Strong Emission Enhancement via Dual-Wavelength Coexcitation in YbTm-Doped Upconverting Nanoparticles for...

Upconversion (UC) emission in lanthanide-doped nanoparticles is typically excited by a single near-infrared (NIR) wavelength, most commonly around 975 nm, which promotes ground-state absorption by Yb³⁺ sensitizer ions and subsequent energy transfer to activator ions such as Tm³⁺. However, due to the presence of multiple long-lived excited states in lanthanide ions, additional excitation wavelengths can activate or modulate further energy-transfer pathways, leading to enrichment or depletion of specific electronic level populations. Despite their significant potential, such possibilities remain underexplored. In this work, we present a dual-wavelength coexcitation approach applied to NaYF₄ upconverting nanoparticles (UCNPs) codoped with Yb³⁺ and Tm³⁺ ions, using simultaneous illumination with a primary 975 nm beam and a secondary, tunable NIR beam. We observed that the addition of NIR light at wavelengths corresponding to two absorption bands of Tm³⁺, centered at 1213 and 1732 nm, to conventional 975 nm excitation yields a strong emission enhancement (up to 800%─far exceeding the additive effect of each excitation wavelength alone). Strikingly, in certain instances, de novo emission is observed under coexcitation, even when neither excitation source alone produces detectable luminescence. We show that this coexcitation mechanism enables visualization of NIR light above 1700 nm, beyond the detection range of silicon and standard InGaAs photodetectors. Furthermore, it allows the modulation of UC emission at excitation intensity levels that are several orders of magnitude lower than those used in super-resolution techniques such as STED. These findings indicate that coexcitation can become a powerful and energy-efficient method for controlling UC emission, with direct implications for subdiffraction imaging, NIR sensing, and advanced photonic applications.