## 3 Results and Discussion  
Figure 1b is the absorption spectra of the triple-band light absorber. As observed, three discrete and narrow-band peaks are obtained in this metamaterial structure. The absorbances of the three peaks at frequencies of 1.316, 3.474 and 4.245 THz are respective 98.38%, 99.89%, and 85.92%. Of particular note here is the bandwidth (we choose the FWHM as its resonance bandwidth) of the triple-band light absorber, the bandwidth of the third absorption peak is only 0.075 THz, which is only 3/10 of the first absorption peak (0.245 THz). If converted into the quality factor \(Q\) (defined as the ratio of the resonance frequency and resonance bandwidth), the \(Q\) value of the third absorption peak can reach 56.600, which is 10.537 times as much as the first absorption peak. That is to say, the \(Q\) value of the absorption at 4.245 THz is an order of magnitude larger than the absorption at 1.316 THz. It is a common sense that large \(Q\) value has potential application in the field of sensing. In fact, the absorption peak at 1.316 THz is the fundamental mode resonance of triple-band absorber, and this resonance mode is most researchers concern. In this manuscript, however, we not only study the fundamental mode resonance absorption peak but also the high-order responses. Figure 1: (**a**) Three-dimension unit cell of triple-band light absorber; (**b**) absorption spectra of the triple-band light absorber having the dielectric constant of lossy (or imaginary part is not zero, see red curve) and loss-free (or imaginary part is zero, see blue curve). As the high-order absorption peak usually has narrow resonance bandwidth (i.e., the large \(Q\) value), its sensing sensitivity is typically much greater than that of the fundamental mode resonance absorption peak. In addition, the absorbances of the three peaks are very low when the imaginary part of the insulation dielectric layer is zero (i.e., Loss-free), see the blue line in Figure 1b. In order to investigate the underlying mechanism of the triple-band light absorber, the field distributions of the three different resonance modes are given in Figure 2. The \(\mid\!Hy\!\mid\) distributions of the three modes in Figure 2(a\({}_{3}\),b\({}_{3}\),c\({}_{3}\)) are all mainly distributed in the insulation dielectric layer of the absorber, which indicates that the three absorption peaks are the localized resonance modes of the pattern Au strip array. Furthermore, for resonance at 1.316 THz, see Figure 2(a\({}_{3}\)), only a strong field distribution region is found in the insulation dielectric layer, while for resonances at 3.474 THz and 4.245 THz, three strong trapped regions in the dielectric layer are observed, see Figure 2(b\({}_{3}\),c\({}_{3}\)). These \(\mid\!Hy\!\mid\) distribution features prove that the first resonance mode at 1.316 THz is caused by the 1-order fundamental mode resonance of the Au strip resonator, while the last two resonance modes are due to the high-order (i.e., the 3-order) responses of the pattern Au array [4]. Although at first glance the \(\mid\!Hy\!\mid\) distributions of the absorption peaks at 3.474 THz and 4.245 THz are the same, the effective lengths and intensity distribution of them are different. As shown in Figure 2(b\({}_{3}\),c\({}_{3}\)), the effective length and field intensity distribution of the second mode (3.474 THz) are larger than that of the third mode at 4.245 THz. Generally, the frequency of metamaterial absorber is proportional to the reciprocal of structure size (or effective length) of the metallic array, and thus, the frequency of the third peak is greater than that of the second peak because the effective length of the third mode is smaller than that of the second mode. Besides the differences in effective length and intensity distribution, the real \(Ez\) distributions of the last two modes are also different.