中国激光, 2023, 50 (17): 1714015, 网络出版: 2023-09-13  

嵌入式二氧化钒超表面对太赫兹谐振模式的动态调控

Dynamic Modulation of Terahertz Resonance Modes by Embedded Vanadium Dioxide Metasurfaces
作者单位
1 南京大学电子科学与工程学院,超导电子学研究所,江苏 南京 210023
2 紫金山实验室,江苏 南京 211111
摘要
提出了一种在超表面中嵌入二氧化钒(VO2)从而对太赫兹波进行动态调控的方法。VO2在68 ℃左右会发生绝缘体到金属的相变,导致其电导率发生4~5个数量级的显著变化。超表面由一个双开口金属谐振环阵列以及金属环开口处嵌入的VO2结构组成。仿真结果表明,通过改变VO2的电导率,太赫兹波可以实现高频与低频之间的谐振模式的切换。值得注意的是,这种模式转换不仅可以在实验中通过直接改变环境温度实现,还可以通过激光和强场太赫兹的泵浦完成。这种多样化的激励为该超表面在实际多场景应用中动态调控太赫兹波提供了参考。
Abstract
Objective

Terahertz (THz) is a electromagnetic wave with the frequency range of 0.1‒10 THz, and is gradually playing an important role in many fields. However, because traditional electronic and optical design methods consider the adjacent microwave and infrared optical bands, the application of the THz band is not easy to directly expand, which will undoubtedly greatly hinder further development of THz technology. Thus, there is an urgent need for new THz device design methods to solve this difficulty. Metamaterials are composed of a series of micro- and nanostructures with artificially designed periodic arrangements, whose size, shape, and distribution can produce optical responses that natural materials do not exhibit after careful design. Two-dimensional metamaterials, i.e., metasurfaces, with a simple process flow and low processing cost, have gradually replaced metamaterials in recent years and have become a popular research topic. The application of metasurfaces to THz technology overcomes the limitations of traditional materials, contributing to their development. With the increasing demand for corresponding applications, researchers have shifted their attention from single passive hypersurfaces to tunable active metasurfaces. These tunable metasurfaces are often dependent on several tunable materials. In particular, in the THz band, vanadium dioxide (VO2) is an excellent tunable material that is being actively investigated by researchers due to its abrupt change in conductivity of four to five orders of magnitude before and after the phase transition temperature, which allows it to complete the insulating to the metallic phase transition.

Methods

A metasurface comprising a periodic array of double-gap split-ring resonators, with VO2 structures embedded in the gaps, was considered in this study. The spectral responses to different VO2 conductivities and electric field distribution images of the corresponding modes were first simulated using the commercial simulation software, CST. Next, samples were obtained via conventional lithography and other micro- and nano-processing techniques, which were then characterized experimentally using THz time-domain spectroscopy (TDS). First, the samples were heated directly using a hot stage, followed by laser pumping, strong THz pumping, and THz detection for mode characterization.

Results and Discussions

The simulation results clearly show that with increasing VO2 conductivity from 10 to 2×105 S/m, the resonant frequency red-shifts from 0.75 to approximately 0.5 THz, the gap of the metal arm is approximately filled, and the whole structure completes the transition from mode 1 to mode 2. Field monitoring shows that before the phase transition, mode 1 is a magnetic dipole resonance with an enhanced electric field at the opening; whereas, after the phase transition, mode 2 is an electric dipole resonance after the conduction of the metal arm, and its electric field is mainly distributed in the upper and lower metal arm regions (Fig. 1). Direct characterization of the sample heating confirmed the simulation results. At temperatures lower than 57 ℃, the resonant frequency of the structure remained at approximately 0.7 THz, indicating that the temperature change at this time could not substantially affect the conductivity of VO2, and the sample was in the mode 1 state. As the temperature increases further, the resonant frequency gradually red-shifts to 0.45 THz, accompanied by a gradual decrease in the amplitude of the resonant peak, reaching a minimum of approximately 0.4 at ~64 ℃. With continuing increase in temperature, the resonant frequency continues to red-shift, and the amplitude becomes larger, indicating that VO2 is transitioning between the insulating phase and the metallic phase. For temperatures higher than 73 ℃, the resonance mode does not change significantly in both amplitude and resonance frequency, and tends to stabilize, at which time the VO2 conductivity tends to saturate and completes the filling of the metal arm gap, indicating that the metasurface is in the state of mode 2. Therefore, by directly heating the sample, a conductive channel at the gap is successfully constructed, the transition from resonant mode 1 to mode 2 of the metasurface is completed, and the variation in the resonant frequency with temperature provides a more direct reflection of the mode switching (Fig.2). Laser pumping requires heating the respective sample to near the phase transition temperature; similarly, different laser powers can induce a VO2 phase transition (Fig.3). Finally, strong THz pumping of samples with different intensities can also produce the VO2 phase change. It is worth noting that although VO2 is in the vicinity of the phase transition temperature, broadband modulation, such as temperature and laser pumping excitation, cannot be fully achieved under strong THz field excitation (Figs.5 and 6).

Conclusions

In this paper, a tunable, embedded VO2 hybrid metasurface is proposed to realize the dynamic switching of resonant modes from a high frequency of around 0.7 THz to a low frequency of around 0.43 THz in the THz band. The VO2 at the opening of the resonant ring undergoes a sudden change in conductivity by more than four orders of magnitude before and after the phase transition, constructing a conductive channel in the metal arm of the resonant ring and thus completing mode switching. The feasibility of this mode-switching was verified experimentally through various applications of thermal, laser, and strong-field THz excitations. Although the physical mechanisms of the former and latter two differ, the multimode dynamic excitation manipulation of THz waves presents a feasible idea for practical applications.

王磊, 李花, 王永杰, 张彩虹, 吴敬波, 范克彬, 金飚兵, 陈健, 吴培亨. 嵌入式二氧化钒超表面对太赫兹谐振模式的动态调控[J]. 中国激光, 2023, 50(17): 1714015. Lei Wang, Hua Li, Yongjie Wang, Caihong Zhang, Jingbo Wu, Kebin Fan, Biaobing Jin, Jian Chen, Peiheng Wu. Dynamic Modulation of Terahertz Resonance Modes by Embedded Vanadium Dioxide Metasurfaces[J]. Chinese Journal of Lasers, 2023, 50(17): 1714015.

引用该论文: TXT   |   EndNote

相关论文

加载中...

关于本站 Cookie 的使用提示

中国光学期刊网使用基于 cookie 的技术来更好地为您提供各项服务,点击此处了解我们的隐私策略。 如您需继续使用本网站,请您授权我们使用本地 cookie 来保存部分信息。
全站搜索
您最值得信赖的光电行业旗舰网络服务平台!