Figure 1. Optical properties of Pr³⁺ ion in YAG.

(a) Unit cell of YAG crystal. Green, white and red sites are occupied by yttrium, aluminium and oxygen, respectively. All together, the unit cell contains eight molecular units Y₃Al₅O₁₂. Praseodymium occupies yttrium sites substitutionally. (b) The local surrounding of one yttrium site. The orientation is identical to (a). While the local z′-axis is along the y crystal axis, the axes x′ and y′ are rotated with respect to x and z crystal axes by 45°. Other five local symmetries can be obtained from this one by one or two successive 90° rotations around x, y and z crystal axes. (c) Electronic level structure of Pr³⁺ ion in a YAG crystal. Relevant optical transitions are indicated by arrows. NR, non-radiative decay channels. (d) Fine structure of ³H₄→³P_0,1,2 transitions giving rise to upconverted ultraviolet fluorescence. Only the lowest three sublevels of ³H₄ manifold populated at room temperature are shown. (e) Four-level model used to explain the shape of photon antibunching signal. The metastable trapping state is assumed to be populated via decay of the emitting state 3.