超材料主动调制太赫兹波,微流集成技术出新招

由人工亚波长结构单元周期排列组成的超材料因其新颖的光学特性引起了科学界的广泛兴趣。其中,表面等离子体的金属开口谐振环(SRRs)作为超材料的基本组成单元,通过几何结构的设计展现了对太赫兹波(THz)优异的调制特性。

然而,固定的结构使得基于SRRs的超材料很难实现对THz波的主动调制。虽然利用外加电场和光场改变SRRs电磁特性,进而实现主动调制的方法已见诸报道,但是复杂的异质结构制备及光路测试系统也给实验带来了一定的困难。

近日,来自天津大学姚建铨院士、张璋博士、枣庄学院梁兰菊教授课题组报道了利用微流集成超材料器件,通过改变有机液体浓度实现对THz波主动调制的最新成果,这为制备基于超材料的THz波调制器提供了另一种新的思路。相关研究成果发表在Photonics Research 2019年第7卷第12期上(Zhang Zhang, Ju Gao, Maosheng Yang, Xin Yan, Yuying Lu, Liang Wu, Jining Li, Dequan Wei, Longhai Liu, Jianhua Xie, Lanju Liang, and Jianquan Yao, Microfluidic integrated metamaterials for active terahertz photonics)。

该项工作将设计的微流道与超材料集成在一起,将含水有机液体作为THz波耗尽层注入到该器件中并使之流动。由于耗尽层中水分子结构的极性引起了THz波的衰减,这种阻尼效应与超材料谐振响应的“并联”式耦合实现了太赫兹波主动调制。通过调节有机液体中的水含量,可以使该器件获得对THz波强度接近90%的调制深度和超过210°的相位调制。

基于连续小波变换的时频联合分析揭示了水含量变化所消耗的能量,并表明水含量的增加产生较小的转动惯量,这可能利于耗尽层弛豫时间的缩短。从根本上讲,这项工作突出了太赫兹器件中水的可利用性,为太赫兹光子学主动调制提供了另一种新的方法。

姚建铨院士、梁兰菊教授相信,该工作成果在新型THz调制器件方面具有重要意义,并且微流道集成方法具有可移植性,对其他功能性的超材料结构设计也同样具有很好的兼容性。

下一步的研究目标是继续深入研究并改善微流道的结构设计及超材料的电磁响应特性,以探索功能更多、性能更好的微流集成超材料调制器及传感器。


微流集成超材料对THz波束的主动调制

Microfluidic integrated metamaterials for active terahertz photonics

Metamaterials, composed of periodic arrangements of artificial sub-wavelength unit cells, have aroused great attention in the scientific community due to their novel optical properties. The well-known basic block is plasmonic split-ring resonators (SRRs). Based on the design of the geometric configuration, they exhibit excellent modulation performance for terahertz waves (THz). However, the fixed structures of SRRs make it difficult to actively modulate THz waves owing to unalterable resonant characteristics. Although the methods of applying external electric and optical fields to change the electromagnetic characteristics of SRRs, and thus achieving active modulation have been reported, the complicated heterostructure fabrication and optical measuring systems have also bring certain difficulties in experiments.

Recently, a research group led by Prof. Jianquan Yao and Prof. Lanju Liang from College of Precision Instruments and Opto-Electronics Engineering, Tianjin University reported the results of active THz modulation by changing the concentration of organic liquids flowing in microfluidic integrated metamaterial devices. This work provides another perspective for THz modulators based on metamaterials, which is published on Photonics Research Vol. 7, Issue 12, 2019 (Zhang Zhang, Ju Gao, Maosheng Yang, Xin Yan, Yuying Lu, Liang Wu, Jining Li, Dequan Wei, Longhai Liu, Jianhua Xie, Lanju Liang, Jianquan Yao. Microfluidic integrated metamaterials for active terahertz photonics[J]. Photonics Research, 2019, 7(12): 12001400).

In this work, a depletion layer played by aqueous organic liquids flowing in a platform of microfluidic integrated metamaterials is experimentally used to actively modulate terahertz (THz) waves. The polar configuration of water molecules in depletion layer gives rise to a damping of THz waves. The parallel coupling of such damping effect induced by depletion layer with the resonant response by metamaterials leads to an excellent modulation depth approaching 90% in intensity and a great difference over 210° in phase shift. Joint time-frequency analysis performed by the continuous wavelet transforms reveals the consumed energy with varying water content, indicating a smaller moment of inertia related to a shortened relaxation time of the depletion layer. This work diametrically highlights the availability of water in THz devices, and provides a new alternative for active modulation of terahertz photonics.

Academician Yao and Professor Liang believe that the results of this work are of great significance in the new THz modulation devices. Due to the transplantable microfluidic integration, it is also compatible with other functional metamaterials. Based on this work, the future goals of Prof. Liang’s group are focused on exploring microfluidic integrated metamaterial modulators and sensors with more functions and better performance by improving the structural design of microfluidics and optimize metamaterial performance.


The active modulation of THz waves realized by microfluidic integrated metamaterials