Tuning of near-infrared-to-near-infrared luminescence from one-photon to two-photon anti-Stokes shift in Ca₃Ga_2- _xCr_xGe₃O₁₂ via varying Cr³⁺ content.

Near-infrared-to-near-infrared (NIR-to-NIR) anti-Stokes luminescence from Cr³⁺ singly doped Ca₃Ga₂Ge₃O₁₂ (CGGG) occurs under the excitation of an 808 nm diode laser. The anti-Stokes processes vary from one photon to two photon, depending on the Cr³⁺ content (x) in Ca₃Ga_2-xCr_xGe₃O₁₂. The results suggest that phonon-assisted anti-Stokes excitation and cooperative energy transfer are involved in the observed upconversion (UC) processes of CGGG:Cr³⁺. The relevant Cr³⁺-doping-concentration-dependent NIR-to-NIR anti-Stokes luminescent mechanism, either one-photon or two-photon UC, is investigated. Such an observation on modulating the UC process via varying the doping concentration is helpful in broadening the understanding of UC phenomena.