The up-conversion luminescence process is an Anti-Stokes process, in which the luminescent ions emit short waves (visible light) by absorbing multiple long waves (infrared light). Upconversion luminescence mechanisms include excited state absorption (ESA), energy transfer (ET), cross relaxation (CR), co-sensitization (CSU) and photon avalanche (PA). The up-conversion luminescence mechanism is shown in Figure 1.4.
- Excited state absorption (ESA) upconversion luminescence mechanism
As shown in Figure 1.4(a), ESA refers to the process in which the same rare earth ion in the ground state transitions to the excited state of a higher energy level by absorbing multiple photons, and then transitions back to the ground state to achieve upconversion luminescence. The specific process is as follows: rare earth ion absorbs a photon energy transition from the ground state E level to the E level, this process is called ground state absorption (GSA), and then the rare earth ion at the E2 level absorbs another photon energy transition to the E3 level, this process is called excited state absorption (ESA) process, and finally the ion transitions from B3 level to the ground state E1 level and radiates photons. Achieve up-conversion luminescence. - Energy transfer (ET) up-conversion luminescence mechanism
As shown in Figure 1.4(b), T refers to the process of producing upconversion luminescence after the 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 by ground state absorption (GSA), and then returns to the ground state by a non-radiative transition, and transfers the energy to the energy acceptor in the form of resonance energy transfer, and the energy donor can continue to be
Energize and transfer energy. The energy receptor transitions from the E level to the B level, the E level then receives the energy transition to the E level, and finally from the E, the energy level transitions back to the ground state E level and radiates photons to achieve up-conversion luminescence. - Cross relaxation (CR) up-conversion luminescence mechanism
As shown in Figure 1.4(c), CR refers to the process in which the energy level matching between the two pairs of energy levels transfers the energy of the higher level to the lower level through energy transfer when the ion in the luminous center is in an excited state. The specific process is as follows: the rare earth ion in the higher excited state transitions from the Ea level to the E level through non-radiative transition, and transfers energy to the rare earth ion in the lower level, so that it is excited from the Et level to the E level.
4, Collaborative sensitization (CSU) up-conversion luminescence mechanism
As shown in Figure 1.4(d), CSU refers to a luminescence mechanism in which two rare earth ions of the same kind can cooperate to transfer energy to the luminescent center ion. The specific process is as follows: the two energy donor ions in the excited state transfer energy through the 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 the radiation transition, so as to achieve up-conversion luminescence. - Photon avalanche (PA) upconversion luminescence mechanism
As shown in Figure 1.4(e),PA refers to the process of combining ESA and energy transfer CR to achieve 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 of the excited state E level, the ion absorption pump energy of the E level transitions to the higher excited state E3 level, and E→E and E→E achieve an effective CR process, which multiplicates the excited electrons on 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 noted that the photon avalanche process occurs only when the rare earth ion doping concentration is high enough. The ions piled up at the E level finally transition to the E level and return to the ground state by radiative transition, thus achieving up-conversion luminescence.
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