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

Abstract

intriguing opportunity. HMMs are uniaxial structures manifesting a hyperbolic dispersion relation whose application range spans from hyperlensing to extreme biosensing. [13][14][15][16][17][18][19][20][21][22][23][24] In order to describe their optical constants, it is convenient to homogenize the dielectric permittivity of HMMs by using the effective medium theory (EMT). If the dimensions of the fundamental components of the HMM, that in our case consists of alternated metal/dielectric layer pairs, are chosen to be deeply subwavelength, a uniaxial anisotropy arises, due to the specific periodic arrangement, with the appearance of two distinct values of dielectric permittivity, one parallel (ε || ) and one perpendicular (ε ⊥ ) to the plane of the layersHere t D and t M are the dielectric and metal thickness, respectively, and ε D and ε M their relative dielectric permittivities. Equation (1) reveals that ε || can vanish at a specific wavelength where (ε D /ε M ) = -(t M /t D ). In the resulting regime, known as epsilon-near-zero (ε NZ ), many fascinating phenomena can occur. [25][26][27][28] Unfortunately, once the fundamental componentsThe 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 TiO 2 , and an organic dye. Due to the hygroscopic nature of unsintered TiO 2 , 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.

Hybridization of epsilon-near-zero modes via resonant tunneling in layered metal-insulator double nanocavities

“…However, for fewlayer metal/insulator stacks, the origin and nature of the ENZ resonances are still under investigation [5,7,39]. For example, the effective medium theory fails to predict their effective permittivity [5,40,41], and it can be debated if the ENZ resonances are photonic or plasmonic or if classical or quantum mechanical models should be applied [7]. In a recent work, we demonstrated that MIM cavities can be seen as a double barrier quantum well and that the ENZ cavity modes can be treated as the resonant tunneling of photons [7].…”

Hyperbolic Metamaterials and Metasurfaces: Fundamentals and Applications

“…Such high refractive index sensitivity is attributed to strong near‐field enhancement of surface plasmon waves on the interface of a metal and a hyperbolic chiral‐sculptured thin film. Besides biosensing application, temperature sensor is also designed based on HMMs using hygroscopic dielectric blend . This dielectric blend consists of a polymer, a sol–gel unsintered TiO 2 , and an organic dye.…”

“…Compared with other optical metamaterials, like chiral and split ring resonator–based metamaterials, HMMs have advantages of relative ease of fabrication at optical frequencies, broadband nonresonant and 3D bulk responses, and flexible wavelength tunability. As a result, HMMs have attracted widespread interest and become a good multifunctional platform for many exotic applications, such as optical negative refraction and light beam steering, subdiffraction‐limited imaging and nanolithography, spontaneous and thermal emission engineering, ultrasensitive optical, biological, and chemical sensing, omnidirectional and broadband optical absorption …”