Luminescence principle of upconversion luminescent materials

1. What is upconversion luminescence?
Stokes law holds that materials can only be excited by high-energy light and emit low-energy light, that is, when excited by light with short wavelength and high frequency, materials emit light with long wavelength and low frequency. Up-conversion luminescence is the opposite. Up-conversion luminescence refers to the type of luminescence that continuously absorbs two or more photons, resulting in the emission wavelength being shorter than the excitation wavelength. We also call it Anti-Stokes.

Up-conversion luminescence is reflected in both organic and inorganic materials, but its principle is different.
The mechanism of photon upconversion of organic molecules is Triplet-triplet annihilation (TTA). Polycyclic aromatic hydrocarbons (PAHs) are typical organic molecules. Among inorganic materials, upconversion luminescence mainly occurs in lanthanide compounds doped with rare earth ions, mainly including NaYF4, NaGdF4, LiYF4, YF3, CaF2 and other fluorides or nanocrystals of Gd2O3 and other oxides. NaYF4 is a typical matrix material in upconversion luminescent materials, such as NaYF4:Er,Yb, that is, when Yb and Er are double doped, Er is used as activator and Yb is used as sensitizer. In this application article, we focus on rare earth doped upconversion phosphors (UC NPS).

Second, the luminescence principle of lanthanide doped rare earth upconversion luminescence
Inorganic materials have three basic luminescence principles: Excited-state absorption, ESA), Energy transfer upconversion, ETU) and Photon avalanche, PA).

(a) excited state absorption
Excited state absorption process (ESA) was proposed by Bloembergen and others in 1959. Its principle is that the same ion reaches the excited state with higher energy from the ground state through continuous multiphoton absorption, which is the basic luminescence process of upconversion luminescence. Such as Figure 3(a), the same rare earth ion transits from the ground state energy level to the excited state energy level through continuous two-photon or multi-photon absorption, and then releases the energy in the form of optical radiation to the ground state energy level.
(b) energy transfer upconversion
Energy transfer refers to the coupling of two excited ions with similar energy through non-radiation process, in which one transfers energy to the other and returns to a low energy state, and the other ion accepts energy and transitions to a higher energy state. The up-conversion of energy transfer can occur between the same ion or between different ions. Energy transfer includes continuous energy transfer (SET), Cooperative Upconversion (CU) and CRoss Relaxation (Cr). one
Igure 4. Three types of energy transfer upconversion
(c) photon avalanche
The upconversion luminescence of “photon avalanche” was first discovered by Chivian et al. in 1979 when they were studying Pr3+ in Lacl3 crystal, which attracted wide attention because it can be used as the excitation mechanism of upconversion laser. The mechanism is based on the fact that particles at one energy level produce pumping effect with quantum efficiency greater than 1 at another energy level through cross relaxation. The “photon avalanche” process is a combination of excited state absorption and energy transfer, but the energy transfer occurs between the same kind of ions.