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The realization of broadband absorption in the terahertz spectra is of significant interest in advanced terahertz applications. In this work, a novel ultrabroadband absorber based on the periodic array of graphene disks and graphene continues sheet is proposed. A developed transmission line theory besides the analytical circuit model of graphene disks and graphene continues sheet are exploited to describe the absorber structure by its circuit model. By using three graphene layers and considering impedance matching concept, the normalized bandwidth of 90% absorption is extended up to 121% of the central frequency. Also, the dependence of the absorption spectra on the structure parameters is investigated and analyzed. Moreover, the performance of the device under possible fabrication errors is discussed. In pursuit of evaluating the efficiency, accuracy, and validity of the proposed method, full-wave numerical modeling is performed by the finite element method. The results show that the proposed circuit approach, in addition to having advantages in terms of computing time and the need for memory resource, is in a good agreement with the full-wave simulations. Furthermore, the structure of the absorber is relatively simple, and it can be manufactured by chemical vapor deposition techniques.

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Utilizing a double bias scheme for graphene patterns as a single layer, a THz meta-surface structure is proposed. The structure includes two dual bias layers and a graphene continuous sheet on top. So five possible biases are available which lead to a highly tunable absorption response. All consisting parts are modeled via passive circuit elements to obtain whole device input impedance. Then impedance matching theory is used to investigate potential frequencies for perfect absorption. Also, ample simulations are performed to show the validity and accuracy of theoretical descriptions. According to the simulation results in the conventional finite element method as a reference approach is in acceptable agreement with the developed circuit model approach. The proposed structure shows nearly perfect absorption in 5, 6, 7, and 8 THz with absorption rate over 90%. Such a tunable reaction is in incredible requests in optical systems and has various applications in structuring sensors, modulators, and photonic functions.

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