Promoting spintronic terahertz radiation via Tamm plasmon coupling
Fig. 1. (a) Schematic illustration of the spin thin films without TPC and with TPC structure for THz radiation. (b) the phase of r1, r2 and r1r2exp[i(4πnSiO2dinsert)/λ] as a function of wavelength, when the thickness of the optical cavity is 57 nm; (c) Simulated reflectance spectra of the dielectric layers as a function of incidence angle and wavelength for TM polarization.
Fig. 2. (a) The illustration of four comparison experiments that pure spin thin films (spin thin films w/o TPC), spin thin films with TPC (spin thin films w/ TPC), the spin thin films with SiO2 substrate (w/o TPC + SiO2), the spin thin films with pure dielectric layers (w/o TPC + dielectric layers). The THz waveforms (b) and the frequency-domain THz signal (c) from the spin thin films w/o TPC (blue line, MAX is 406), the spin thin films w/ TPC (red line, MAX is 1072), w/o TPC + SiO2 (black line, 354), and w/o TPC + dielectric layers (green line, MAX is 356). at the pump fluence of 12.7 μJ/cm2. (d) The simulation absorptance spectra of the spin thin films with TPC and without TPC. (e) The maximum amplitude of generated THz electric field as a function of the pump laser power when its spot diameter is 1 cm.
Terahertz (THz) frequency occupies a unique position in the electromagnetic spectrum, serving as a transitional band between macro-scale electronics and micro-scale photonics. As an important electromagnetic wave, terahertz (THz) wave has strong practicability in non-destructive testing, wireless communication and imaging sensor technology. Ultra-high data rates in communication technology push toward exploring broadband and commercially available terahertz (THz) signal sources. The application and development of terahertz (THz) technology are largely limited by the level of THz sources. Therefore, obtaining stable and high-performance THz sources has been a major focus of current research.
Due to the feature of high stability and economic efficiency, ultrabroad bandwidth, spintronic THz emitters (STEs) have been a hot topic in the field of THz sources. This approach offers advantages such as independence from the driving wavelength, ultra-wide bandwidth, and high damage threshold. The principle involves using femtosecond laser pumping on spintronic THz heterostructures composed of ferromagnetic and non-magnetic thin films. The ferromagnetic layer undergoes ultrafast demagnetization, thereby inducing the generation of ultrafast spin currents. The ultrafast spin currents diffuse from the ferromagnetic layer into the non-magnetic layer, where they undergo spin-orbit coupling, transforming into ultrafast charge currents in non-magnetic thin films. This leads to THz radiation.
The mechanism of spintronics THz emission and its modulation has been extensively studied over the years and made a significant progress. However, spintronics THz emitters and THz time domain system still face some challenges: (1) limited utilization of pump light resulting in weak THz generation, and (2) the inability to effectively employ high-energy pump light in the THz spectroscopic system, leading to potential damage to subsequent optical components. Moreover, the current attenuators contribute to undesirable THz wave attenuation.
To solve the aforementioned challenges, the group of Prof. Weisheng Zhao and Xiaoqiang Zhang from Beihang University proposed a high performance spintronics THz emitter. Here, through the combination of optical physics and ultrafast photonics, the Tamm plasmon coupling (TPC) facilitating THz radiation is realized between spin THz thin films and photonic crystal structures. The relevant research results were published in Photonics Research, Volume 11, No. 6, 2023 (Yunqing Jiang, Hongqing Li, Xiaoqiang Zhang, Fan Zhang, Yong Xu, Yongguang Xiao, Fengguang Liu, Anting Wang, Qiwen Zhan, Weisheng Zhao. Promoting spintronic terahertz radiation via Tamm plasmon coupling[J]. Photonics Research, 2023, 11(6): 1057).
