Pascal Gerner, Karl Krämer and Hans U. Güdel
Broad-band Cr⁵⁺-sensitized Er³⁺ luminescence in YVO₄
J. Lumin. 102-103, 112-118 (2003)                 

Abstract: In nominally 2% Cr⁵⁺ and 10% Er³⁺ co-doped YVO₄, a highly efficient nonradiative energy transfer Cr^{5+ 2}B₂Er^{3+ 4}I_13/2 is active, which is competitive with the multiphonon relaxation of Cr⁵⁺. A new type of near-infrared-to-visible upconversion process is identified. It consists of a Cr^{5+ 2}A₁²E excitation, nonradiative ²E to ²B₂ relaxation, energy transfer to Er^{3+ 4}I_13/2 and finally excited state absorption processes on Er^{3+ 4}I_13/2⁴S_3/2, ⁴F_7/2, ²H_11/2. In addition, Er^{3+ 4}I_11/2Cr^{5+ 2}B₂ energy transfer occurs. The pathway through Cr^{5+ 2}B₂ leads to an enhanced nonradiative relaxation rate from the Er^{3+ 4}I_11/2 to the laser relevant ⁴I_13/2 energy level. The rate constants of the energy transfer processes are estimated.