Fig. 1: Basic properties of Tm³⁺/Yb³⁺ codoped phosphors.

a Emission spectrum of YBO₃ doped with 0.1% Tm³⁺ and 2% Yb³⁺ upon excitation of the Tm^{3+ 1}G₄ level at 465 nm. The red-shaded band is due to the ²F_5/2 → ²F_7/2 emission of Yb³⁺, and the green-shaded band is due to the overlapping ¹G₄ → ³H₅ and ³H₄ → ³H₆ emissions of Tm³⁺³¹. b Corresponding excitation spectrum, measured by scanning through the Tm^{3+ 3}H₆ → ¹G₄ absorption transition and recording the intensity of the ¹G₄ → ³F₄ emission at 653 nm (15,300 cm⁻¹). c Cooperative energy transfer involves the distribution of the excited-state energy in the Tm^{3+ 1}G₄ level over two nearby Yb³⁺ ions. d This can eventually yield two near-infrared photons of 1000 nm emitted by Yb³⁺ per blue photon absorption event. e A Tm³⁺ ion in the ¹G₄ level can alternatively transfer part of its energy to a single nearby Yb³⁺ ion through phonon-assisted cross-relaxation. f This produces at most one near-infrared photon of 1000 nm. g–j Scanning electron micrographs of the four host materials in order of decreasing highest phonon energy: g YBO₃, h YAG, i Y₂O₃, and j NaYF₄. The scale bars represent 5 μm