In this work, most of the light radiation is trapped on the metal/dielectric interface and absorbed by the metal. Unlike the surface plasmonic polariton that has been found in spin-thin films and a dielectric layer with complicated structure, e.g. prisms, the TPC can be excited directly between the metal/dielectric interface. To design this innovative spintronics THz emitter, the research group initially optimized the parameters of the one-dimensional photonic crystal and spintronics THz thin films using the transfer matrix theory and Tamm plasmon theory. According to the Tamm plasmon theory, the excitation requires satisfying the phase matching condition, given by Arg[r1r2 ei(4πnSiO2dinsert)/λ)]≈2πN, (N=0,1,2…), where r1, r2, dinsert represent the reflection coefficients of the spintronics THz thin films, the reflection coefficients of the one-dimensional photonic crystal, and the thickness of the intermediate layer between the spintronics THz thin films and the one-dimensional photonic crystal, respectively. The research group conducted simulations and investigations of the reflection phases at different wavelengths for the spintronics THz thin films, the one-dimensional photonic crystal, and the value of Arg[r1r2 ei(4πnSiO2dinsert)/λ)] at a thickness of 57 nm for the inserted layer. They found that TPC state can be generated at a wavelength of 780 nm. Furthermore, the research results demonstrated efficient transmission of THz waves through the designed one-dimensional photonic crystal, providing a theoretical basis for subsequent device fabrication.
To prepare the Tamm plasmon-enhanced STE, the one-dimensional photonic crystal dielectric multilayer consisting of an insert layer (SiO2) and twenty groups of alternating layers (SiO2 and Si3N4) is fabricated by the plasma-enhanced chemical vapor deposition (PECVD) firstly. Then, the spin thin films (Pt(4 nm)/Co(4 nm)/MgO(4 nm)) for THz emission are deposited on the top of the dielectric layers using magnetron sputtering. The research group conducted tests on the spectral absorption of the spintronics THz emitter, providing theoretical and experimental verification, that it can achieve Tamm plasmon coupling state.
Subsequently, using a self-built terahertz time-domain spectroscopy system, the research group evaluated the terahertz emission performance of this novel spintronics THz emitter. Compared to traditional Pt/Co spintronics THz emitters, the new spintronics THz emitter exhibited a significant improvement in pump light utilization, increasing from 36.8% to 94.3%. It generated terahertz waves with an amplitude 2.64 times higher than that of traditional spintronics THz emitters. The efficient absorption of the pump light and the presence of the one-dimensional photonic crystal structure enabled the emitter to effectively block the transmission of the pump light while facilitating efficient transmission of the generated terahertz waves.
