Mechanism and Controlling Factors of Infrared-to-Visible Conversion Process in Er³⁺ and Yb³⁺-Doped Phosphors

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Luminescence and related optical properties of Er³⁺ and Yb³⁺-doped YF₃, Y₃OCl₇ and YOCl phosphors are precisely and extensively measured in the range from 4.2 K to room temperature, in order to investigate the mechanism and controlling factors of infrared-to-visible conversion. It is concluded that the red luminescence emitted from Er:⁴F_9⁄2 is produced by two energy transfers from Yb³⁺ to Er³⁺ ions, and that ⁴F_9⁄2 is directly populated from Er:⁴I_13⁄2 by the second energy transfer. The cutoff phonon energy and electron-phonon coupling constant of the hosts, which dominate the probabilities of internal relaxation and phonon-assisted energy transfer, are indicated to be important controlling factors for the conversion process, in addition to the relative energy positions of Yb:²F_5⁄2 and Er:⁴I_11⁄2. The hosts having larger values of these factors, such as Y₃OCl₇, are advantageous to the red luminescence, while those with the opposite tendency, such as YF₃, are favorable to the green luminescence.