# Numerical Study of an Ultra-Broadband All-Silicon Terahertz Absorber
## 3 Simulated Results and Discussion
(c,g) show the electric field intensity distributions at the x–z cross-section when y = 10 \(\upmu\)m, while (d,h) indicate the magnetic field intensity distributions at the y–z cross-section when x = 10 \(\upmu\)m.**  
Next, we adjusted the thickness of the diamond metamaterial layer, but all the other parameters remained unchanged. Figure 5a presents the numerically simulated absorption spectrum for different thickness values \(t\). We can observe that each absorption spectrum has two resonance peaks, one at the lower frequency and the other at a higher frequency. The two resonant peaks undergo a right shift as the thickness of the diamond layer decreases, while the peak absorption is almost unchanged. As shown in Figure 3c,g, the change of the thickness of the diamond layer has a more profound influence on the electric field at the first resonant peak. Thus, peak 1 exhibits a significant deviation. Therefore, we can design the thickness of the metamaterial layer according to the actual needs to obtain increased absorption responses in specific bands. We then discuss the length of the diamond diagonal on the absorption efficiency where the period \(p\) remains unchanged at 170 \(\upmu\)m. When the lengths of the diagonal are 170, 150, 130, and 110 \(\upmu\)m, this is the same as when the sides \(a\) of the diamond are \(85\sqrt{2}\), \(75\sqrt{2}\), \(65\sqrt{2}\), and \(55\sqrt{2}\)\(\upmu\)m. As shown in Figure 5b, when the length of the diamond decreases from \(85\sqrt{2}\) to \(55\sqrt{2}\)\(\upmu\)m, the first absorption peak is gradually increased and then decreased, while the second absorption peak has no obvious change. This is because changing the length of the diamond has a greater effect on the electric field at peak 1, as shown in Figure 3c,g, so peak 1 has a greater change. It is worth noting that when the length of the diamond is \(75\sqrt{2}\)\(\upmu\)m, the first absorption peak reaches nearly perfect absorption. This is because the electric field is localized better, thus achieving the enhancement of electric resonance. Also, we plotted the absorption spectrum in Figure 5c to assess the influence of the value of the periodic \(p\). As shown in Figure 5c, when we increase the length from 170 to 200 \(\upmu\)m, and with all the other parameters unchanged, there is no significant change in the first resonant peak. By contrast, the second peak shifts significantly to the left and yields an absorption efficiency \(>\)97%. Because the change of the periodic has a considerable effect on the magnetic field of peak 2, as shown in Figure 3b,f, the second resonant peak exhibits an obvious deviation. Finally, we discuss the effect of the thickness of the substrate on absorption, with the thickness values \(h\) of 150, 200, 250, and 300 \(\upmu\)m, respectively. It can be clearly seen from Figure 5d that when the thickness of the substrate is reduced from 300 to 150 \(\upmu\)m, there is no significant effect on the absorption. This is because when the substrate is thick enough to block the transmission of electromagnetic waves, there is no significant effect on absorption. In order to achieve maximum absorption in the THz regions, the optimized structural parameters of the all-silicon diamond absorber are \(p\) = 170 \(\upmu\)m, \(h\) = 250 \(\upmu\)m, \(t\) = 60 \(\upmu\)m, and \(a\) = 75 \(\sqrt{2}\)\(\upmu\)m. The structural parameters of the absorber can be reasonably designed to obtain a specific absorption broadband range. Figure 4: (**a**) Absorption spectra of only the silicon substrate compared with the diamond absorber. (**b**) Absorption spectra in the case of TE polarization for five different thickness values \(b\) of a non-patterned (continuous) top layer placed on the substrate. The aforementioned analysis refers to the absorption characteristic of incident light at a normal angle. In practice, the light source is incident at an oblique angle on the device, and it is thus valuable to design an absorber with a strong absorption at a large angle.