Decoupling excitation pathways: A dual-wavelength strategy for ultraviolet upconversion in Tm3+ systems

Rare-earth-doped materials exhibiting ultraviolet upconversion luminescence (UV-UCL) are attractive for advanced photonic applications, yet developing flexible, controllable, and efficient systems remains challenging. Here, we introduce a dual-wavelength strategy that decouples excitation pathways for UV-UCL, in which UV emission occurs only when both excitation wavelengths are applied simultaneously; neither source alone produces detectable UV output. Our investigations, conducted on a model system composed of Tm3+-doped Mg3Y2Ge3O12 garnet, reveal that this behavior originates from sequential excited-state absorption. Initially, infrared photons populate the long-lived 3H4 or 3F4 intermediate levels. When a second photon in the blue-violet range is absorbed, it promotes the system to the UV-emitting 1I6 or 1D2 states. The UV emission wavelength is governed by the choice of blue-violet pump wavelength, while the emission intensity scales linearly with the visible-light power above a low threshold (∼10 μW cm−2). These results elucidate the photophysics of multi-wavelength upconversion in rare-earth systems and provide a practical route toward wavelength-tunable UV emitters for anti-counterfeiting, optical switching, and UV photonics.