# Combined Mie Resonance Metasurface for Wideband Terahertz Absorber
## 2 Structure Design and Simulation
All in all, the magnetic resonances occurring inside blocks of different sizes caused absorption peaks at different frequencies, while the special resonance caused by the near-field coupling between blocks led to relatively weaker absorption valleys between absorption peaks. Therefore, incorporating similar structures of different sizes into each unit cell could broaden the absorption bandwidth. Figure 4: The cross-sectional magnetic field distribution in the \(x\)-\(z\) plane with \(y=40\) μm, \(0\) μm, \(-40\) μm and cross-sectional electric field distribution in the \(y\)-\(z\) plane with \(x=-40\) μm, \(0\) μm, \(40\) μm for our combined metasurface absorber at the frequency of (**a**) 0.85 THz; (**b**) 0.867 THz, and (**c**) 0.883 THz, respectively. We compared the absorption spectra of the metasurface absorber with single dielectric resonator--four or nine dielectric resonators in one unit cell--as shown in Figure 6a. Bandwidth when absorption rate over 0.8 is also marked out. The size of this single dielectric resonator was \(33\times 33\times 30\)\(\upmu\)m\({}^{3}\). These four resonators of the combined metasurface absorber were \(32.1\times 32.1\times 30\)\(\upmu\)m\({}^{3}\), \(32.7\times 32.7\times 30\)\(\upmu\)m\({}^{3}\), \(33.3\times 33.3\times 30\)\(\upmu\)m\({}^{3}\), and \(33.9\times 33.9\times 30\)\(\upmu\)m\({}^{3}\). The combined metasurface absorber with nine resonators was the same as mentioned above. As we can see, as the number of different resonators increased in each unit cell, the frequency domain of absorption also widened significantly. The bandwidth of absorption rate above 0.8 was 0.012 THz, 0.024 THz, and 0.047 THz. Figure 6b shows the absorption spectra of the single resonator metasurface with different side lengths (from 31 \(\upmu\)m to 35 \(\upmu\)m). Comparing Figures 6a and 6b, we can see that the absorption bandwidth of the metasurface combining nine resonators was comparable with the bandwidth covered by nine adjacent absorption spectra (Figure 6b), which was the same as the four blocks combined. Moreover, according to Figure 6, we can also observe that the greater the number of combined structures (or the larger the range of the structure size), the wider was the absorption frequency range of this combined metasurface absorber. Figure 5: The cross-sectional magnetic field distribution in the \(x\)-\(z\) plane with \(y=40\)\(\upmu\)m, \(0\)\(\upmu\)m, \(-40\)\(\upmu\)m and the cross-sectional electric field distribution in the \(y\)-\(z\) plane with \(x=-40\)\(\upmu\)m, \(0\)\(\upmu\)m, \(40\)\(\upmu\)m for our combined metasurface absorber at the frequency of (**a**) 0.857 THz and (**b**) 0.875 THz. It is well known that the dielectric resonator can restrict the electromagnetic field into its structure, which is also the basis of silicon-based waveguides. In this design, when the size of the block (the proportion of the block in the unit cell) is slightly reduced, the electromagnetic energy will be limited to a smaller area, and the resonance inside the structure will be stronger due to higher energy density. We checked the electromagnetic field of different sizes and found that the maximum magnetic field amplitude of the smaller structure would be larger. For example, the maximums of the magnetic field distribution were 40,491 A/m, 38,070 A/m, and 36,051 A/m when the side lengths of the single block structure were 31 \(\upmu\)m, 33 \(\upmu\)m, and 35 \(\upmu\)m, respectively. This absorber was based on the magnetic resonance of the dielectric structure and was mainly affected by the magnetic field energy. Thus, the smaller size corresponded to higher absorption rate, and there was a slope in the absorption level for different sizes. It is worth noting that this phenomenon is based on the premise that structural changes do not break the resonance. Figure 6: (**a**) Absorption spectra of the Mie resonance-based metasurface absorber with one, four, and nine dielectric resonator in one unit cell.