Genetically optimized dual-wavelength all-dielectric metasurface based on double-layer epsilon-near-zero indium-tin-oxide films

Abstract

Following the pioneering works on electrically tunable conducting oxide-based reflectarray metasurfaces, it has been shown that maximum phase modulation can be realized at a wavelength, where the transition from over- to under-coupling regimes coincides with the epsilon-near-zero (ENZ) phenomenon inside the indium-tin-oxide (ITO) active layer. However, the ENZ transition is restricted to a narrow bandwidth in the near-infrared regime, which limits the maximal achievable phase span at the wavelengths exterior to this bandwidth. Here, we present the realization of a dual wavelength all-dielectric metasurface with a large wavelength-contrast ratio between the operating channels, which is integrated by double-layer ITO films. The doping densities inside the ITO films are judiciously controlled to facilitate the ENZ-crossing of the relative permittivities at the corresponding working wavelengths. The all-dielectric metasurface is comprised of the arrays of cross-shaped holes made inside a high-index silicon slab supporting two resonances that are 300 nm apart. Numerical analysis of the near-field resonant modes reveals the excitation of guided-mode and magnetic dipole resonances, which strongly overlap with ITO active layers. Leveraging from the double-ENZ effect, considerable phase modulations of almost 220° and 240° are attained with a single metasurface platform at the wavelengths of λ1=1200 nm and λ2=1500 nm under the bias voltage application. The design parameters including the geometrical sizes and plasma frequencies of the differently doped ITO layers are carefully optimized by multi-objective genetic algorithm. The proposed metasurface illustrates a great promise in tunable beam splitting of the reflected light and dynamic conversion of the polarization states.