Abstract
The optical properties are independent of the intensity of exciting light in the case of linear optics. The case in which the materials optical properties depend on the radiant flux density of excitation light is nonlinear optics. Such kind of phenomenon is the upconversion process. The fluorescence follows the principle of Stokes law in which the exciting photons are of higher energy than emitted photons. Upconversion is an anti-Stokes process, which concerns with an excess of energies of emitted photons. This phenomenon shows regulating the incident photon wavelength and thus could be used as a potential candidate for heat regulating material. It is usually characterized by the conversion of long-wavelength radiation, for example, infrared or NIR radiation to short-wavelength radiation in the visible range.
