智能太赫兹器件:可光控的超材料透明效应

太赫兹(THz)波为介于微波与红外光之间、频率位于 0.1 THz 到 10 THz 之间的电磁波,具有穿透性强和大带宽等特点。开发调制范围大、速度快的太赫兹元器件对促进太赫兹技术的发展具有十分重要的意义。

在光与物质相互作用中,当光足够强且同时满足其他条件时,介质的折射率有可能为零,即实现介质的完全透明,该现象被称为电磁感应透明(EIT)。

太赫兹超材料中的类电磁感应透明(EIT)效应也是一种对太赫兹电磁场调制实现的特殊电磁现象。超材料中存在的超辐射(亮模式)和亚辐射(暗模式)模式能够发生强烈的干涉,从而形成高Q值的透明窗口,使穿透过的电磁波在一定的频谱范围内具有高透明、低损耗、强色散的传播特性。相较于传统EIT,太赫兹类EIT效应在慢光技术、光存储、生物传感等领域有广泛的应用。

迄今为止,大部分的超材料在加工完成后无法对电磁波的调制特性进行调整。然而,包括自由空间光通信、深度传感在内的众多应用,对超材料类电磁感应透明进行多样化的动态调控都有着迫切的需求。

针对上述问题,国防科技大学的孙豪、胡瑜泽和唐玉华研究员等人在 Chinese Optics Letters2020 年第 18 卷第 9 期发表的工作中通过引入主动可控材料,展示了太赫兹频域内超材料类 EIT 效应的主动形成过程,为研制高性能光控太赫兹功能器件提供了可行方案(H. Sun, et al., Active Formatting Modulation of Electromagnetically Induced Transparency in Metamaterials)。

在此超材料的设计中,切割线谐振器(SCW)与开环谐振器(SRR)分别实现了亮模式与暗模式的电磁场谐振,而两种模式的干涉实现了类 EIT 效应。通过将硅半导体层嵌入到SCW 中,实现了对亮模式的主动光控,进而成功实现了类 EIT 效应的主动形成过程。

实验结果表明,随着波长为 800 nm 的泵浦光的引入,原本 0.87 THz 处的透射谷受到了抑制。随着泵浦光功率的增加,更多激发出的光生载流子将 SCW 导通,亮模式及其与暗模式之间的干涉得以实现,并在 0.89 THz 处实现了类EIT 透明窗口,并伴随显著的慢光效应。使用光泵浦太赫兹探测技术研究该现象的动力学过程,类 EIT 现象的主动形成过程能够在皮秒时间尺度内完成。

后续的工作可以进一步探究类 EIT 效应的各向异性的形成过程与抑制过程的主动调制。使用超材料实现对类 EIT 效应的主动调控不仅仅局限于太赫兹范围,对于从红外到微波的光学应用,譬如多波段传感、超宽带无线通信以及非线性光学等也具有重要借鉴意义。

国防科技创新研究院的郑鑫副研究员认为,相比之前的工作主要关注类 EIT效应的抑制过程,该工作首次展示了类 EIT效应的主动形成过程,对于无线光通信的发展具有很大意义。

(a) 变泵浦功率条件下实验测量超材料器件得到的透射光谱。(b)类 EIT效应主动形成过程的伪彩色图。

Active Formatting Modulation of Electromagnetically Induced Transparency in Metamaterials

Terahertz (THz) waves are electromagnetic waves between microwaves and infrared light, possessing a frequency range from 0.1 THz to 10 THz. Particularly, THz waves share the characteristics of both microwave and light waves, and exhibit distinguished physical properties, such as low quantum energy, strong penetrability and large bandwidth, granting themselves great potentials for applications in wireless transmission of large-capacity data. Furthermore, the THz spectrum band is the key to the development of 6G mobile communication networks that require the wireless data transmitting speed to exceed TB per second. Fortunately, the THz spectrum band provides a higher usable bandwidth, and hence meets the ever-increasing demand for higher data transmission rates. Due to the high absorption rate of the water vapor to THz waves, the propagation of THz waves in the atmosphere attenuates drastically with distance, which is conducive to the achievement of secure communication in space. On the other hand, the THz technology also has broad application prospects infields of biomedicine, environmental science, material science, public security, and national defense and military. Therefore, the researching of THz components with a large modulation range and a fast operation speed is of great significance to promote the development of THz technology. At present, due to the lack of widespread practical applications in the range of THe frequency, the THz band is also called "THz gap". It is currently very difficult to obtain active and efficient active-control THz devices, which hinders the advancement and applications of the THz technology.

Metamaterials are artificially designed periodic structures composed of subwavelength resonators, presenting electromagnetic properties that are absence in natural materials, which can achieve flexible control of the incident electromagnetic field in the frequency domain of interest. The analog of Electromagnetically Induced Transparency (EIT) effect in THz metamaterials is a special electromagnetic phenomenon realized by using metamaterials to modulate the free space THz electromagnetic field. The destructive interference between the super-radiation (bright mode) and sub- radiation (dark mode) modes supported by the metamaterials forms a transparent window with a high value of quality factor (Q factor), which endows transmitted electromagnetic waves with high transparency, low loss, and strong dispersion in a certain frequency spectrum. Metamaterials have been widely used in the fields of slow light technology, optical storage, and biosensing. Up to date, the majority of metamaterials cannot be changed or altered after being fabricated. Thus, the dynamic control of optical properties of metamaterials is in great need for applications including free space optical communications and depth sensing.

To solve the above problems, Hao Sun et al. from the National University of Defence Technology demonstrated in Chinese Optics Letters, Volume 18, Issue 9 (H. Sun, et al., Active Formating Modulation of Electromagnetically Induced Transparency in Metamaterials) that by introducing active controllable materials, they realized the active formation process of analog of EIT effects in the THz frequency domain for the first time, which provided a feasible solution for the development of high- performance optically controlled THz functional devices.

In the design of the metamaterials, the cut wire resonator (SCW) and the split ring resonator (SRR) realize the electromagnetic field resonance of the bright mode and the dark mode, respectively, while the interference between two modes realizes the EIT-like effect. By embedding the silicon semiconductor layer in the SCW, the active optical control of the bright mode is realized, and then the active formation process of the EIT-like effect is successfully realized as well. Experimental results show that with the introduction of pump light with a wavelength of 800 nm, the transmission dip at 0.87 THz is suppressed. As the power of the optical pump increases, more photo-generated carriers are excited to turn on the SCW, realizing the bright mode and the interference between the dark mode and the bright mode. An EIT-like transparent window is realized at 0.89 THz, accompanied with a remarkable slow light effect. The dynamic process of this phenomenon is studied by using the optical-

pump THz-probe (OPTP) technology, and the active formation process of which can be completed in the picosecond time scale. In subsequent works, based on the active formatting process of the EIT-like effect, the polarization-dependent modulation of the formation and suppression process of the EIT-like effect can be researched, utilizing the polarization property of the free space electromagnetic field. The active control of the EIT-like effect utilizing metamaterials is not only limited to the THz range, but also perform much meaning reference in optical applications from infrared to microwave, such as multi- band sensing, ultra-wideband wireless communications, and nonlinear optics.

Associate Professor Xin Zheng from the National Innovation Institute of Defense Technology believes that when compared with previous work on EIT-like effects, which mainly focused on the process of suppression, this work successfully realized the active formation process for the first time, which is extremely important and attractive to the development and promotion of wireless optical communications.

(a) Experimentally measured transmission spectrum of metadevice considering a series of selected optical fluences. (b) Color map showing the active formatting process of PIT effect.