In the compelling race of finding alternative plasmonic material, metallic sodium tungsten bronzes, Na_xWO_3 with x>0.25, hostpromising optoelectronic properties emerging from the insulator-metal transition (IMT), such as strong interband transition andintense near-infrared plasmonic absorption. So far, studies have focused on tuning the IR plasmonic properties for therealization of functional devices, ranging from biosensors to smart windows. However, the utilization of the transparency bandwhere the permittivity approaches zero still remains largely unexplored. Here, we show preliminary results which indicates anepsilon-near-zero (ENZ) behavior at optical frequencies of Na_xWO_3 which arises from the minimization of the total scatteringcross-section. Additionally, as a proof of concept, we explore this material for sensing applications and we establish aperformant optical sensor with sensitivity of 150 nm/RIU and showing a threefold enhancement with respect to traditional Aunanospheres. The peculiar sensing mechanism is investigated both experimentally and theoretically by means ofelectrodynamic and first principle calculations. Combined with the high quality of the Na_xWO_3 single crystals, ENZ propertiesin the ~400-600 nm region and low losses, these new insights offer great promise for the inexpensive realization of newgenerations of electro-optical devices with application ranging from ultrasensitive biosensors and light harvesting to exoticcloaking materials.
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