Ultraviolet Light Generation through Lanthanide Upconversion

ConspectusUpconversion is a nonlinear optical process in which long-wavelength photons are absorbed by specific material systems and converted into shorter-wavelength light. Yb³⁺-Ln³⁺ (Ln: Er/Ho/Tm) pairs are the most widely studied upconversion systems, demonstrating great success in efficient near-infrared-to-visible light conversion. Nevertheless, further exploration of upconversion luminescence toward shorter wavelengths, especially in the UV region, has achieved limited progress. In comparison with visible light, UV radiation suffers from minimized interference from natural and most artificial light sources. By shifting the emission to the deep UV band, for example, solar interference could be circumvented, enabling highly valuable applications such as solar-blind imaging and labeling. Additionally, due to the higher photon energy in this spectral range, the system could be simultaneously employed for sterilization, phototherapy, and plastic degradation.To unlock the application potentials of UV-emitting upconversion materials, substantial research efforts have been undertaken in recent years. Specifically, classic visible upconverting Er³⁺ and Tm³⁺ ions have been repurposed for UV emission due to their rich energy levels extending to the UV spectrum region. To effectively populate the high-lying excited states, systematic investigations into doping concentrations, host lattice compositions, and excitation schemes have been conducted. In parallel, Pr³⁺─typically ineffective for near-infrared to visible upconversion─has been established as a prominent candidate for UV upconversion under blue-light excitation. By precisely tuning its 4f¹5d¹ state through host lattice engineering, both the upconversion dynamics and emission characteristics can be strategically optimized.In this Account, we focus on recent advances in UV upconversion through lanthanide-doped inorganic crystals, primarily drawing upon our research group's advancements over the past few years. We begin by summarizing the methods for constructing UV upconversion materials based on rational selection of dopant ions and host crystals, including Er³⁺-, Tm³⁺-, and Pr³⁺-based systems. Building on these foundations, we introduce emerging methods for enhancing the UV upconversion emission intensity, encompassing dielectric coupling, plasmonic modulation, and organic surface coating, which all have a certain degree of universality. The subsequent section will focus on the frontier applications of UV upconversion in lighting, imaging, and environmental sciences. In the end, we conclude by providing a summary and a perspective on future directions.