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

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.