Upconverting nanoparticles

Upconverting Nanoparticles (UCNPs) are nanoscale particles (1–100 nm) that exhibit photon upconversion. In photon upconversion, two or more incident photons of relatively low energy are absorbed and converted into one emitted photon with higher energy. Generally, absorption occurs in the infrared while emission occurs in the visible or ultraviolet regions of the electromagnetic spectrum. UCNPs are usually composed of lanthanide- or actinide-doped transition metals and are of particular interest for their applications in bio-imaging and bio-sensing at the deep tissue level. They also have potential applications in photovoltaics and security, such as infrared detection of hazardous materials.

Before 1959, the Anti-Stokes shift was believed to describe all situations in which emitted photons have larger energies than the corresponding incident photons. An anti-Stokes shift occurs when a thermally excited ground state is electronically excited, leading to a shift of only a few kBT where kB is the Boltzmann constant and T is temperature. At room temperature, kBT is 25.7 meV. In 1959, Nicolaas Bloembergen proposed an energy diagram for crystals containing ionic impurities (Figure 1). Bloembergen described the system as having excited state emissions with energy differences much greater than kBT, in contrast to the anti-Stokes shift.

Advances in laser technology in the 1960s allowed for the observation of non-linear optical effects such as upconversion., This led to the experimental discovery of photon upconversion in 1966, by François Auzel. Auzel showed that a photon of infrared light could be upconverted into a photon of visible light in Ytterbium–Erbium and Ytterbium–Thulium systems. In a transition metal lattice doped with rare-earth metals, an excited state charge transfer exists between two excited ions. Auzel observed that this charge transfer allows for an emitted photon with much higher energy than the corresponding absorbed photon. Thus, upconversion can occur through a stable and real excited state, supporting Bloembergen's earlier work. This result catapulted upconversion research in lattices doped with rare earth metals. One of the first examples of efficient lanthanide doping, the Yb/Er doped fluoride lattice, was achieved in 1972 by Menyuk et al.

Photon upconversion belongs to a larger class of processes by which light incident on a material induces anti-Stokes emission. Multiple quanta of energy such as photons or phonons are absorbed, and a single photon with the summed energy is emitted. It is important to make the distinction between photon upconversion, where real metastable excited states allow for sequential absorption, and other nonlinear processes like second-harmonic generation or two-photon excited fluorescence which involve virtual intermediate states such as the "simultaneous" absorption of two or more photons. It is also distinct from more weakly anti-Stokes processes like thermoluminescence or anti-Stokes Raman emission, which are due to initial thermal population of low-lying excited states and consequently show emission energies only a few kBT above the excitation. Photon upconversion is distinctly characterized by emission-excitation differences of 10–100 kBT.

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