The upconversion mechanism in UCNPs involves the sequential absorption of two or more low-energy photons, which are then converted into a single higher-energy photon. This process is facilitated by the energy levels of the rare-earth ions, which allow for multi-photon absorption and emission. When excited by near-infrared (NIR) light, the nanoparticles emit visible or ultraviolet light, making them useful in diverse fields like bioimaging and photodynamic therapy.
