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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.〔''Design of Luminescent Inorganic Materials: New Photophysical Processes Studied by Optical Spectroscopy'' Daniel R. Gamelin and Hans U. Güdel Acc. Chem. Res., 2000, 33 (4), pp 235–242 〕 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〔F. Auzel, ''C. R. Acad" Sci'' 1966, 262, 1016〕〔F. Auzel, ''C. R. Acad Sci'' 1966, 263, 819〕 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. These surface properties can be realized through designs of photonic crystal, and theories and experiments have been demonstrated on thermophotovoltaics and radiation cooling. Under best criterion, energy conversion efficiency from solar radiation to electricity by introducing up-converter can go up to 73% using AM1.5D spectrum and 76% considering sun as a blackbody source at 6000K for a single junction cell.〔S.V. Boriskina, G. Chen, 2014, 314, 71–78, 〕 == Upconversion nanoparticles == ===Lanthanide-doped nanoparticles=== 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 opportunities 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. The development of a reproducible, high yield synthetic route that allows controlled growth of rare earth halide nanoparticles has enabled the development and commercialization of upconversion nanoparticles in many different bioapplications described above. The first worldwide, commercially available upconversion nanoparticles were developed by Intelligent Material Solutions, Inc. and distributed through Sigma-Aldrich. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Photon upconversion」の詳細全文を読む スポンサード リンク
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