With the rapid development of terahertz sources, highly sensitive detectors, and new terahertz functional devices, the practical application fields of terahertz technology are constantly expanding. Terahertz absorbers are of important application value in the fields of high-sensitivity terahertz detectors, terahertz radar stealth, and electromagnetic radiation protection, and they have become a research hotspot in the field of terahertz in the past decade. In this study, we report a nickel-based composite film (NCF) terahertz broadband absorber with an absorptivity of more than 0.9 in the range of 0.17-3.5 THz, and it still has excellent absorptivity and absorption bandwidth under high temperature and compression. At present, the terahertz absorbers are mainly two-dimensional structures. We use a three-dimensional nickel foam to reduce surface reflection and find that it has a wider absorption bandwidth than two-dimensional structures. At the same time, compared with three-dimensional graphene absorbers and aerogel absorbers, NCFs are easier to prepare, with lower cost, and they can be applied on a large scale. Therefore, our work contributes to the design and fabrication of terahertz broadband absorbers.

We prepare NCF terahertz broadband absorbers from nickel foam, polydimethylsiloxane (PDMS), and few-layer graphene. First of all, we mix the main agent and curing agent of PDMS according to the ratio of 1∶10, put different mass fractions of few-layer graphene into PDMS to prepare PDMS/few-layer graphene mixtures, and stir magnetically for 30 min to mix PDMS and few-layer graphene evenly. Then, we use a pipette to add the mixture dropwise to the surface of the nickel foam and let it stand for 10 min to allow the mixture to fully enter the pores of the nickel foam. Subsequently, a homogenizer is used for spin coating at 500 r/s for 20 s, so as to evenly distribute the mixture in the nickel foam. The mixture is defoamed in a vacuum drying for 30 min. Finally, NCFs can be obtained by placing the PDMS on a thermostatic heating stage and curing the PDMS at 75 ℃ for 30 min. A mixture of PDMS/few-layer graphene is thus prepared, and the mixture is added to nickel foam to obtain NCFs. The transmittance and reflectance of the NCFs are measured using a terahertz time-domain spectrometer (CCT-1800), and the absorption is calculated using a fast Fourier transform formula. We analyze the effects of PDMS/few-layer graphene mass fraction, temperature, compression, and nickel foam thickness on the absorption properties of NCFs. The characteristics of broadband absorption are discussed from the perspectives of impedance matching theory, electromagnetic wave theory, and multiple interference theory.

The absorptivity of 0.5 mm NCF (2%) exceeds 0.9 (Fig. 4) in the range of 0.3-3.5 THz, and the average absorptivity reaches 0.944 (Fig. 5). At the same time, the effects of temperature, compression, and thickness on the absorption performance are studied, and the results show that the average absorptivity of NCF at 100 ℃ reaches 0.966 when the mass fraction of few-layer graphene is 2%, which is 0.022 higher than that at room temperature (Fig. 5). When the 1.5 mm and 1 mm nickel foam are compressed to 0.5 mm, the NCF still has excellent terahertz broadband absorption performance, and the absorptivity remains above 0.8 in the range of 0.3-3.5 THz (Fig. 7). In addition, the absorption bandwidth of NCFs with different thicknesses can be expanded by adjusting the mass fraction of few-layer graphene, and the qualified bandwidth of 1 mm NCF (0.5%) with an absorptivity of more than 0.9 is 0.2-3.5 THz, which is 0.1 THz higher compared with the qualified bandwidth of 0.5 mm NCF (2%). The qualified bandwidth of 1.5 mm NCF (0.5%) with an absorptivity of more than 0.9 ranges from 0.17 THz to 3.5 THz, which is 0.13 THz higher than the qualified bandwidth of 0.5 mm NCF (2%).

In this paper, NCFs are prepared by using the spin coating method. There are three main reasons why NCFs have broadband absorption, Firstly, the three-dimensional porous structure facilitates good impedance matching, allowing as much of the incident terahertz wave to enter the absorber as possible. At the same time, terahertz waves will also be reflected and scattered many times in the three-dimensional porous structure, which prolongs the attenuation path of terahertz waves and enhances the attenuation ability of terahertz waves. Secondly, the interconnected nickel skeletons provide an efficient way for electron jumping and migration so that terahertz waves are consumed in the form of conductive losses. Finally, the addition of the mixture can provide a large number of heterogeneous interfaces (PDMS/nickel foam, PDMS/few-layer graphene, and few-layer graphene/nickel foam), where the accumulated charges result in the interface polarization loss because of their different permittivity. On the study surface, in nickel foams, broadband absorption of terahertz waves can be achieved by manipulating the PDMS/few-layer graphene mass fraction.