A Type I Hyperbolic Metamaterial Driven by Phonons on ZnO

Negative refraction index can be achieved with uniaxial, type I hyperbolic metamaterials (HMMs) featuring ɛ_∥>0 and ɛ_⊥<0. A strategy to accomplish this has been to use surface plasmon polaritons (SPPs) in stacked doped/undoped semiconductor layers. Alternately, surface phonon polaritons (SPhPs) have emerged as a promising low-loss alternative. In this work, a phonon-driven type I HMM using ZnO/(Zn,Mg)O heterostructures is designed, demonstrating control over the hyperbolic behavior through the careful choice of Mg content and layer thicknesses. This study shows that increasing the Mg concentration in the ternary layers enhances type I behavior, while the optimal layer thickness varies depending on the Mg content. After analyzing the conditions for achieving type I hyperbolic dispersion, this concept is experimentally demonstrated with three samples. The structures are characterized by means of polarized reflectance spectroscopy and attenuated total reflectance spectroscopy is used to report the presence of a SPhP mode within the type I region. Employing the transfer matrix method, it is demonstrated that this mode exhibits negative frequency dispersion, a hallmark of type I hyperbolic modes, and isofrequency curve calculations further confirm this behavior. Controlling the design of a phononic type I HMM lays the groundwork for exploring low-loss, sub-diffraction-limited optical modes using SPhP excitations.