Self-Sensitized Shortwave Infrared Upconversion and Photon-Looping in Holmium-Doped Nanoparticles

Abstract Photon upconversion in lanthanide-doped nanoparticles provides an ultra-large anti-Stokes shift crucial for biomedical and photonic applications, but conventional architectures are typically limited to excitation wavelengths below 1700 nm. Here, we report efficient, direct 2 μm shortwave infrared (SWIR) upconversion to visible red light (640 nm) using self-sensitized holmium-doped core–shell nanoparticles. By exploiting the long-lived 5I7 and 5I6 intermediate energy states of Ho3+, we achieve high-order 4-photon upconversion with a relative upconversion-to-downconversion power conversion efficiency ratio of 13 ± 6%, approaching half the theoretical maximum for a 4-photon process. Crucially, we unveil two distinct operating regimes dictated by spatial boundary conditions, high-efficiency energy transfer upconversion (ETU) in well-passivated core–shell nanoparticles, and a previously unreported surface-defect-mediated photon-looping/avalanche regime in core-only nanoparticles. Finally, we demonstrate the utility of these optimized core–shell nanoconverters for high-contrast, high-resolution SWIR upconversion bioimaging through highly scattering media using low-cost silicon-based detection.