Fig. 11: Upconversion nanoparticles and their optical properties.

a Schematic depiction of UCNPs. b ETU process of activators and sensitizers in UCNPs. c Typical TEM image of UCNPs (Scale bar, 50 nm). Reproduced with permission from ref. ²⁹¹. Copyright 2019, Springer Nature. d Tunable emissions of UCNPs excited with NIR light. Reproduced with permission from ref. ²⁹². Copyright 2008, American Chemical Society. e Schematic representation for multi-dimension information involved in chiral upconversion nanomaterials, including the CPL signal, the wavelength of emission, and excitation in photon upconversion. Reproduced with permission from ref. ²⁹³. Copyright 2021, The Chemical Society of Japan