Experimental realization of ultrathin, double-sided metamaterial perfect absorber at terahertz gap through stochastic design process

We design and demonstrate a flexible, ultrathin and double-sided metamaterial perfect absorber (MPA) at 2.39 terahertz (THz), which enables excellent light absorbance under incidences from two opposite sides. Herein, the MPA is fabricated on a λ0/10.1-thick flexible polyethylene terephthalate substrate of εr = 2.75 × (1 + 0.12i), sandwiched by two identical randomized metallic patterns by our stochastic design process. Such an MPA provides tailored permittivity and permeability to approach the impedance of free space for minimizing reflectance and a great imaginary part of the refractive index for reducing transmittance and finally results in high absorbance. Both experimental measurement and numerical simulation are in a good agreement. The flexible, ultrathin and double-sided MPA significantly differs from traditional quarter-wavelength absorbers and other single-sided perfect absorbers, paving a way toward practical THz applications in thermal emission, sensing and imaging, communications, stealth technique, and even energy harvesting.

To realize influences from loss tangent of a substrate, we plot the corresponding changes of transmittance, reflectance and absorbance as well with varied tan  from 0 to 0.12 as portrayed in Fig. S1(a), S1(b) and S1(c), respectively. From these figures, we focus on the frequency of 2.4 THz where the absorbance reaches its maximum. In Fig. S2 i.e., the wave impedance of the MPA. Figure S2 shows a retrieval result of an MPA with tan = 0.06 revealing that such an MPA possess an excellent matching of wave impedance (1.018+0.047i for tan  = 0.06; 1.001+0.172i for tan = 0.12), resulting in smaller reflectance and then higher absorbance.
As for the broadband response of the double-sided MPA, we attributed this broadband characteristic to two possible reasons; one is the quality factor of the MPA and the other is the mergence of two resonance peaks. To examine the first reason, the quality factor, we should scrutinize Fig. S1(c) again and observe that the bandwidth of absorbance becomes much broader as tan δ of PET increases, originating from a lower quality factor based on = 0 . Also, in Fig. S1(b), the reflectance spectrum with loss-free PET reveals that there coexist two reflectance dips, developing minor and major absorbance bands at 2.30 THz and 2.40 THz, respectively. Once loss tangent of PET becomes higher and higher, these two absorbance peaks appear broader and broader and finally merging together, resulting in a broadband absorbance for the case of tan  = 0.12 in our work. respect to x-y view. As expected, away from the metallic structure, there appear few thermal losses within PET.
We calculated the corresponding volume loss = 0 ∫ | | 2 , and surface loss = 1 2 √ ∫ | | 2 , respectively (i.e., total loss power P = P v + P s ). As expected, the thermal losses mainly concentrate around the metallic patterns and are gradually decreasing away from metal due to higher conductivity of metal compared to the dielectric spacer as shown in Fig. S3.
It is worth mentioning once we insert a layer of an energy reservoir such as thin film solar cells within the spacer S1 , hence electromagnetic energy trapped by our absorber could be transformed into electricity and heat dissipation would be no longer the only route for Supplementary Information energy conversion. This is why we only put the field distribution in the main content and claim that most fields are confined within the spacer region.
 Thickness-dependent response of the MPA Figure S4. Thickness dependence of the double-sided MPA. An obvious red shift is observed due to increases of an optical path with increasing thicknesses of PET. Such optical path changes contribute to a significant and a minor decrease of maximum absorbance as thicknesses of PET are thinner and greater than 12 m, respectively.
 Angle-dependent response of the MPA Figure S5. Angle-dependence of the double-sided MPA for (a) TE and (b) TM cases. Our double-sided MPA could tolerate oblique incident angles up to around 10-degree. After this angle, the absorbance band becomes split with smaller absorbance peaks for both TE and TM cases.