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<p>photon upconversion occurs through <a href="page.php?w=energy_transfer_upconversion">energy transfer upconversion</a> (ETU), <b>excited-state absorption</b> (<b>ESA</b>) and <b>photon avalanche</b> (<b>PA</b>). Such processes can be observed in materials with very different sizes and structures, including optical fibers, bulk crystals or <a href="page.php?w=nanoparticle">nanoparticle</a>s, as long as they contain any of the active ions mentioned above. Organic molecules can upconvert photons through sensitized triplet-triplet annihilation (sTTA)</p><p>
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