Environmental Control of the Topological Transition in Metal/Photoemissive‐Blend Metamaterials

Abstract The quest for unconventional optical materials finds natural answers in the field of plasmonics. Here, special composites can manifest singularities in their dielectric permittivity. The so‐called epsilon‐near‐zero (ε_NZ) condition is typically encountered in artificial materials called hyperbolic metamaterials (HMMs). Unfortunately, tuning the HMMs ε_NZ is still challenging. Here it is demonstrated how the ε_NZ frequency of an HMM can be reversibly tuned via thermally induced water absorption/desorption. The key element is a dielectric hygroscopic material, consisting of a blend of a polymer, a sol–gel unsintered TiO₂, and an organic dye. Due to the hygroscopic nature of unsintered TiO₂, an increase of temperature induces a reversible physical contraction of the thickness of the dielectric blend, as well as an increase of refractive index. This causes a remarkable 45 nm shift of the absorption peak of the embedded dye, acting as a chromatic label. When such a blend is embedded in an HMM, a reversible thermal tuning of the overall optical response, as well as an epsilon‐near‐zero wavelength shift by about 25 nm, is induced. The remarkable tuning range shown here, besides obvious HMM‐based temperature sensing applications, paves the way toward a plethora of new functions in which tunable ε_NZ materials are needed.