NIR to VUV: Seven-photon upconversion emissions from Gd³⁺ ions in fluoride nanocrystals

Here we show that a near-infrared (NIR) diode laser is capable of generating vacuum ultraviolet (VUV) emissions in fluoride nanocrystals through photon upconversion (UC) processes. By using Yb³⁺ and Tm³⁺ as sensitizers, we successfully obtained the VUV photons with the energy exceeding 6 eV in YF₃: Yb, Tm, and Gd nanocrystals. The seven photon UC fluorescence from the ⁶G_J → ⁸S_7/2 transitions of Gd³⁺ ions and the possible VUV UC mechanism were reported along with the calculation of the branching ratio under diff erent pumping power excitation. Practically, it offers a promising solution for VUV light generation without cryogens and expensive instrumentations. Fundamentally, the extremely high-order UC processes will intrigue great interest in exploring unusual high-energy radiative transitions in rare earth ions.