Abstract

Upconversion (UC) is a nonlinear process in which the excitation energy is absorbed by two or more photons that leads to the emission of light at shorter wavelength than the excitation wavelength. In this case, near-infrared (NIR) light, that is, 980 nm (lower energy, longer wavelength) is upconverted to visible light, 400–700 nm (higher energy, shorter wavelength) by UC material. In contrast to other emission processes, UC has high internal quantum efficiency (Haase and Schäfer 2011). In the 1950s, the UC effect was first discovered and studied intensively during the 1960s but up until 2003 it was observed only in bulk, transition metals or rare earth-doped phosphors which were suggested to have applications in the field of detectors, display, and laser materials (Bünzli and Eliseeva 2011). Since then UC materials have found their application in solar cells, waveguide amplifiers and later, they became applicable in rewritable optical storage nondestructive optical memory and many more potential scientific applications; and in the last decade, UC materials became well known in the biological field for bio sensing, drug delivery, etc. (Ho et al. 2003; Kim et al 2009; Shalav et al. 2005; Suyver et al. 2005; Vennerberg and Lin 2011; Wang and Bass 2004; Wang and Liu 2009; Wang et al. 2010; Zhou et al. 2015).