Long-term efficiency preservation for gradient phase metasurface diffraction gratings in the visible

Metal-dielectric metasurfaces with an engineered phase response provide a versatile technology for the next generation of optical systems. However, metal oxidation is detrimental to the performance and lifespan of these devices. Here we demonstrate the design and fabrication of a 1200 lp/mm coated reflective metal-dielectric metasurface grating for use with 650 nm illumination. An 80 nm PMMA coating protects the sample from metal oxidation and sample degradation. The fabricated samples exhibit ~45% efficiency in the first diffractive order over 0-50° angle of incidence consistent with the electromagnetic simulations. The coated samples preserve the performance over 25 weeks in standard room temperature and humidity while the unprotected samples show over an 80% drop in efficiency.