Luminescence of Tb_1−xY_xPO₄ under the Strong and Selective Excitation of ⁵D₄ of Tb³⁺ Ion

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Luminescence of Tb_1−xY_xPO₄ under the strong and selective excitation of ⁵D₄ manifold of Tb³⁺ ion is investigated using the 488 nm light from an argon ion laser at 1.8 K. Besides the strong luminescence from ⁵D₄, weak anti-Stokes luminescence from ⁵D₃ is observed. On the basis of the arguments in which the dependence of the pumping rate of ⁵D₃ on the Tb³⁺ concentration is analyzed, it is verified that due to the resonance energy transfer between two excited Tb³⁺ ions, one ion is excited into higher states, thus yielding the observed anti-Stokes luminescence. The Probability of this energy transfer for the nearest neighbor ion pair is experimentally estimated as 10⁶ sec⁻¹.