Understanding and tuning blue-to-near-infrared photon cutting by the Tm3+/Yb3+couple

Light conversion: Insights into photon cutting Improved understanding of 'photon cutting', in which a high-energy photon is absorbed by a phosphor material and converted into two or more lower-energy photons, could lead to new applications in fields ranging from experimental optics to commercial light displays. Qinyuan Zhang at South China University of Technology and Freddy Rabouw at Utrecht University in the Netherlands, with colleagues, explored photon cutting in crystals doped with ions of the lanthanide elements Thulium (Tm3+) and Ytterbium (Yb3+). They identified key energy transfer mechanisms and methods for fine-tuning the process by exploring the conversion of photons of blue light into near-infrared photons, in several phosphors. The research confirms the potential of Tm3+/Yb(3+)doped materials for achieving light energy conversion efficiencies above 100 percent, but suggests earlier reports of surprisingly high efficiencies should be treated with caution. Lanthanide-based photon-cutting phosphors absorb high-energy photons and 'cut' them into multiple smaller excitation quanta. These quanta are subsequently emitted, resulting in photon-conversion efficiencies exceeding unity. The photon-cutting process relies on energy transfer between optically active lanthanide ions doped in the phosphor. However, it is not always easy to determine, let alone predict, which energy-transfer mechanisms are operative in a particular phosphor. This makes the identification and design of new promising photon-cutting phosphors difficult. Here we unravel the possibility of using the Tm3+/Yb(3+)lanthanide couple for photon cutting. We compare the performance of this couple in four different host materials. Cooperative energy transfer from Tm(3+)to Yb(3+)would enable blue-to-near-infrared conversion with 200% efficiency. However, we identify phonon-assisted cross-relaxation as the dominant Tm3+-to-Yb(3+)energy-transfer mechanism in YBO3, YAG, and Y2O3. In NaYF4, in contrast, the low maximum phonon energy renders phonon-assisted cross-relaxation impossible, making the desired cooperative mechanism the dominant energy-transfer pathway. Our work demonstrates that previous claims of high photon-cutting efficiencies obtained with the Tm3+/Yb(3+)couple must be interpreted with care. Nevertheless, the Tm3+/Yb(3+)couple is potentially promising, but the host material-more specifically, its maximum phonon energy-has a critical effect on the energy-transfer mechanisms and thereby on the photon-cutting performance.