Dual‐Wavelength‐Driven Upconversion for Tunable UV Photonic Security

ABSTRACT Controlling UV emission through upconversion luminescence (UCL) presents an exciting pathway for developing secure photonic technologies, yet conventional single‐wavelength excitation methods leave encoded features vulnerable to replication. Here, a dual‐wavelength excitation strategy is proposed, in which UV emission is generated via excited‐state absorption (ESA) upon simultaneous illumination with infrared and visible light. Using a model system based on Tm 3+ ‐doped Y 3 Ga 5 O 12 , this approach enables distinct UV emissions (e.g., at 294, 350, or 365 nm) under concurrent 800 nm and blue‐violet excitation. Spectroscopic investigations reveal that 800 nm light pre‐populates long‐lived intermediate states ( 3 H 4 and 3 F 4 ), enabling low‐power visible photons to reach the UV‐emitting levels ( 1 I 6 or 1 D 2 ). Remarkably, this process operates at exceptionally low visible‐light power thresholds (∼0.05 mW∙cm −2 ), comparable to typical indoor lighting levels. Additionally, by co‐doping the material with Gd 3+ ions, we demonstrate the capability to further tailor the UV emission profiles, thereby offering increased flexibility for applications such as multi‐mode UV anti‐counterfeiting. These findings suggest that our dual‐wavelength excitation strategy could serve as a complementary addition to existing UV encoding techniques, providing an extra layer of photonic security.