Zero‐Light Thermal Scanning of Multi‐Level Security Information via Localized Emissivity Modulation

ABSTRACT Machine‐readable security information requires higher encoding capacity against counterfeiting and readout without external illumination in automated dark factories, where visible‐light cameras fail. Although encoding in the thermal domain removes the illumination requirement, existing strategies rely on phase‐change materials, multilayer stacks, nanoimprint masters, or deep cavity ablation, thus complicating fabrication and limiting pixel‐level programmability. Here, we report a zero‐light‐readable, multi‐level thermal security platform based on maskless direct‐write laser exposure of a 20 nm‐thick Ag film on Si. Varying the per‐pixel exposure time from 0.05 to 1 s induces photothermal dewetting of the continuous Ag film into nanoparticles and micro‐islands. The resulting surface roughness enhances broadband infrared emissivity through localized plasmonic field confinement within subwavelength metallic voids. We fabricate 3‐ and 4‐level tags, each a 10 × 10 pixel array within a 1 mm × 1 mm footprint, achieving an overall emissivity tuning range of 0.23 across the laser‐exposed levels. A long‐wave infrared camera decodes the pixelated tags in complete darkness and at sample temperatures of up to 100°C. This maskless, single‐layer fabrication provides a programmable route to multi‐level infrared security information, and offers a basis for future object identification and authentication in zero‐light industrial environments.