Two-step quantum cutting efficiency in Pr3+-Yb3+codoped KY3F10

Abstract

The efficiency of a two-step quantum cutting (QC) mechanism for solar downconversion via two consecutive energy transfers from Pr${}^{3+}$ to Yb${}^{3+}$ is investigated in Pr${}^{3+}$-Yb${}^{3+}$ codoped KY${}_{3}$F${}_{10}$ crystals. The efficiency of the QC second energy transfer involving the Pr${}^{3+}$ (${}^{1}$${G}_{4}$) and Yb${}^{3+}$ (${}^{2}$${F}_{5/2}$) levels is particularly difficult to determine in a precise manner since these two levels are coupled by reciprocal energy transfer. We present a specific rate equation modeling that accurately describes Pr${}^{3+}$ and Yb${}^{3+}$ dynamics and intensity ratios measured as a function of the Yb${}^{3+}$ concentration. The modeling enables to calculate energy transfer rates for all the processes and shows the competition between the two-step QC mechanism and parasitic processes. The first-step of the QC mechanism is, as expected, particularly efficient reaching 96.9$%$ and is not challenged by competing processes. The second step of the QC, while reaching an intrinsic efficiency of 75.9$%$, is plagued by backtransfers from Yb${}^{3+}$ (${}^{2}$${F}_{5/2}$) to Pr${}^{3+}$ (${}^{1}$${G}_{4}$) along with efficient concentration quenching mechanisms draining the Yb${}^{3+}$ excited-state. In addition, the investigation of intensity ratios with the Yb${}^{3+}$ concentration shows the importance of energy migration among Yb${}^{3+}$ ions that further enhances the detrimental effect of the Yb${}^{3+}$ quenching processes. As a result, the intrinsic QC efficiency (QCE) of the QC first and second energy transfers increases with the Yb${}^{3+}$ doping level up to 173$%$ in KY${}_{3}$F${}_{10}$: 0.5$%$Pr${}^{3+}$-20$%$Yb${}^{3+}$, while the real QCE reduced by migration-assisted quenching processes is only 6.2$%$ in this sample. These results clarify apparently conflicting reports found in the literature showing, on one hand, very high QCEs derived from spectroscopic experiments and, on the other hand, very low efficiency values obtained from integrating sphere measurements.