The research group stated, "This work fully demonstrates the advantages of Tamm plasmon coupling in enhancing spintronics THz emission and promoting integration in THz systems. Moreover, this approach offers possibilities for addressing the compatibility issues between optical structure design and low energy consumption in ultrafast THz opto-spintronics and similar devices. In the future, the team will further engage in the research and development of spintronics THz emitters to enhance device stability and improve THz generation efficiency."
封面|强化自旋太赫兹源
图1（a）传统自旋太赫兹源和集成塔姆等离元的新型自旋太赫兹源；（b）不同波长下自旋太赫兹薄膜的反射相位、一维光子晶体的反射相位及中间插入层厚度57nm时Arg[r1r2 ei(4πnSiO2dinsert)/λ)]的大小；（c）介质层反射光谱随TM波入射角度和波长变化的仿真结果
图2（a）太赫兹时域光谱系统测量四个比较实验的图解；（b）测得的太赫兹时域光谱图；（c）基于傅里叶变换得到的太赫兹频域图；（c）不同泵浦波长下普通自旋太赫兹源和集成塔姆等离激元得新型自旋太赫兹源得吸收谱；（d）不同泵浦功率下普通自旋太赫兹源和新型自旋太赫兹源产生太赫兹波振幅大小。
太赫兹频段位于电磁波谱上的特殊位置，是宏观电子学与微观光子学的过渡频段，兼具宽带性、低能性、高透性、唯一性等诸多优势特性，在无损检测、卫星通信、医疗诊断、军事等领域具有重大的科学价值和广阔的应用前景。太赫兹科学与技术的发展和应用很大程度上受限于太赫兹源的水平，获得稳定且高性能的太赫兹源，一直是当下研究的重点。近年来，采用自旋电子学方法产生太赫兹波，具有不受驱动波长限制、超宽带、损伤阈值高等特性，为太赫兹源研发开辟了一条新的道路。其基本原理是飞秒激光泵浦由铁磁、非磁材料构成的自旋太赫兹异质结薄膜，使铁磁层产生超快退磁，从而激发超快自旋流；产生的超快自旋流从铁磁层扩散注入非磁层，由于自旋轨道耦合效应，产生的超快自旋流转换为非磁层中的超快电荷流，进而对外辐射太赫兹波。
经过多年的研究，自旋太赫兹的产生机制以及调控已取得长足发展。然而，现有自旋太赫兹源及相关太赫兹光谱系统也存在亟待解决的问题：（1）泵浦光利用率较低，产生太赫兹较弱；（2）在太赫兹光谱系统中无法利用的高能泵浦光对后续光学元件产生损伤，并且目前的阻光元件会产生太赫兹波衰减。
为解决上述问题，北京航空航天大学集成电路科学与工程学院赵巍胜教授团队张晓强副研究员团队基于光学中塔姆等离激元理论，提出了一种基于塔姆等离激元增强的新型自旋太赫兹发射器，太赫兹产生效率被大大提升。该工作得到了上海理工大学詹其文教授及中国科学技术大学王安廷副教授的指导。相关研究成果发表于Photonics Research 2023年第6期。
塔姆等离激元是一种存在于金属与一维光子晶体界面处的一种高度局域的光学表面态。与表面等离激元需要棱镜或者光栅激发不同，垂直入射的TE波或TM波都可直接激发塔姆等离激元，并具有损耗低、易制备、易激发等特点。为设计该新型自旋太赫兹发射器，研究团队首先基于传输矩阵理论及光学塔姆等离激元理论，开展了一维光子晶体及自旋太赫兹金属薄膜参数优化设计。根据塔姆等离激元理论，激发塔姆等离激元需满足相位匹配公式Arg[r1r2 ei(4πnSiO2dinsert)/λ)]≈2πN, (N=0,1,2…)，其中r1、r2、dinsert分别为自旋太赫兹薄膜的反射系数、一维光子晶体的反射系数和自旋太赫兹薄膜与一维光子晶体间中间插入层厚度。研究团队仿真研究了不同波长下自旋太赫兹薄膜的反射相位、一维光子晶体的反射相位和插入层厚度57 nm时Arg[r1r2 ei(4πnSiO2dinsert)/λ)]大小，发现在780nm波长可激发塔姆等离激元。同时，研究结果表明，太赫兹波可高效通过设计的一维光子晶体，为后续器件制备提供了理论基础。
在理论分析基础上，研究团队首先采用等离子体增强化学气相沉积法制备了一维光子晶体结构；其次借助磁控溅射工艺，在制备的一维光子晶体结构上生长厚度分别为4nm的Pt/Co金属异质结作为自旋太赫兹源，最终制备出该器件。研究团队通过研究和测试该器件的光谱吸收情况，理论和实验双重验证了该器件可实现塔姆等离激元。
随后采用自主搭建的太赫兹时域光谱系统，研究团队测试了该新型自旋太赫兹发射器的太赫兹发射性能。相比传统Pt/Co自旋太赫兹发射器，该新型自旋太赫兹发射器泵浦光利用率从36.8%提升到94.3%，产生太赫兹波强度是传统自旋太赫兹发射器的2.64倍。同时泵浦光的高效吸收及一维光子晶体的存在，使得该发射器兼具高效阻碍泵浦光透过和产生的太赫兹薄高效透过的特性。
研究团队表示：“该工作充分展示了塔姆等离激元在增强自旋太赫兹发射及提升太赫兹系统集成化方面的优势，同时该方案也为克服光学结构设计与超快太赫兹光自旋电子学和其他类似器件的低能耗之间的兼容性问题提供了可能性。后续团队将进一步开展自旋太赫兹发射器的相关研发，提升器件的稳定性和太赫兹的产生效率。”