In this paper, a tunable dielectric metamaterial absorber with temperature-based vanadium dioxide (VO 2 ) is proposed. In contrast to previous studies, both the metal phase of VO 2 and the semiconductor phase are applied to manipulate the Mie resonant modes in the dielectric cubes. By embedding VO 2 in the main resonant structure, the control over Mie resonant modes in dielectric metamaterials is realized. Each resonant mode is analyzed through field distribution and explains why the phase switch of VO 2 could affect the absorbance spectrum. This use of tunable materials could create another new methodology for the manipulation of the Mie resonance-based dielectric cubes and make them closer in essence to isotropic metamaterials.Symmetry 2018, 10, 423 2 of 12 composite material functions as a sub-wavelength metamaterial with the tunable operating material phase permitting the realization of an absorber.
Modeling and Design PrincipleThis study starts with the cross-shaped dielectric metamaterial absorption. For metamaterial or metasurface absorbers, one of the most important conditions is the exploitation of epsilon-near-zero (ENZ) materials [26]. A cross-shaped absorber was developed from the cube absorber [27]. By adding other components into the unit cell, ENZ materials can be obtained by the Mie resonances inside the cross-shaped structure.First, this study starts with the Mie resonance inside the dielectric cube. The incident wave excites different Mie resonant modes at different frequencies. As there are different application requirements, full control over the resonant modes is needed. Learning from the broadband antenna or left-handed metamaterial design, adding some other structures into the unit cell could manipulate the resonant modes by tuning the nearby resonances together. Based on previous experience with absorbers, the cross-shaped dielectric absorber could have three absorbance peaks corresponding to three resonant frequencies and resonant modes. At each resonant frequency, the incident energy is trapped in the center of the shape and both dielectric arms, on the basis of the electric wave. This multi-mode resonance structure has great advantages in wave control, and also makes it easier to accomplish different electromagnetic goals. Compared to the similar shapes [28,29], the cross-shaped absorber can also be regarded as a complementary dielectric cube absorber. Based on the equivalent circuit theory, the cross-shaped structure is smaller while keeping the same resonant performance. Additionally, the unit cell is composed of dielectric ceramics, which means low ohmic loss and a greater suitability for high-temperature and high-power conditions. Considering that temperature is one of the most important parameters in this study, it is necessary to keep the host unit cell shape and constitutive parameters stable while tuning the temperature. Therefore, the cross shape is the best candidate for the study of a hybrid VO 2 and dielectric metamaterial absorber.