Conventional photoluminescence (PL) yields at most one emitted photon for each absorption event. Downconversion (or quantum cutting) materials can yield more than one photon by virtue of energy transfer processes between luminescent centers. In this work, we introduce Gd₂O₂S:Tm³⁺ as a multi-photon quantum cutter. It can convert near-infrared, visible, or ultraviolet photons into two, three, or four infrared photons of ~1800 nm, respectively. The cross-relaxation steps between Tm³⁺ ions that lead to quantum cutting are identified from (time-resolved) PL as a function of the Tm³⁺ concentration in the crystal. A model is presented that reproduces the way in which the Tm³⁺ concentration affects both the relative intensities of the various emission lines and the excited state dynamics and providing insight in the quantum cutting efficiency. Finally, we discuss the potential application of Gd₂O₂S:Tm³⁺ for spectral conversion to improve the efficiency of next-generation photovoltaics.

The present study reports for the first time the optimization of the infrared (1523 nm) to near-infrared (980 nm) upconversion quantum yield (UC-QY) of hexagonal trivalent erbium doped sodium yttrium fluoride (β-NaYF₄:Er³⁺) in a perfluorocyclobutane (PFCB) host matrix under monochromatic excitation. Maximum internal and external UC-QYs of 8.4% ± 0.8% and 6.5% ± 0.7%, respectively, have been achieved for 1523 nm excitation of 970 ± 43 Wm⁻² for an optimum Er³⁺ concentration of 25 mol% and a phosphor concentration of 84.9 w/w% in the matrix. These results correspond to normalized internal and external efficiencies of 0.86 ± 0.12 cm² W⁻¹ and 0.67 ± 0.10 cm² W⁻¹, respectively. These are the highest values ever reported for β-NaYF₄:Er³⁺ under monochromatic excitation. The special characteristics of both the UC phosphor β-NaYF₄:Er³⁺ and the PFCB matrix give rise to this outstanding property. Detailed power and time dependent luminescence measurements reveal energy transfer upconversion as the dominant UC mechanism.