The combination of metamaterials and terahertz(THz) technology has yielded remarkable advancements in the design of efficient optical devices, including filters [1], [2], modulators [3], senors [4], switches [5], and absorbers [6], [7], [8]. Metamaterial perfect absorbers (MPAs) have garnered significant attention in electromagnetic shielding, optoelectronic sensing, and optical stealth due to their exceptional electromagnetic wave absorption properties [9], [10]. In 2008, Landy et al. [11]. demonstrated MPAs experimentally and proposed a classical sandwich structure utilizing a metamaterial resonant cavity to absorb incident electric and magnetic fields. MPAs are typically achieved through a combination of periodic structures and consumable materials, with the fundamental mechanism being the confinement and dissipation of incident electromagnetic fields within the consumable material [11], [12], [13]. To achieve broadband absorption, researchers have explored various strategies. One common approach is the multi-resonator method, wherein broadband absorption spectra are realized by incorporating multiple resonators with slightly different geometrical dimensions in the unit cell, creating a succession of minor differences [14], [15]. Another practical broadband structure approach involves stacking dielectric layers with varying geometrical parameters and thicknesses [16], [17]. In current research, the most serious shortcoming of the majority of broadband absorbers lies in the fact that the second harmonic generated by reflected oscillations cannot produce the same perfect absorption effect. Many broadband absorbers using a single guided mode resonance band, resulting in extremely high power losses. However, the fundamental and second harmonic can be realized simultaneously theoretically, which will lead to perfect double broadband absorption.

Graphene, a novel two-dimensional material with a honeycomb lattice structure comprising closely arranged carbon atoms through sp2 hybridization [18]. As a two-dimensional crystal with remarkable optical properties, graphene finds extensive use in the electromagnetic field. Its Fermi energy levels can be dynamically modulated by applying a gate voltage [19], [20], and a change in relaxation time allows for a wide-narrow band transition [21], [22].

This paper introduces an unpolarized broadband absorber capable of achieving near-unity absorption in the THz range. The approach combines a graphene sheet with a unit cell structure. Localizing the resonance frequency of the second harmonic in the hollow-out structure achieves a perfect absorption of the two broad bands. The absorber exhibits normalized bandwidths of 1.1 THz and 1.14 THz for 90% terahertz absorption. The absorber demonstrates perfect absorption peaks reaching 99% within the bands of 0.62 THz and 0.46 THz. To gain insight into the mechanism of broadband absorption, the paper extensively investigates the structure’s electric field distribution, surface cur-rent distribution, and magnetic field distribution, probing the accuracy of the underlying dipole resonance and plasma resonance in the text. Moreover, the study explores the tunability of the absorber. It successfully achieves absorption peak tuning from 17.00% to 99.99% by adjusting the Fermi energy level of graphene ranging from 0.1 eV to 0.9 eV. The absorption performance and frequency shift characteristic of the absorber is studied in greater detail by adjusting the structural parameters. The optimal characteristic of our proposed broadband absorber lies in the realization of perfect utilization of the second harmonic with excellent absorption characteristics.