As digital information technology, artificial intelligence, and next-generation communication systems continue to advance, there is an increasing demand of dynamically manipulating electromagnetic waves with high speed and efficiency. Metasurface can precisely control the amplitude, phase, polarization and beam direction of incident electromagnetic waves in a two-dimensional plane. Consequently, significant efforts have been devoted to integrating metasurfaces with tunable materials, enabling active control through electrical, optical, and temperature excitations. However, these active metasurfaces primarily rely on the tuning of a single electrical resonance (ER) and make less use of magnetic resonance (MR), thereby hindering the reduction of insertion loss and limiting the realization of superior characteristics such as high transmittance, reflection suppression, and complete phase coverage. Compared with single ER metasurfaces, the Huygens metasurfaces exhibits synchronous electrical magnetic resonance (EMR), and by adjusting the intensity of the ER-MR coupling, the insertion loss of the device can be significantly reduced, the transmission rate can be enhanced, and the electromagnetic wave can be manipulated more flexibly.

Huygens metasurfaces have demonstrated remarkable potential in perfect transmission and precise wavefront modulation through the synergistic integration of electric resonance and magnetic resonance. However, prevailing active or reconfigurable Huygens metasurfaces, based on all-optical systems, encounter formidable challenges associated with the intricate control of bulk dielectric using laser equipment and the presence of residual thermal effects, leading to limitations in continuous modulation speeds. T In response to this problem, under the leadership of academician Cui Tiejun, a team from the University of Electronic Science and Technology of China and Southeast University combined GaN-2DEG to propose a new ultrafast bilayer Huygens metasurface. Though precisely controlling the carrier concentration in the 2DEG layer, can effectively govern the current distribution on the metasurface, inducing variations in electromagnetic resonance modes to modulate THz waves. This modulation mechanism can not only reduce the insertion loss, but also broaden the bandwidth and increase the modulation speed. Experimental investigations demonstrate continuous modulation capabilities of up to 6 GHz, an amplitude modulation efficiency of 90%, a transmission of 91%, and a remarkable relative operating bandwidth of 55.5%. Relevant research results were recently published in Photonics Research, Volume 12, Issue 5, 2024.[ Hongxin Zeng, Xuan Cong, Shiqi Wang, Sen Gong, Lin Huang, Lan Wang, Huajie Liang, Feng Lan, Haoyi Cao, Zheng Wang, Weipeng Wang, Shixiong Liang, Zhihong Feng, Ziqiang Yang, Yaxin Zhang, Tie Jun Cui, "Ultrafast modulable 2DEG Huygens metasurface," Photonics Res. 12, 1004 (2024) ]

The proposed dynamic Huygens metasurface is a sandwich structure composed of a double layer artificial microstructure and a SiC substrate, as shown in Figure 1. The total thickness of the metasurface is about 0.11λ (where λ represents the operating wavelength in free space). The top unit of the metamaterials consists of four "I" shaped metal structures in series, each "I" shaped metal structure embedded with AlGaN/GaN HEMT, each row of HEMT is linked by a gate while sharing a common source and drain, thereby creating a gate-controlled 2DEG transistor array. Additionally, the bottom unit of the metasurface comprises two split resonant rings (SRRs). The 2DEG, serving as the pivotal structure for dynamically controlling the electron distribution of the metasurfaces, enables adjustment of the phase difference between the top and bottom surface currents by regulating its concentration, so that the resonant state of the modulation unit can be switched between ER and EMR. Based on this concept, when the modulated signal is applied to a 2DEG transistor array on the Huygens metasurface, the transmission intensity of the incident THz wave in free space can be modulated with special speed and efficiency.

Figure 1 Diagram of the Huygens metasurface

The high-speed dynamic Huygens metasurface prepared based on this principle achieves a monophonic 6 GHz modulation speed, 90% modulation efficiency, and 91% transmission. This breakthrough not only surpasses the limitations of traditional multi-layer modulated dynamic Huygens metasurfaces but also addresses the issue of slow response speeds encountered with all-dielectric dynamic Huygens metasurfaces.

THz presents a unique advantage in the field of communication. Firstly, compared with microwave and millimeter waves, THz waves have a larger bandwidth and higher transmission rate. Secondly, the high carrier frequency makes it have anti-interference performance, especially suitable for inter-satellite secure communication and wireless local area communication. In the 6G era, THz communication has become one of the important communication technologies. THz communication involves many disciplines and professional fields, and requires the development of communication technology, as well as the development of high-performance devices, especially low-loss and high-rate THz modulators. Due to the lack of natural materials with excellent response characteristics to THz waves, it is a major challenge to achieve high performance modulation of THz waves. The development of THz modulators based on metamaterial provides an effective way to solve this challenge. The combination of metasurface and GaN-HEMT is one of the important ways to realize quasi-optical high-rate modulation. The combination of reconfigurable metasurface and Huygens principle can not only reduce insertion loss and improve modulation efficiency, but also improve the bandwidth and modulation rate of quasi-optical modulator.

The realization of bilayer ultrafast Huygens metasurfaces in the THz band is challenging for the following reasons: Firstly, metal-based Huygens metasurfaces usually require multi-layer metal structures. In the microwave band, electroresonant structures and magnetic resonant structures can be prepared respectively through mature circuit board printing technology to form multi-layer Huygens metasurfaces. However, with the increase of operating frequency and the decrease of wavelength, it is difficult to prepare multilayer structures on the substrate. Secondly, to obtain the dynamic Huygens metasurfaces in the THz frequency band, it is necessary to combine the electric resonance with the magnetic resonance and design on the double-layer structure. Since the resonant structure acts as both an electrical resonant structure and a magnetic resonant structure, it is necessary to carefully design the structure to balance the strength of the two resonances and the corresponding relationship. This method solves the problems of low modulation efficiency and slow modulation rate of quasi-optical modulators, and has a wide application prospect in high-speed wireless communication, superresolution imaging and efficient high-speed beam manipulation.

In the next step, the team will attempt to apply this design principle to other metastructures and heterojunction components to achieve higher modulation rates and modulation efficiency. And try to combine this approach with other functional devices to lay the foundation for efficient multifunctional THz modulatory devices.