Key Takeaways & Executive Findings
- •• • The engineered NaGdF4:10%Ho@NaYF4 core/shell PA NPs (~19.6 nm) exhibit a high nonlinearity (N) of 17–22 and a mild excitation threshold (P_th) of ~22 kW cm−2, enabling super-resolution imaging with reduced phototoxicity and improved signal-to-noise ratio. • • The parallel PA mechanism leverages dual reservoir levels (5I7 and 5I6) of Ho3+, achieving simultaneous red-green-blue emissions with a large N > 20, which is critical for multicolor sub-diffraction imaging and multiplexed detection. • • The extremely high sigma_ESA/sigma_GSA ratio of ~75000 (exceeding the PA criterion of 10000) ensures efficient excited-state absorption and low ground-state absorption, enabling low-threshold PA at the nanoscale. • • The fast response time of 343–371 ms and tunable chromaticity via host lattice screening and co-doping (Ce3+, Tm3+) allow dynamic multicolor imaging and potential applications in optical data storage and super-resolution microscopy.
Abstract
Photon avalanche (PA) is a nonlinear optical phenomenon characterized by steep upconversion emission growth with excitation power. Since the first demonstration of PA on nanoscale and at room temperature in 2021, PA luminescence of lanthanides has attracted considerable attention in nano- and bio-photonics. However, PA nanoparticles (NPs) remain restricted to a limited range of material systems and lanthanide ions, facing challenges including inadequate nonlinearity (N), high excitation threshold (P_th), and deficient chromaticity. In a recent publication in Nature Photonics, Dong and co-workers reported a new class of PA nanosystem based on Ho3+-doped fluoride NPs, which afforded tunable PA chromaticity for multicolor sub-diffraction imaging. Unlike conventional PA systems reliant on a single reservoir level, this study introduced a novel 'parallel PA' (PPA) mechanism leveraging the dual long-lived intermediate reservoir levels (5I7 and 5I6) of Ho3+, facilitating simultaneous operation of two PA loops to generate multicolor emissions. Under 965 nm continuous-wave excitation, the PPA of Ho3+ produced simultaneous red-green-blue emissions with large N (>20). Differential rate equation modelling identified all PA signatures, including clear P_th, S-shaped curves, extremely high sigma_ESA/sigma_GSA ratio of ~75000, and volcano-shaped rise time. The engineered NaGdF4:10%Ho@NaYF4 core/shell PA NPs (~19.6 nm) exhibited remarkably high N of 17–22 with mild P_th of ~22 kW cm−2 and fast response time of 343–371 ms. By screening host lattices and introducing co-dopants, PA chromaticity was precisely tailored, demonstrating unparalleled emission tunability for multicolor super-resolution imaging.
1. Introduction
Photon avalanche (PA) nanoparticles have emerged as a powerful tool for super-resolution imaging, but their practical deployment has been hindered by limited material systems, inadequate nonlinearity (N), high excitation thresholds (P_th), and poor chromaticity. Conventional PA systems rely on a single reservoir level, restricting emission colors and complicating multiplexed imaging. The need for multicolor sub-diffraction imaging demands novel PA mechanisms that can generate multiple emission bands simultaneously while maintaining high N and low P_th.
This study addresses these bottlenecks by introducing a parallel PA (PPA) mechanism in Ho3+-doped fluoride nanoparticles, which leverages dual long-lived intermediate reservoir levels (5I7 and 5I6) to drive two PA loops concurrently. This design enables simultaneous red-green-blue emissions under a single 965 nm excitation, achieving high N (>20) and low P_th (~22 kW cm−2). The engineered core/shell architecture and optimized doping concentration further enhance performance, offering a viable route to multicolor super-resolution imaging with fast response times and tunable chromaticity.
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LU Yixuan, HUANG Ping, CHEN Xueyuan (2026). Photon-Avalanching Holmium Nanoparticles for Multicolor Super-Resolution Imaging. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3556-6
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Frequently Asked Questions
What is the mechanism behind the parallel photon avalanche (PPA) in Ho3+ nanoparticles, and how does it enable multicolor emission?
The PPA mechanism leverages two long-lived intermediate reservoir levels (5I7 and 5I6) of Ho3+. Under 965 nm excitation, two PA loops operate simultaneously: PA-I (red emission at 645 nm) is triggered by 5I7, and PA-II (green emission at 540 nm) is driven by 5I6. Additional excited-state absorptions and cross-relaxations populate the blue-emitting level (485 nm), resulting in simultaneous red-green-blue emissions. This dual-loop design allows tunable chromaticity by adjusting the relative populations of the reservoir levels.
How do the nonlinearity (N) and excitation threshold (P_th) of the engineered NaGdF4:10%Ho@NaYF4 nanoparticles compare to previously reported PA systems?
The engineered nanoparticles exhibit a high N of 17–22 and a mild P_th of ~22 kW cm−2. This P_th is significantly lower than many earlier PA systems, which often required higher thresholds. The high N ensures steep nonlinear response, enabling super-resolution imaging with lower excitation intensities and reduced photobleaching.
What is the role of the core/shell architecture and doping concentration in optimizing PA performance?
Larger NP size and thicker shell suppress non-radiative relaxation associated with surface defects, thereby improving N and reducing P_th. The Ho3+ doping concentration is optimized to ensure adequate cross-relaxation for PA while avoiding energy dissipation from overloaded CR. The optimized NaGdF4:10%Ho@NaYF4 core/shell structure achieves a balance between high N and low P_th.
How is the chromaticity of the PA emission tuned, and what is the impact on multicolor imaging?
Chromaticity is tuned by screening host lattices (β-NaGdF4, α-NaYF4, LiYF4, Y2O3) and introducing co-dopants such as Ce3+ and Tm3+. These modifications alter the energy transfer pathways and relative emission intensities, enabling precise control over the red-green-blue output. This tunability is essential for multicolor sub-diffraction imaging, allowing multiplexed labeling and improved contrast.
What are the potential scalability and stability challenges for translating these PA nanoparticles into practical super-resolution imaging applications?
The synthesis of core/shell nanoparticles with precise size and doping control is scalable via established wet-chemical methods. However, long-term stability under continuous excitation and in biological environments must be assessed. The fast response time (343–371 ms) and low P_th are favorable for imaging, but photostability and toxicity need further evaluation for in vivo applications.
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