Diffraction‐Free Omnidirectional Antireflection Binary Metasurface via Femtosecond Laser Hybrid Etching

ABSTRACT An ideal anti‐reflection window requires both diffraction‐free transmission and mechanical robustness, which poses significant challenges for hard and brittle materials. Conventional subwavelength structures often achieve diffraction‐free transmission by either reducing the periodicity or increasing aspect ratio yet low processing efficiency hinders scalable fabrication. Herein, we propose a novel cone‐cylinder binary metasurface that overcomes the limitations of traditional subwavelength designs by exploiting multi‐mode resonance and phase compensation. This synergy enables non‐diffractive transmission in a scalable non‐subwavelength sapphire architecture. To realize the metasurface, we developed a femtosecond laser penetrating hybrid etching (FsLPE) technique, which enables the fabrication of centimeter‐scale mid‐infrared broadband windows on sapphire. The resulting device exhibits a peak transmittance of 98.3% at 5 µm, an average transmittance exceeding 92.0%, and minimal degradation (0.3%) even at a 50° incidence angle. Moreover, the window withstands temperatures up to 1200°C, demonstrating great potential for applications in infrared optical systems and optical sensing under extreme environmental conditions.