Upconversion

Upconversion is a nonlinear optical process in which two or more low-energy photons are absorbed and a single photon of higher energy is emitted. In practical terms, upconversion converts near-infrared or infrared light into visible or higher-frequency radiation. It is commonly observed in lanthanide-doped host materials and, more recently, in certain nanomaterials. Upconversion is a form of anti-Stokes emission, where the emitted photon has greater energy than the excitation photons, and it often requires intermediate long-lived excited states to accumulate energy before emission.

The dominant mechanisms in lanthanide-doped systems are energy transfer upconversion (ETU) and excited-state absorption (ESA). In ETU, one ion acts as a sensitizer, absorbing a photon and transferring energy to an activator ion, which then emits. Ytterbium (Yb3+) is frequently used as a sensitizer for 980 nm excitation, transferring energy to erbium (Er3+), thulium (Tm3+), or holmium (Ho3+) activators to produce visible or near-infrared emissions. ESA involves the same activator absorbing two photons directly in sequence. Cooperative sensitization and cross-relaxation pathways can also contribute in certain dopant combinations. The host lattice, dopant concentrations, and surface effects strongly influence efficiency.

Common hosts include fluoride materials such as NaYF4, often in core-shell nanoparticle forms to suppress surface quenching. Core-shell engineering, dopant optimization, and surface passivation are used to improve luminescence efficiency and stability.

Applications of upconversion span bioimaging and phototherapy, photovoltaics and solar concentration, anti-counterfeiting, and various optical sensing technologies. While upconversion enables operation under low-energy excitation and deep-tissue penetration in biomedical contexts, its practical use is limited by relatively low quantum yields and the need for sufficient pump intensity, though advances in materials design and nanostructuring continue to address these challenges.