# Numerical Study of an Ultra-Broadband All-Silicon Terahertz Absorber
## 3 Simulated Results and Discussion  
First, the polarization characteristic was analyzed, as shown in Figure 2. For TE polarization, the electric field is transverse along the x-axis, and for TM polarization, the magnetic field is also transverse along the x-axis. It can be observed that the spectral curves of the two polarizations are highly coincident owing to the highly structural symmetry. As observed from the figure shown above, absorption rates of nearly 100% and 97.8% can be achieved at the resonant frequencies of 1 and 1.72 THz, respectively. The all-silicon terahertz absorber has an absorption bandwidth of 1.3 THz with an absorption efficiency \(>\)90%. We can conclude that it has a polarization independence characteristic and a broad absorption bandwidth. Figure 2: Reflection, transmission, and absorption spectral outcomes of transverse electric (TE) and transverse magnetic (TM) polarizations. To gain a better understanding of this absorption effect, we also investigated the electromagnetic field distributions at the two resonant peaks in the presence of TE polarization with a normal incidence. Figure 3a,b,e,f respectively show the electric and magnetic field distributions on the upper surface of the silicon layer at the resonant frequencies of 1 and 1.72 THz. Figure 3c,g respectively show the electric field distribution at the x-z cross-section when y = 10 \(\upmu\)m at the two resonant frequencies. Figure 3d,h respectively show the magnetic field distribution at the y-z cross-section when x = 10 \(\upmu\)m at the two resonant frequencies. Figure 3a,c show that the electric field is mainly concentrated in the diamond metamaterial edges and the air-slot between the adjacent unit cells at the resonant frequency of 1 THz, thus indicating the occurrence of the electric resonance [34]. Additionally, Figure 3b,d show that the magnetic field is mainly located in the diamond layer. This indicates that the magnetic resonance is also excited [34]. However, as shown in Figure 3f,h, the magnetic field intensity increases considerably at the resonant frequency of 1.72 THz compared with the first resonant peak. This indicates that the magnetic resonance is significantly enhanced. It is worth noting that the electric field is stronger than the magnetic field, so both absorption peaks are dominated by electric resonance. As shown in Figure 3a,c,e,g, the strength of the electrical resonance of peak 1 is higher than that of the peak 2, so the absorption is higher. In summary, the appearances of the electric and magnetic resonances lead to an almost perfect broadband absorption. We also analyzed the influence of the geometrical shapes on the absorption properties. First, we analyzed the effect of the diamond metamaterial layer on the absorption characteristic. The simulated results of the absorber for a normal incidence for TE polarization using only the substrate (red) and the diamond absorber (black) are shown in Figure 4a. The size of the Si substrate has not changed. It can be observed that the two resonant peaks and the bandwidth for 90% absorbance disappear in the absence of a diamond metamaterial layer. In order to reflect the absorption effect of the diamond metamaterial layer, we also analyzed the absorption effect of a non-patterned (continuous) top layer placed on the substrate, with the thickness values \(b\) of 40, 50, 60, 70, and 80 \(\upmu\)m, respectively. As shown in Figure 4b, it can be clearly seen that increasing the thickness of the top layer has no effect on the absorption efficiency, and the absorption is less than 80%. This may be because increasing the thickness of the top layer is equal to increasing the thickness of the substrate. Therefore, compared with Figure 4a, it can be concluded that the diamond metamaterial layer enhances the absorption. Figure 3: **(a,b), and (e,f), respectively, indicate the electric and magnetic field intensities on the upper surface of the silicon layer at the two resonant peaks.