Critical Power Density: A Metric To Compare the Excitation Power Density Dependence of Photon Upconversion in Different Inorganic Host Materials

Abstract

In photon upconversion (UC) based on triplet−triplet annihilation, the upconversion photoluminescent quantum yield (UC-PLQY) depends on the excitation power density in a way that can be described by a single figure of merit. This figure of merit, the threshold value, allows the excitation power density required for efficient UC-PLQY to be compared between different triplet−triplet annihilation systems. Here, we investigate the excitation power density dependence of two-photon UC processes in a series of four lanthanide-doped inorganic host materials (oxides, fluorides, and chlorides) all doped with 18 mol % Yb 3+ sensitizer ions and 2 mol % Er 3+ activator ions. We demonstrate that an analogous figure of merit, which we call the critical power density (CPD), accurately describes the UC power dependence of these samples. Better CPD values are obtained when the lifetime of the intermediate states is long. The UC-PLQY at the CPD is linked to the saturation UC-PLQY. Thus, a measurement of the UC-PLQY at this low power density can be used to estimate the theoretical saturation UC-PLQY in the absence of deleterious effects such as laser-induced heating. This is compared to another method to estimate the saturation based on the CPD model, namely, taking half of the level's PLQY under direct excitation. Our careful analysis of the upconversion spectra as a function of excitation power density gives several insights into the differing upconversion pathways in the hosts and proves to be a useful tool for their comparison.

An up-conversion luminophore with high quantum yield and brightness based on BaF2:Yb3+,Er3+ single crystals

“…The beneficial low CPD values of the 4 S 3/2 ⟶ 4 I 15/2 transition of the Er 3+ ions were earlier reported for the most efficient UC materials as 0.7 W/cm 2 in β-NaYF 4 , 1.0 W/ cm 2 in YF 3 and 1.0 W/cm 2 in La 2 O 3 . 60 The smallest CPD value of the 4 S 3/2 ⟶ 4 I 15/2 transition in BaF 2 crystal (with 2 % of Er 3+ , 15 % of Yb 3+ ions) is 1.1 W/cm 2 , which is just fractionally higher than values calculated for the best UC materials. Table S5 shows the results for the samples that provided the best fit.…”

“…Please do not adjust margins The setup and the methodology for estimation of  UC under 976 nm excitation have been described earlier. 23,60 To record  DS of the 4 S 3/2 ⟶ 4 I 15/2 and 4 F 9/2 ⟶ 4 I 15/2 transitions of the Er 3+ ions a tunable CW laser (Solstis with EMM-Vis, M-Squared Lasers Ltd) pumped by 532 nm laser (Verdi-V18, Coherent) was utilized. The system was tuned to 522 nm for the direct excitation of the 4 S 3/2 level and to 652 nm for the direct excitation of the 4 F 9/2 level.…”

“…The rest of the setup was the same as described in our earlier publication. 60 Luminescence lifetimes of the emissive levels were measured with a home-built optical system described previously. 61 Briefly, 525 nm, 976 nm, and 633 nm (Roithner) and 1550 nm (Thorlabs) laser diodes mounted in temperature stabilized mounts (TCLDM9, Thorlabs) and driven by a laser diode controller (ITC4001, Thorlabs) as well as 375 nm LED driven also by controller (ITC4001, Thorlabs) were used as the excitation sources.…”

“…78 As another UC figure-of-merit parameter, the critical power density (CPD) is calculated for each sample using an earlier published method. 60 This parameter describes the saturation  UC and facilitates the comparison of different UC materials.…”

Ratiometric Luminescent Thermometry with Excellent Sensitivity over a Broad Temperature Range Utilizing Thermally‐Assisted and Multiphoton Upconversion in Triply‐Doped La2O3:Yb3+/Er3+/Nd3+

“…On the other hand, the Er 3+ : 4 F 9/2 is populated by two photon mechanisms that do not require thermal activation. [ 21 ] However, the non‐radiative rate increases with temperature thus decreasing the 660 nm band emission. In Figure 1, we schematically show this as non‐radiative transfer between Er 3+ : 4 F 9/2 and Er 3+ : 4 I 9/2 for simplicity, but this reduction can be thought to encompass all additional non‐radiative losses from precursor levels.…”