Non-Periodic Epsilon-Near-Zero Metamaterials at Visible Wavelengths for Efficient Non-Resonant Optical Sensing.

Epsilon-near-zero (ENZ) materials offer unique properties for the engineering of advanced optoelectronic devices with applications extending from optical clocking to loss-less high-speed optical communication technology. To date, the fabrication of ENZ materials at visible wavelengths relies mostly on the use of periodic structures, providing some manufacturing and material challenges. Here, we present the engineering of non-periodic sodium tungsten bronzes (NaxWO3) metamaterials featuring epsilon-near-zero (ENZ) properties in the visible spectrum. We showcase their use as efficient and highly performing optical sensors demonstrating a non-resonant sensing mechanism based on refractive index matching. Our optimized ENZ metamaterials display an unconventional blue-shift to increasing refractive index of the surrounding environment, achieving sensitivity as high as 150 nm/RIU, without the high radiative losses and heat dissipation associated with metallic plasmonic materials. Our theoretical and experimental investigations provide first insights on this sensing mechanism establishing guidelines for the future engineering and implementation of efficient ENZ sensors. The unique optoelectronic properties demonstrated by this class of tuneable NaxWO3 materials, such as metal-insulator transition and visible transparency bands, bear potential for various other applications ranging from light harvesting to optical filters and photodetectors.