Multi-stimulated far-UVC luminescence for solar-blind imaging

Lanthanides-doped luminescent materials have gathered considerable attention due to their application potential in stress sensing, lighting and display, anti-counterfeiting technology and so forth. However, existing materials mainly cover the 380-1540 nm range, with slight extension to the UV region, impeding their applications in solar-blind imaging, background-free tracking, concealed communication, etc. To address this challenge, here we propose guidelines for far-UVC (200-230 nm) optical design. Accordingly, we achieve multi-stimulated far-UVC luminescence at ~222 nm in Pr³⁺-doped SrF₂, stemming from the inter-configurational 4f5d → 4f ² transition of Pr³⁺. Besides Pr³⁺, the SrF₂ host shows high tolerance to Ce³⁺, Nd³⁺, Sm³⁺, Eu^2+,3+, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺ and Yb³⁺, vastly extending the emission wavelength across the entire spectral range from 200 to 1700 nm. Particularly, these materials exhibit self-recoverable mechanoluminescence by direct mechanical excitation, along with thermally and mechanically stimulated emission after X-ray irradiation. We demonstrate that these lanthanides-doped SrF₂ crystals offer unique opportunities for high-contrast marking and structural health monitoring in complex environments.