Femtosecond laser maskless direct writing of dual-band crosstalk-free information for all-in-one high-security encryption metasurface

Metasurface fabrication still faces critical processing challenges in balancing the structural order and disorder, achieving high-speed patterning, and extending material compatibility to refractory metals for operation under extreme conditions. This study demonstrates that femtosecond laser maskless direct writing (fs-LMDW) offers a versatile platform for engineering multispectral information, all-in-one metasurfaces on pure zirconium (Zr) substrates. Through sequential fs-LMDW in air and ethylene glycol (EG), deceptive grey-colored visible information is superimposedly encoded with the infrared (IR)-encrypted information (invisible among black-color structured background), which leverages the crosstalk-free structural modulation of singular-band IR and visible light. The metasurface exhibits robust thermal stability and high-security encryption capability across a wide temperature range, with IR-concealed information (such as QR code) remaining securely encrypted until thermally activated at 300 °C for smartphone-readable information decryption. Furthermore, the visible information is both erasable through 300 °C oxidation heating in air and rewritable via fs-LMDW in EG without compromising IR encryption security. Particularly, the one-time complete erasability makes it possible to identify whether the encrypted IR-information has been decrypted, underscoring the robustness and high security of the platform. The presented hierarchical micro/nanostructuring methodology is deemed to be applicable to a large material matrix to gain high-security, and multifunctional metasurfaces that are more difficult and complex for current metasurface fabrication techniques.