upconverting

Upconversion is a nonlinear optical process in which two or more photons of relatively low energy are absorbed and a single photon of higher energy is emitted. It is a form of anti-Stokes luminescence, in contrast to conventional Stokes-shifted emission, where emitted light has lower energy than the absorbed light. Upconversion is observed in a variety of materials, including inorganic lanthanide-doped crystals and organic compounds, and can occur via several mechanisms.

In inorganic systems, sensitizer ions such as Yb3+ absorb near-infrared photons and transfer energy to activator ions such as Er3+, Tm3+, or Ho3+, producing emission in the visible or near-IR. Common mechanisms include energy transfer upconversion and excited-state absorption. In organic systems, triplet-triplet annihilation upconversion involves a sensitizer that harvests photons to generate triplet states on an annihilator, where two triplets combine to emit a higher-energy photon.

Materials often used include NaYF4:Er3+,Yb3+ and NaYF4:Yb3+,Tm3+ with core-shell nanostructures to reduce surface quenching. Other platforms include quantum dots, metal-organic frameworks, and organic semiconductor blends. Efficiency depends on dopant concentrations, host lattice, surface passivation, and excitation intensity.

Applications span solar energy to bioimaging. In photovoltaics, upconversion layers aim to convert sub-bandgap photons for silicon cells, though practical implementation remains challenging due to low quantum yields and high requirements for excitation power. In biomedicine, upconversion nanoparticles enable deep-tissue imaging using near-infrared light.

Challenges include nonradiative losses, concentration quenching, and thermal effects; research focuses on optimized host materials, core-shell designs, and novel sensitizers to improve brightness and stability.