fig2

Figure 2. (A) Schematic of the intrinsic luminescence mechanism in LiNbO₃; (B and C) Luminescence mechanisms in rare earth-doped LiNbO₃. Reproduced from Ref.^[12] with permission from Elsevier and Ref.^[68] with permission from American Chemical Society Publications; (D) Excited state absorption UCL mechanism; (E) Energy transfer UCL mechanism; (F) Cross-relaxation UCL mechanism; (G) Photon avalanche UCL mechanism. (D-G) Reproduced from Ref.^[69] with permission from Elsevier; (H) Schematic illustration showing the design strategy for creating recoverable ML in piezoelectrics. Reproduced from Ref.^[70] with permission from the American Chemical Society; (I) ML mechanism in LiNbO₃:Pr³⁺. CB: Conduction band; VB: valence band; ESA: excited state absorption; GSA: ground state absorption; CR: cross relaxation; UV: ultraviolet; CTB: charge-transfer band; IVCT: intervalence charge-transfer; ML: mechanoluminescence; UCL: upconversion luminescence; LiNbO₃: lithium niobate; Pr: praseodymium.