Infrared-Visible Wavelength Conversion by Upconversion Phosphors and Materials Development

Abstract

Heavy rare-earth ion doped upconversion phosphors can emit blue-green-red visible light with higher energy than the excitation light by multi-photon multi-step excitation by irradiating near-infrared light around 980 nm. Yb³⁺ is used to absorb near-infrared light at 980 nm, and the energy is transferred to Ho³⁺, Er³⁺, and Tm³⁺, which are responsible for luminescence. The excited states of f-electrons have long lifetimes, and the ions responsible for luminescence are excited again by another energy transfer from Yb³⁺. The radiative relaxation of the f electrons in the luminescent ions, which are multi-photon multi-step excited by these processes, emits visible light with higher energy than the excitation light. This is an extremely unique luminescence phenomena and is used in security inks. The high bio-permeability of infrared light used for excitation has led to research into its application in bioimaging, and it is expected to be used in combination with solar cells to improve power generation efficiency by converting infrared light that cannot be used for power generation into visible light.