The upconversion luminescence process is an Anti-Stokes process, that is, luminescent ions emit short waves (visible light) by absorbing multiple long waves (infrared light). Up-conversion luminescence mechanisms include excited state absorption (ESA), energy transfer (ET), cross relaxation (CR), synergistic sensitization (CSU) and photon avalanche (PA). The schematic diagram of upconversion luminescence mechanism is shown in Figure 1.4.
- excited state absorption (ESA) upconversion luminescence mechanism
As shown in fig. 1.4(a), ESA refers to the process that the same rare earth ion in the ground state absorbs multiple photons, transitions to an excited state at a higher energy level, and then transitions from the excited state back to the ground state to realize upconversion luminescence. The specific process is as follows: rare earth ions absorb a photon energy from the ground state E level to transition to the E level, which is called ground state absorption (GSA), and then the rare earth ions at the E2 level absorb another photon energy to transition to the E3 level, which is called excited state absorption (ESA). Finally, the ions transition from the B3 level to the ground state E1 level and radiate photons to realize upconversion luminescence. - Energy transfer (ET) up-conversion luminescence mechanism
As shown in Figure 1.4(b), that is, T refers to the process of up-conversion luminescence after interaction between rare earth ions, which usually occurs between different rare earth ions with matched energy levels. The specific process is as follows: the energy donor in the ground state E level is excited to the E level through ground state absorption (GSA), then returns to the ground state through non-radiative transition, and transfers energy to the energy acceptor through resonant energy transfer, and the energy donor can continue to be
Excite and transfer energy. The energy acceptor jumps from the E level to the B level, and the E level then receives energy and jumps to the E level, and finally jumps back to the ground E level from the E level and radiates photons, thus realizing up-conversion luminescence. - cross relaxation (CR) upconversion luminescence mechanism
As shown in fig. 1.4(c), CR refers to the process that two pairs of energy levels with matching energy levels transfer the energy of the higher energy level to the lower energy level through energy transfer when the ions in the luminescence center are in the excited state. The specific process is as follows: rare earth ions in higher excited state transition from Ea energy level to E energy level without radiation, and transfer energy to rare earth ions in low energy level, so that they are excited from Et energy level to E energy level. - Up-conversion luminescence mechanism of CSU.
As shown in Figure 1.4(d), CSU refers to a luminescence mechanism in which two rare earth ions of the same species can cooperate to transfer energy to the luminescence center ions together. The specific process is as follows: the two energy donor ions in the excited state transfer energy through non-radiative transition, and at the same time transfer energy to the energy acceptor ions, so that the energy acceptor ions are excited from the E level to the E level, and finally return to the ground state through radiative transition, thus realizing up-conversion luminescence. - Photon avalanche (PA) upconversion luminescence mechanism
As shown in fig. 1.4(e), PA refers to the process of combining ESA with energy transfer CR to realize short-wavelength luminescence. The specific process is as follows: when the absorption of pump energy by the ground state E level in the matrix is weaker than that by the excited state E level, the ion of the E level absorbs the pump energy and transitions to the higher excited state E3 level, and E→E and E→E realize an effective CR process, which multiplies the excited electrons at the E level, resulting in the accumulation of rare earth ions at the E level like an avalanche, so it is called a photon avalanche. It should also be pointed out that the photon avalanche process will only occur when the doping concentration of rare earth ions is high enough. Ions accumulated at the E level finally jump to the E level and return to the ground state through radiation transition, thus realizing up-conversion luminescence.