Photon upconversion

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 or f-block elements. Examples of these ions are Ln³⁺, Ti²⁺, Ni²⁺, Mo³⁺, Re⁴⁺, Os⁴⁺, and so on.

Three basic mechanisms can contribute to photon upconversion: energy transfer upconversion (ETU), excited-state absorption (ESA) and photon avalanche (PA). Such processes can be observed in materials with very different sizes and structures, including optical fibers, bulk crystals or nanoparticles, as long as they contain any of the active ions mentioned above. Upconversion should be distinguished from two-photon absorption and second-harmonic generation. These two physical processes have a similar outcome than photon upconversion (emission of photons of shorter wavelength that the excitation) but the mechanism behind is different. An early proposal (a solid-state IR quantum counter) was made by N. Bloembergen in 1959 and 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. 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 black body source at 6000K for a single junction cell.

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