Alignment-insensitive bilayer THz metasurface absorbers exceeding 100% bandwidth

Metasurface absorber with ultra-thin thickness designed for a terahertz focal plane array detector

Terahertz (THz) refers to electromagnetic waves with frequency from 0.1 to 10 THz, which lies between millimeter waves and infrared light. This paper proposes an ultra-thin metasurface absorber which is perfectly suited to be the signal coupling part of terahertz focal plane array (FPA) detector. The absorptance of the proposed metasurface is higher than 80% from 4.46 to 5.76 THz (25.4%) while the thickness is merely 1.12 µm (0.018 λ). Since the metasurface absorber will be applied to terahertz FPA detector which requires planar array formation, it is divided into meta-atoms. Each meta-atom consists of the same unit cell layout, and air gaps are introduced between adjacent meta-atoms to enhance the thermal isolation, which is crucial for FPA detector to obtain desired imaging results. Due to the symmetrical layout of meta-atoms, absorptance keeps stable for different polarized waves, moreover, good absorptance could also be achieved for incidence angles range of ± 30 °. Spectral measurements show good agreement with the simulation. As a result, features of ultra-thin thickness, polarization insensitivity, and high absorptance make the proposed metasurface absorber well suited to highly efficient coupling of terahertz signals in FPA detector.

Ultra-broadband metamaterial absorbers from long to very long infrared regime

Broadband metamaterials absorbers with high absorption, ultrathin thickness and easy configurations are in great demand for many potential applications. In this paper, we first analyse the coupling resonances in a Ti/Ge/Ti three-layer absorber, which can realise broadband absorption from 8 to 12 μm. Then we experimentally demonstrate two types of absorbers based on the Ti/Ge/Si3N4/Ti configuration. By taking advantage of coupling surface plasmon resonances and intrinsic absorption of lossy material Si3N4, the average absorptions of two types of absorbers achieve almost 95% from 8 to 14 μm (experiment result: 78% from 6.5 to 13.5 μm). In order to expand the absorption bandwidth, we further propose two Ti/Si/SiO2/Ti absorbers which can absorb 92% and 87% of ultra-broadband light in the 14-30 μm and 8-30 μm spectral range, respectively. Our findings establish general and systematic strategies for guiding the design of metamaterial absorbers with excellent broadband absorption and pave the way for enhancing the optical performance in applications of infrared thermal emitters, imaging and photodetectors.

Ultrabroadband metamaterial absorbers from ultraviolet to near-infrared based on multiple resonances for harvesting solar energy

In this paper, a metal-dielectric metamaterial absorber is proposed to achieve ultrabroadband absorption at frequencies from ultraviolet to near-infrared. Based on finite element method solutions, the average absorption of the absorber is 97.75% from 382 nm to 1100 nm, with a maximum of 99.92%, resulting from multiple resonance coupling. The influences of geometric parameters and incident conditions on absorption are investigated. Broadband and narrowband absorption changes are realized by changing incident light polarization. Polarization-independent properties can be realized by changing the dielectric structure to centrosymmetric. The average absorption of the polarization-independent structure is 97.11% from 250 nm to 1115 nm, with a maximum of 99.98%. The proposed absorber structure has wide optical applications including solar energy harvesting and light-emitting devices.

Tunable Broadband Terahertz Waveband Absorbers Based on Fractal Technology of Graphene Metamaterial

In this paper, a metasurface Terahertz absorber based on the fractal technology of a graphene geometry resonator to realize ultra-wideband, ultrathin, adjustable double-layer cross-fractal formation is introduced. This paper proposes a dynamically tuned graphene absorbing material. The structure is composed of one- to four-level-fractal graphene pattern layers, MgF₂ layers and metal reflective layers to form a two-sided mirror of an asymmetric Fabry–Perot cavity. To confine the terahertz electromagnetic wave, four different fractals are integrated into a supercell, and the coupling and superposition of adjacent resonant cavities form a broadband high-absorption absorber. Using finite element-based full-wave electromagnetic simulation software to simulate the response frequency of 0.4–2.0 THz, we found that the absorber achieves a broadband 1.26 THz range (absorption > 80%) and a relative bandwidth of 106.8%. By adjusting the Fermi energy, it can realize free switching and expand to wider broadband terahertz absorption, by adjusting the polarization angle (Φ) from 0 to 90° to prove that the structure is not sensitive to polarization, the absorber provides a 60° large angle of incidence, polarization for TE and TM the absorption pattern remains basically the same. Compared with the previous work, our proposed structure uses fractal technology to expand the bandwidth and provide dynamic adjustable characteristics with great degrees of freedom. The appearance of the fractal structure reduces the difficulty of actual processing.

A Polarization-Insensitive and Wide-Angle Terahertz Absorber with Ring-Porous Patterned Graphene Metasurface

A broadband terahertz (THz) absorber, based on a graphene metasurface, which consists of a layer of ring-porous patterned structure array and a metallic mirror separated by an ultrathin SiO₂ dielectric layer, is proposed and studied by numerical simulation. The simulated results show that the absorptivity of the absorber reaches 90% in the range of 0.91–1.86 THz, and the normalized bandwidth of the absorptivity is 68.6% under normal incidence. In the simulation, the effects of the geometric parameters of the structure on the absorption band have been investigated. The results show that the absorber is insensitive to the incident polarization angle for both transverse electric (TE) and transverse magnetic (TM) under normal incidence. In addition, the absorber is not sensitive to oblique incidence of the light source under TE polarization conditions, and has an approximately stable absorption bandwidth at the incident angle from 0° to 50°. The absorption band can be adjusted by changing the bias voltage of the graphene Fermi level without varying the nanostructure. Furthermore, we propose that a two-layer graphene structure with the same geometric parameters is separated by a dielectric layer of appropriate thickness. The simulated results show that the absorptivity of the two-layer absorber reaches 90% in the range of 0.83-2.04 THz and the normalized bandwidth of the absorptivity is 84.3% under normal incidence. Because of its excellent characteristics based on graphene metamaterial absorbers, it has an important application value in the field of subwavelength photonic devices.

Small-sized long wavelength infrared absorber with perfect ultra-broadband absorptivity

Two types of ultra-broadband long wavelength infrared (LWIR) absorbers with small period and super thin thickness are designed. The absorption with high absorptivity and large bandwidth is achieved through combined propagating and localized surfaced plasmon resonances. We first design a three-layer absorber with a Ti-Ge-Ti configuration, the period of the structure is only 1.4 µm (nearly 1/8 of the center wavelength), the thickness of its dielectric is only 0.5 µm (1/22 of the center wavelength), and the average absorption is 87.9% under normal incident from 8µm to 14µm. Furthermore, the four-layer absorber with a Ti-Ge-Si₃N₄-Ti configuration is designed to obtain more average absorption increasing to 94.5% from 8 µm to 14µm under normal incident, the period of the structure increases to 1.6 µm and the total thickness of dielectric increases to 0.6µm. The proposed absorber is polarization-independent and possesses a good tolerance of incident angle. We calculate that the average absorption of the four-layer absorber for both TE- and TM-modes still exceeds 90% up to an incident angle of θ = 40° (90.7% for TE-mode, 91.9% for TM-mode), and exceed 80% up to an incident angle of θ = 60° (80.2% for TE-mode, 82.1% for TM-mode).