When Two (Infrared) Beams Are Better Than One: Controlling Emission in Thulium-Doped Upconverting Nanoparticles

Upconverting nanoparticles (UCNPs) are luminescent materials capable of absorbing low-energy radiation (e.g., near-infrared) and emitting higher-energy photons in the visible range. Their emission is typically tuned through structural design while keeping the excitation scheme conventional, based on a single infrared beam. A dual-excitation approach using two infrared sources reveals a surplus emission effect, where co-excitation produces a strong enhancement or induces emission otherwise absent. This synergistic behavior allows visualization of NIR radiation above 1700 nm and offers new opportunities for optical logic and controlled light-matter interactions in nanoscale systems.