New Type of Near-Infrared to Visible Photon Upconversion in Tm²⁺-Doped CsCaI₃

Tm2+ doped in CsCaI3 displays unusual optical properties that are characterized by the existence of two metastable 4f-5d excited states in the near-infrared and visible spectral region, respectively. For the first time, a photon upconversion process based on sequential absorption of light by 4f-5d states is reported. The large absorption cross-section of the involved transitions allows highly efficient pumping in the NIR. An efficiency of 11% for the green upconversion luminescence is reached at 10 K, and the upconversion luminescence remains visible by eye up to room temperature. The energy positions of the relevant 4f-5d states and thus the photophysical and light emission properties can be tuned by chemical variation, such as placing the Tm2+ ion into the isostructural CsCaBr3 and CsCaCl3 lattices.