WARSAW, Poland, Sept. 15, 2025 — Researchers from the Ultrafast Phenomena Lab at the Faculty of Physics, University of Warsaw, in collaboration with a team from the Institute of Low Temperature and Structure Research, the Polish Academy of Sciences, have discovered an enhancement effect in the emission of upconverting nanoparticles. The researchers demonstrated that simultaneous excitation of these nanostructures with two near-infrared beams of laser light leads to a significant increase in emission intensity.

Under carefully chosen conditions, visible emission emerges only when both beams are applied together, even though neither beam alone produces any emission at all. The researchers then showed how this technique can be used to visualize infrared radiation beyond the sensitivity range of standard detectors.

Among photoactive materials used in photonic technologies, those that absorb lower-energy photons and emit higher-energy ones stand out. This process is made possible by sequential absorption of multiple photons, followed by the emission of a single photon with higher energy. While photon up-conversion remains one of the most widely used features of these materials, other applications arise from their nonlinear response, that is, the intensity of the emitted light is not a linear function of the excitation intensity. This nonlinearity makes lanthanide-doped upconverting nanoparticles particularly useful in enhancing the resolution of microscopic imaging. 

The current study, spearheaded by Paulina Rajchel-Mieldzioc, a Ph.D. candidate at the Ultrafast Phenomena Lab at the Institute of Experimental Physics, leveraged the fact that rare-earth metal ions, the photoactive core of upconverting nanoparticles, exhibit a complex structure of energy levels, allowing them to interact with light across a wide range of wavelengths. The study found that when these nanoparticles are illuminated not only with light of a wavelength typically used for excitation but also with additional beams in the NIR range, the emitted light intensity can increase dramatically, sometimes by several-fold.

 
“Furthermore, under specific conditions, visible light emission can be triggered only through the joint action of two NIR beams — neither of which produces the effect on its own” said Rajchel-Mieldzioc.

The work, according to the team, could have applications beyond infrared detection and its conversion to visible light, including in the development of novel microscopy techniques and purely optical computing.

This research was published in ACS Publications (www.doi.org/10.1021/acsnano.5c08510).

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