Quadrature frequency resolved spectroscopy of upconversion photoluminescence in GeGaS:Er³⁺; II. elucidating excitation mechanisms of red emission besides green emission

The paper addresses the quadrature frequency resolved spectroscopy (QFRS) on the red (⁴ F _9/2 → ⁴ I _15/2) upconverted photoluminescence (UCPL) as well as the green (⁴ S _3/2 → ⁴ I _15/2) UCPL of Er³⁺ in GeGaS chalcogenide glasses at the pump wavelength 975 nm. On the basis of the perturbed rate equations for the generally accepted 5-level model of the green and red UCPLs of Er³⁺, we have derived the formulas for the QFRS on the green and red UCPLs of the two differently Er-doped samples. Adopting the respective formulas as model functions for the nonlinear regressions on the QFRS data of both the UCPLs, we have determined the relevant parameters such as relaxation rates at the four excited levels, the excited absorption cross section and energy transfer parameters. Thereby the differences of UCPL dynamics between the green and red UCPLs as well as between the two differently Er-doped Ge₂₅Ga₁₀S₆₅ samples are interpreted in terms of the determined parameters.