Graphene-Based Enhanced Active Terahertz Wavefront Modulator: Provides Potential for Compact THz Spatial Light Modulators

Terahertz (THz) waves are known as electromagnetic waves located between infrared rays and microwaves, and THz technology has made significant advances in biomedicine, non-destructive testing, imaging, and high-speed wireless communications. However, the lack of modulators is still one of the bottlenecks in the development of THz technology. Recently, researchers adopted metasurfaces based on tunable materials to solve this problem. Graphene is a two-dimensional tunable material. Since the band structure and optical properties of graphene can be modulated by applying a gate voltage, graphene becomes a good tunable optoelectronic material. Graphene-based active metasurfaces have the advantages of fast tuning speed, high transmittance, ultra-small thickness, and good response to THz waves due to intra-band transitions, which offer great application potential for THz wavefront control.

However, graphene-based THz metasurfaces with metal antennas still face two problems. One is the lack of independent tuning modulation. Most of these metasurfaces use an entire layer of graphene and apply a uniform gate voltage for tuning, thus the modulation of each unit on the metasurface cannot be changed independently. The other problem is that the interaction between THz waves and graphene-based active metasurfaces is still not good enough.

To solve the above problems, the research group of Bin Hu from Beijing Institute of Technology and the research group of Yan Zhang from Capital Normal University proposed an electrically controlled enhanced active THz wavefront modulator. The device is composed of a 5×5 array capacitor formed by 10 graphene ribbons and a metasurface composed of a double C-shaped metal antenna array. The transmitted THz wavefront can be reconstructed by electronically modulating the graphene capacitor in an addressing way, thus realizing independent pixel modulation of the THz wavefront. The quasi-bound states in the continuum (QBIC) mode are introduced by designing a double C-shaped metal antenna array to enhance the interaction between THz waves and the metasurface, thereby achieving efficiency enhancements. The research results were published in Photonics Research Volume 11, No. 7, 2023(Jianzhou Huang, Bin Hu, Guocuo Wang, Zongyuan Wang, Jinlong Li, Juan Liu, and Yan Zhang. BICs-Enhanced Active Terahertz Wavefront Modulator Enabled by Laser-Cut Graphene Ribbons[J]. Photonics Research, (2023), 11(7): 1185).

This work integrates the two functions of efficiency improvement and dynamic reconfigurable of THz waves into a single device. The schematic structure of this THz modulator is shown in Fig. 1(a), which includes two layers. The top layer is a metal double C-shaped antenna metasurface, and the bottom layer is a 5×5 array of graphene capacitors. The double C-shaped antenna array interacts with THz waves to generate QBIC resonance, which enhances the conversion efficiency. To realize the phase-tunable effect, a 5×5 graphene capacitor array is placed close to the antenna metasurface, which consists of two groups of 5 parallel graphene micro ribbons. Therefore, pixelized THz wavefront modulation based on BICs resonance enhancement can be achieved when different gate voltages are applied to ten graphene ribbons.

To increase the efficiency of the interaction between the metasurface and the THz wave, the researchers proposed a double C-shaped antenna, as shown in Fig.2 (a)-(b). This structure utilizes the FW-BIC resonance to localize the incident x-polarized THz wave in a bound state on the surface of the structure, thereby enhancing the interaction with the metasurface and achieving conversion efficiency enhancement under different graphene chemical potentials. In addition, to design a reconfigurable THz wavefront phase modulator, dynamic phase modulation is also necessary. Therefore, based on the laser cutting graphene technology, the researchers fabricated a 5×5 pixelated reconfigurable THz wavefront modulator. The phase profile can be modulated by switching the mode of the graphene gate voltages, and the experimental results are shown in Fig.2 (c)-(j). Under different applied voltage modes, the device can realize the focusing of the THz wave and change the focal length.

Dr. Bin Hu said: "Terahertz technology can be widely used in wireless communication, biomedical detection, remote sensing, environmental monitoring, and other fields, which has great application value. The terahertz modulator proposed in this work provides an efficient strategy for flexible manipulation of terahertz waves, which provides new opportunities for high-degree-of-freedom terahertz detection and imaging, as well as high-rate 6G communication."

