Terahertz (THz) technology is useful in applications such as biomedical imaging, telecommunications and advanced sensing systems. However, due to the unique nature of electromagnetic waves in the 0.1 to 10 terahertz range, it has been difficult to develop high-performance components that demonstrate the true potential of terahertz technology. Even the design of essentially componentless parts such as filters and absorbers remains a huge challenge.
Fortunately, the rise of metamaterials may lead to innovative solutions to these problems. Thanks to advances in fabrication and processing technologies, it is now possible to fabricate two-dimensional (2D) patterned microstructures with unique electromagnetic properties in the terahertz range, allowing unprecedented control of signals at these frequencies.
Although a variety of 2D metamaterials (or "hypersurfaces") have been proposed for wave-absorbing materials, most of them still have serious limitations. A common problem is that once the structural modes of a hypersurface absorbing material have been identified and fabricated, its electromagnetic properties are fixed.
This non-tunability limits the possible applications of such devices. On the other hand, although tunable metal-based hypersurface absorbers exist, the use of thin metal layers is discouraged. This is due to several drawbacks, such as the difficulty of fabricating the necessary structures and the poor performance due to the inherent properties of metals.
In this context, Dr. Wenhan Cao's team at Shanghai University of Science and Technology has developed a novel carbon-based tunable metasurface absorber with an ultra-wide tunable bandwidth in the terahertz range. This research directed by Dr. Wenhan Cao was recently published in Advanced Photonics Nexus.
"The core of the absorber uses graphene and graphite microstructures as resonators and graphite layers as back-reflecting surfaces." Dr. Wenhan Cao explains, "The repeating subunits (or 'cells') in this terahertz metasurface absorber have been strategically designed to optimize the absorption efficiency based on four main factors: geometry, material properties, polarization sensitivity, and tuning mechanism."
In terms of geometry, the absorber consists of three thin layers. The top layer is a patterned conductive layer containing concentric graphite rings interconnected by graphene wires; the second layer is a simple dielectric that helps dissipate unwanted electromagnetic waves; and the third layer is an absorbing layer that prevents terahertz waves from penetrating directly through the device, thereby maximizing absorption efficiency.
The material selection and geometric design of the absorber are optimized by numerical analysis and simulation, which contributes to its remarkable absorption in the terahertz range. Notably, a key property of the proposed absorber is its tunability, which stems from the tunable Fermi energy levels. This parameter is crucial in materials and semiconductor technology as it determines the distribution of electrons in different energy levels.
By applying a voltage to a graphene layer, it is possible to change its Fermi energy level and thus easily fine-tune the absorption bandwidth. Dr. Wenhan Cao emphasized, "At a Fermi energy level of 1 eV, the proposed absorber can achieve an astonishingly wide bandwidth of 8.99 THz, providing more than 90% absorption in the frequency range of 7.24 to 16.23 THz, with two distinct resonance peaks at 8.35 THz and 14.70 THz."
Another significant advantage of the proposed design is its insensitivity to the polarization angle of the incident radiation. The use of concentric circles in the cell of the absorber naturally yields this favorable property. The circle, being a perfectly symmetrical shape, allows the absorber to maintain a high absorption rate at incident angles up to 50°.
In short, the many advantages of the proposed design combined with its simplicity represent a true breakthrough in terahertz technology." The proposed absorber offers an ultra-thin, simple metal-free structure with a wide tunable absorption bandwidth at a low thickness, which greatly enhances its applicability. These advantages surpass other reported absorbers.
In the near future, terahertz devices will become part of everyday technology, especially in areas such as medicine and communications, as well as more research-oriented fields such as materials science and biology.
