Transfer (FRET) that relies on robust energy transfer between donor and acceptor, it is of substance to ensure that those agents do not photobleach nor display any unwanted spectral characteristics. Once again, the employment of PA capable lanthanide-based nanoparticles proved to be a significant step forward in terms of improving the resolution & sensitivity, limiting the toxicity as well as eliminating the background, owing to efficient anti-Stokes emission, which can’t be realized in organic dyes nor quantum dots. Underlined features enable fairly safe and effective way of sensing in biological systems of such species as proteins, DNA or antigen-antibody interaction with in vitro & in vivo regime.

We predict the PA phenomenon may further find interest in numerous other sensing and imagining applications as well as neuromorphic computing, optical data processing etc.

by Jastin Popławski, Zuzanna Korczak & Artur Bednarkiewicz

Sources and further reading:
1. Bednarkiewicz, Artur, Chan, Emory M. and Prorok, Katarzyna. "Enhancing FRET biosensing beyond 10 nm with photon avalanche nanoparticles". Nanoscale Adv. 2. (2020): 4863-4872.
2. Bednarkiewicz, Artur, Chan, Emory M. Kotulska, Agata, Marciniak, Lukasz, and Prorok, Katarzyna. "Photon avalanche in lanthanide doped nanoparticles for biomedical applications: super-resolution imaging". Nanoscale Horiz. 4. (2019): 881-889.
3. Lee, C., Xu, E.Z., Liu, Y. et al . Giant nonlinear optical responses from photon-avalanching nanoparticles. Nature 589 , 230–235 (2021).
4. Liu, Y., Lu, Y., Yang, X. et al . Amplified stimulated emission in upconversion nanoparticles for super-resolution nanoscopy. Nature 543 , 229–233 (2017).