White-light excitable NaYF ₄ :Nd ³⁺ for ultraviolet anti-counterfeiting applications

Using luminescent technology for security verification holds promise for optical anti-counterfeiting. However, existing strategies often struggle in brightly lit environments due to ambient light interference. To address this issue, we introduce NaYF₄:Nd³⁺ as a flexible platform for ultraviolet (UV) luminescence in security applications. This phosphor exhibits readily detectable upconversion luminescence under intense white-light illumination, emitting at 354 nm-a wavelength beyond the typical indoor spectrum-for background-free signal detection. Driven by a two-photon excitation mechanism with an illumination threshold of 80 mW·cm^-2, UV imaging under daylight validates its reliability. This approach offers a robust, real-world solution for effective anti-counterfeiting.