Photon upconversion (UC) is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength. It is an anti-Stokes type emission. An example is the conversion of infrared light to visible light. Materials by which upconversion can take place often contain ions of d-block and f-block elements. Examples of these ions are Ti2+, Ni2+, Mo3+, Re4+, and Os4+.
Three basic mechanisms are energy transfer upconversion, excited-state absorption (ESA) and photon avalanche (PA). Upconversion should be distinguished from two-photon absorption and second-harmonic generation. An early proposal (a solid-state IR quantum counter) was made by N. Bloembergen in 1959 The process was first observed by F. Auzel in 1966
Thermal upconversion mechanism has also been proposed. This mechanism is based on the absorption of photons with low energies in the upconverter, which heats up and re-emits photons with higher energies. To make this process possible, the density of optical states of the upconverter has to be carefully engineered to provide frequency- and angularly-selective emission characteristics. For example, a planar thermal upconverting platform can have a front surface that absorbs low-energy photons incident within a narrow angular range, and a back surface that efficiently emits only high-energy photons. 
Lanthanide-doped nanoparticles emerged in the late 1990s due to the prevalent work on nanotechnology, marking a turning point in the landscape of modern lanthanide research. Although the optical transitions in lanthanide-doped nanoparticles essentially resemble those in bulk materials, the nanostructure amenable to surface modifications provides new oportunities for research. Particularly, these nanoparticles are promising alternatives to molecular fluorophores for bioapplications. Their unique optical properties, such as large Stokes shift and nonblinking, have enabled them to rival conventional luminescent probes in challenging tasks including single-molecule tracking and deep tissue imaging. Despite the promising aspects of these nanomaterials, one urgent task that confronts materials chemists lies in the synthesis of nanoparticles with tunable emissions, which are essential for applications in multiplexed imaging and sensing.
Applications and examples:
At present, there is great interest in luminescent materials for efficient frequency conversion from infrared to visible radiation, mainly because a visible source pumped by a near infrared laser is useful for high-capacity data storage optical devices. This process can be obtained by upconversion mechanisms, where several infrared photons can be absorbed by the material doped with rare earth ions (RE) in order to populate more energetic levels. Therefore, both the fluorescence lifetime and the stimulated emission cross-section of the RE excited level should be maximized, whereas the nonradiative decay mechanisms should be minimized. Oxyfluoride glass ceramics are ambivalent materials. Despite the fact that they are mainly oxide glasses, they can exhibit optical properties of fluoride single crystals when they are doped with rare earth ions. They are often called nanocomposite materials. Their weird character is obtained by a classical melting and quenching preparation in air followed by an adapted thermal treatment during which fluoride phases are crystallized. The size, size distribution, and volume concentration of fluoride crystallites are crucial for photonic applications. For example, to be a promising optical functional material, the size of the crystallites should be smaller than at least half of the wavelength of the light used while the size distribution should be narrow and the crystallites should possess a homogeneous spatial distribution. In this way, according to the scattering theory developed by Rayleigh, complete transparency of a light transmitting material can be attained. A refractive index difference between the amorphous and crystalline phases of less than 0.1 is also required. However, according to Beall and Pinckney, based on Hopper’s model, crystal sizes of 30 nm and differences in refractive index of 0.3 may be acceptable, provided that the crystal spacing is not larger than six times the average crystal size. Transparent Glass Ceramic (TGC) can also be obtained with even larger crystal sizes if optical isotropy is achieved within the glass ceramic. Consequently, the selection of the oxide glass composition and the fluoride phase composition is the key factor in obtaining the desired glass ceramic materials. The Er3+ ions are specially interesting due to their emission at 1.5 μm and the green upconversion obtained under near infrared excitation. In order to improve these emissions, the sensitization of this nanocomposite with Yb3+ ions may be a good choice because of the efficient energy transfer process from Yb3+ to Er3+ ions.
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