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Researchers from the National University of Singapore (NUS) have developed a new class of lanthanide-doped nanomaterials that exhibit extraordinarily high optical nonlinearity, exceeding a magnitude of 500. This achievement establishes a new global benchmark for photon avalanche nanophotonics. The paper is in Nature.

Photon avalanche is an exceptionally rare and powerful photophysical effect characterized by its nonlinear response to light excitation. In conventional fluorescence, the intensity of emitted light typically increases proportionally to the excitation intensity, reflecting a linear input-output relationship. However, photon avalanche systems display an abrupt, nonlinear behavior: once the excitation intensity surpasses a critical threshold, even a minor increase leads to a sudden, exponential surge in emission output.

This effect is known as an "avalanche" because it behaves like a runaway chain reaction, driven by energy transfer processes within the material, which rapidly amplify the light output. Thus, photon avalanche materials can turn tiny changes in light input into large increases in emission intensity, making them ideal for ultrasensitive detection and advanced imaging.

The research team led by Professor Liu Xiaogang from the NUS Department of Chemistry in collaboration with Professor Liang Liangliang from Xiamen University, China, discovered that modifying the structure of lanthanide-doped nanocrystals, specifically by substituting lutetium into the crystal lattice to introduce pronounced local crystal field distortions, greatly boosted photon avalanche nonlinearity.

Scientifically, this occurs through a positive feedback loop involving excited-state absorption and energy transfer between lanthanide ions such as Tm³⁺. These processes mutually reinforce each other, enabling ultrahigh-order nonlinear optical responses that are orders of magnitude more sensitive than traditional fluorophores.

As a result, photon avalanche materials can act as optical amplifiers, where even subtle changes in input, such as light intensity, temperature or environmental conditions, can produce dramatic shifts in emission.

By engineering these materials at the nanoscale, the research team introduced local distortions in the crystal lattice that enhanced energy transfer interactions, particularly cross-relaxation among thulium ions. This approach led to a very high optical nonlinearity (over 150 in 27 nm nanocrystals), making it possible to achieve super-resolution imaging using just a single light beam.

The resolution reached 33 nm across and 80 nm deep, similar to what Stimulated Emission Depletion Microscopy can do, but without the need for complex instrumentation. When they used a larger 170-nm nanodisk, the nonlinearity increased even further, going beyond 500. Moreover, it revealed spatially distinct nonlinear emission patterns within individual nanocrystals, allowing imaging with resolution beyond the physical dimensions of the emitters themselves.

This work opens up transformative opportunities across optical and biomedical fields, offering a cost-effective route to super-resolution imaging, enabling highly sensitive chemical and biosensing. It also provides a robust platform for quantum photonics applications such as optical switches and light sources, and supports advanced data storage and encryption through spatially and intensity-resolved emission control.

Prof Liu said, "By combining photon avalanche effects with precise nanomaterials design, we are redefining the boundaries of nonlinear optics. This work lays the foundation for a new generation of light-based technologies that are faster, more compact, and more sensitive than ever before."