Abstract

Most emissions are down conversion (DC) in nature, which means that the emission energy is lower than the excitation energy. In this book, we introduce an opposite concept—upconversion (UC) emission—the emission of one higher energy photon upon the excitation of several lower energy photons. The latter is very attractive for applications in data storage, multicolor displays, photovoltaic devices, biological field, etc. In recent years, the development of nanotechnology has been increasingly inviting scientific interest, especially the interest of the biomedical field, in relevant material systems such as lanthanide (Ln) ions doped materials. In these materials, near-infrared (NIR) photons are converted to higher energy photons ranging from ultraviolet (UV) to NIR. Here, the excitation wavelengths are especially seductive to the biomedical field since it falls in the so-called “optical window” (~650–1300 nm), that is, the spectral range of minimal absorption of human tissue. Besides, auto-fluorescence of the biological background is negligible under the NIR excitation. All these unique features make these materials, once in nanometer sizes, ideal candidates for contrast agents of luminescence imaging. Their application potential is also expanded to solar energy utilization by converting the NIR part of the solar spectrum into visible range in order to match the absorption of commercially available solar cells.