This research was supported by Beijing Municipal Natural Science Foundation (L223031) and National Natural Science Foundation of China (61875010).

封面|增强型太赫兹空间光调制器：基于石墨烯实现寻址式波前重构

太赫兹波是频段介于红外光和微波之间的电磁波，在生物医学、无损检测、成像及高速无线通信等领域具有广阔的应用前景。当前，缺乏调制器件仍是太赫兹技术发展的瓶颈问题之一。基于可调谐材料的超表面结构为解决这一问题提供了新的方法。其中，石墨烯是一种新型的半导体可调谐材料，由于能带结构和光学性质可以通过外加门电压调控，使其成为一种良好的宽带可调光电子材料。基于石墨烯的有源超表面具有调谐速度快、透射率高、超薄厚度等优点，以及电子带内跃迁带来的其对太赫兹波良好响应，为太赫兹波前控制创造了巨大的应用潜力。

然而，结合石墨烯和金属天线的可调超表面器件仍然面临两个问题。一个是缺乏独立的调谐单元，此类器件大多使用一整片石墨烯，并均匀施加栅极电压进行调谐，无法实现每个单元的独立调制。另一个问题是太赫兹波与有源超表面之间的相互作用仍然较弱。

为解决上述问题，北京理工大学胡滨研究组和首都师范大学张岩研究组合作，设计了一种电控有源太赫兹波空间相位调制器。该器件由10个石墨烯条带形成的5×5阵列电容和双C型金属天线阵列组成的超表面级联而成，通过寻址的方式电控调制石墨烯电容，可以对透射的太赫兹波前进行重构，从而实现对太赫兹波前的独立像素调制；通过设计双C型金属天线阵列结构引入准连续域束缚态（QBIC）模式，增强太赫兹波和超表面的相互作用，从而实现效率的提升。相关研究成果发表于Photonics Research 2023年第7期（Jianzhou Huang, Bin Hu, Guocuo Wang, Zongyuan Wang, Jinlong Li, Juan Liu, and Yan Zhang. BICs-Enhanced Active Terahertz Wavefront Modulator Enabled by Laser-Cut Graphene Ribbons[J]. Photonics Research, (2023), 11(7): 1185）。

该工作将效率提升和太赫兹波前的动态重构这两个功能集成到同一个器件中。由两层结构构成的调制器如图1所示，上层结构是双C型天线阵列的超表面，下层结构是5×5阵列的石墨烯电容器。双C型金属天线阵列与太赫兹波相互作用产生QBIC共振，实现转换效率的增强；石墨烯电容器由两组相互垂直的石墨烯条带组成，通过电寻址的方式，在不同区域实现像素化的电磁响应，进而重构太赫兹波前。

针对石墨烯结合金属天线超表面与太赫兹波作用效率低的问题，研究人员提出了双C型金属天线阵列结构，如图2（a）-（b）所示。该结构利用FW-BIC共振，使入射的x偏振光在结构表面处于局域态，从而增强与超表面的相互作用，实现在不同石墨烯电化学势下转换效率的增强。同时，为了设计一个可重构的太赫兹波前相位调制器，动态的相位调制也是必要的。因此，研究人基于激光切割石墨烯的技术加工了5×5像素化的可重构太赫兹波前调制器，使相位轮廓可以通过切换石墨烯栅极电压模式的方式实现调制，实验结果如图2（c）-（j）所示。在不同电压施加方式下，器件可实现太赫兹波的聚焦，并改变焦距。

胡滨教授表示：“太赫兹技术可广泛应用于无线通信、生物医药检测、遥感、环境监测等领域，具有重大的应用价值。这项工作所提出的太赫兹调制器提供了一种有效的策略来灵活地操纵太赫兹波。这为实现高自由度的太赫兹探测和成像，以及高速率的6G通信提供了新的契机。”

该项研究工作获得了北京市自然科学基金-小米创新联合基金（L223031）及国家自然科学基金面上项目（61875010）的资助。