Rare earth elements constitute a class of elements found in the 6th and 7th periods of the periodic table along with lanthanum and actinium. It is divided into two main series known as the lanthanide
and actinide series. The elements in the lanthanide series have gained attention over the past few years due to their unique optical characteristics, especially in the area of biological applications. Most of the lanthanides exist in a trivalent state (except for cerium and europium), and their unique optical properties arise from electronic transitions within the 4f shell or from 4f-5d shell. Many synthesized rare earth nanomaterials consisted of one rare earth element, or combinations of more than one rare earth elements, to produce a wide range of spectral characteristics. They can be tuned to emit from the UV to the near-infrared (NIR) region by the process of upconversion or downconversion depending on the composition of nanoparticles. Downconversion is a process of absorbing a highenergy photon and emitting low-energy photons. On the contrary, upconversion is a process in which two or more low-energy photons are absorbed to produce a single high-energy photon. The mechanism of upconversion and downconversion is explained in Fig. 3.1.