Nonresonant ENZ metamaterial at visible wavelength for superior refractive index matching sensing

In the compelling race of finding alternative plasmonic material, metallic sodium tungsten bronzes, Na_xWO₃ with x<0.25, host promising optoelectronic properties emerging from the insulator-metal transition (IMT), such as strong interband transition and intense near-infrared plasmonic absorption. So far, studies have focused on tuning the IR plasmonic properties for the realization of functional devices, ranging from biosensors to smart windows. However, the utilization of the transparency band where the permittivity approaches zero still remains largely unexplored. Here, we show preliminary results which indicates an epsilon-near-zero (ENZ) behavior at optical frequencies of Na_xWO₃ which arises from the minimization of the total scattering cross-section. Additionally, as a proof of concept, we explore this material for sensing applications and we establish a performant optical sensor with sensitivity of 150 nm/RIU and showing a threefold enhancement with respect to traditional Au nanospheres. The peculiar sensing mechanism is investigated both experimentally and theoretically by means of electrodynamic and first principle calculations. Combined with the high quality of the Na_xWO₃ single crystals, ENZ properties in the ∼400-600 nm region and low losses, these new insights offer great promise for the inexpensive realization of new generations of electro-optical devices with application ranging from ultrasensitive biosensors and light harvesting to exotic cloaking materials.