Upconverted

Upconverted refers to a photophysical process in which two or more lower-energy photons are absorbed and a single higher-energy photon is emitted. This anti-Stokes emission, known as upconversion luminescence, is observed in certain materials where absorbed energy steps up through intermediate states before radiative decay. It is distinct from downconversion, where a high-energy photon splits into multiple lower-energy photons.

Two main mechanisms underlie upconversion. Energy transfer upconversion (ETU) occurs in rare-earth-doped systems, typically with a sensitizer ion such as Yb3+ that absorbs a photon and transfers energy to activator ions like Er3+, Tm3+, or Ho3+, promoting them to higher excited states. A second mechanism is excited-state absorption or two-photon absorption, where a single ion sequentially absorbs photons, reaching higher levels before emitting. The result can be visible or near-infrared emission, depending on the dopants and host lattice.

Materials that exhibit upconversion are often lanthanide-doped nanocrystals, known as upconversion nanoparticles (UCNPs). Common hosts include NaYF4, NaGdF4, and related fluorides, with Yb3+ as a typical sensitizer and Er3+/Tm3+/Ho3+ as activators. UCNPs are frequently passivated with surface coatings to improve stability and biocompatibility for biological applications. Emission colors are characteristic of the dopants, such as green and red from Er3+, or blue from Tm3+.

Applications of upconversion span bioimaging, photodynamic therapy, solar energy harvesting, anti-counterfeiting, and specialty displays. Challenges include relatively low quantum yields under conventional excitation and surface-related quenching, prompting ongoing research into optimized host lattices, dopant ratios, and core–shell architectures to enhance efficiency.