SPECIAL TOPIC—Terahertz spintronic optoelectronics
太赫兹自旋光电子专题编者按
DOI: 10.7498/aps.69.200101
太赫兹辐射在电磁波谱上位于红外和微波之间, 频率通常划定为0.1—10.0 THz. 太赫兹光子能量低(约4.1 meV (1 THz)), 对应半导体带内载流子动力学和非线性光学效应所需能量, 对应关联电子体系中众多重要的单粒子和集体激发能量尺度, 对应液态水氢键网络的慢弛豫转动能级, 生物大分子集体振动频率, 宇宙大爆炸背景辐射的主要能量等. 太赫兹技术不仅可用于研究关联电子体系的量子多体问题, 水科学和复杂生物体系的能量转移和转化问题, 宇宙起源和生命起源的本质问题, 而且在移动通信、检测病毒、医疗成像、安检反恐、探索宇宙等方面有着极其重要的应用前景.随着5G 的普及, 6G 应用已提上日程. 6G 将全面进入太赫兹时代, 太赫兹技术也逐渐走进了大众的视野, 成为人类认识世界的“第三只眼睛”.
1971 年, 人们在铌酸锂晶体中获得了人类历史上第一束太赫兹脉冲激光. 经过近半个世纪的快速发展, 虽然部分太赫兹技术已逐步从实验室研究向应用阶段过渡, 但太赫兹领域的关键瓶颈问题依然没有得到很好的解决. 高效率辐射源、高灵敏度探测器和功能器件的缺乏, 直接阻碍了太赫兹科学与技术的发展, 成为电磁场与电磁波领域的关键核心问题之一. 事实上, 一个国家的太赫兹技术水平很大程度上取决于该国的太赫兹源水平, 进而牵动了其他相关领域的发展.
由于太赫兹源的缺乏, 现有太赫兹研究大多处于弱场被动检测的线性区, 然而产生强场太赫兹辐射并用其驱动物质发生相变或精确操控物质量子状态已经成为太赫兹领域重要发展方向.
自旋电子学与太赫兹科技在近二十年来经历了它们狂热的青春期, 都领略着自己空前繁荣的辉煌时代. 随着学科交叉融合的深入, 太赫兹与自旋电子学的联手正在创造更多的惊喜. 例如, 1) 超快激光泵浦的自旋激发太赫兹辐射为低成本、超宽带、易集成、偏振可调谐的太赫兹辐射源提供了思路; 2) 太赫兹时域光谱技术及其与低温和强磁场的结合, 为磁性系统自旋动力学表征和研究提供了新的方法; 3) 利用强太赫兹辐射的电场或磁场分量对磁性或其他物质系统实现非绝热量子状态调控是令人非常着迷而热门的研究课题.
为进一步促进国内同行的交流, 《物理学报》组织出版了“太赫兹自旋光电子”专题, 邀请活跃在本领域的部分专家, 从太赫兹与自旋体系的物理和材料方面, 以不同的视角介绍本领域的最新进展和未来趋势. 鉴于太赫兹科学与技术和自旋电子学属于交叉学科, 具有多样性及复杂性的特点,本专题只能重点介绍太赫兹自旋光电子领域的部分研究成果, 与读者和同行分享. 从研究内容上,目前可大致分为两类: 一是探索自旋太赫兹发射物理规律, 寻找下一代新型太赫兹辐射材料; 二是探索太赫兹电磁场和电磁波与自旋材料的相互作用物理和应用.
希望本专题能有助于扩大太赫兹自旋光电子学在海内外华人学者中的影响, 吸引更多学者, 尤其是年轻学者的关注和加入, 为我国在本领域的蓬勃发展增添新生力量.
2020, 69 (20): 200703.
doi: 10.7498/aps.69.20200623
Abstract +
The terahertz frequency band is located between infrared and microwave in the electromagnetic spectrum. The interesting properties such as broadband, low energy, high permeability, fingerprint, etc. make terahertz wave important for applications in the fields of aerospace, wireless communications, security, materials science, biomedicine, etc. The development and application of terahertz science and technology are largely limited by the terahertz sources, therefore it is crucial to develop new terahertz radiation sources. Recently, it was shown that terahertz spintronic not only provides the possibility of physically controlling the femtosecond spin current, but also expects to be the next-generation ultra-wideband, low-cost, high-efficiency terahertz sources. In this paper we systematically review the historical development, experimental devices, emission mechanisms, material selections, and future prospects of the spintronic terahertz sources. We present the research advances in the physical mechanisms of ultrafast spin current induced by femtosecond laser, the spin charge conversion at ferromagnetic and non-magnetic interfaces, and the terahertz emission triggered by ultrafast pulses. This review also introduces spintronic terahertz sources based on heavy metals, topological insulators, Rashba interfaces, and semiconductor systems.
2020, 69 (20): 200704.
doi: 10.7498/aps.69.20200680
Abstract +
Ferromagnet/nonmagnet (FM/NM) heterostructure under the excitation of femtosecond laser has proved to be a potential candidate for high-efficiency terahertz (THz) emission. Topological insulator (TI) is a novel two-dimensional (2D) material with a strong spin-orbital coupling, which endows this material with an extremely large spin-Hall angle. Thus, TI appears to be an attractive alternative to achieving higher-performance spintronic THz emitter when integrated with ferromagnetic material. In this paper, we discuss the ultrafast photocurrent response mechanism in TI film on the basis of the analysis of its crystal and band structures. The discussion of the mechanism reveals a relationship between THz radiation and external conditions, such as crystal orientation, polarized direction and chirality of the laser. Furthermore, we review the spintronic THz emission and manipulation in FM/NM heterostructure. The disclosed relationship between THz radiation and magnetization directions enables an effective control of the THz polarization by optimizing the system, such as by applying twisted magnetic field or fabricating cascade emitters. After integration, the FM/TI heterostructure presents a high efficiency and easy operation in THz radiation. This high-performance topological spintronic THz emitter presents a potential for the achievement of arbitrary polarization-shaping terahertz radiation.
2020, 69 (20): 204202.
doi: 10.7498/aps.69.20200715
Abstract +
Terahertz technology shows great potential applications in imaging, sensing and security. As is well known, the conventional solid-state broadband terahertz sources rely primarily on the nonlinear optical crystals and photoconductive antennas. Therefore, one major challenge for the next generation of terahertz technology is to develop the high-efficient, ultra-broadband and low-cost terahertz sources. In recent years, much attention has been paid to the spintronic terahertz emitters made of the metallic magnetic heterostructures on a nanometer scale. In this paper, the underlying physical mechanisms associated with this type of terahertz emitter is discussed. They mainly include the ultrafast demagnetization and the spin-charge interconversion processes. In order to further improve the terahertz emission efficiency, three main aspects are considered: appropriate choice of the materials (including conditions of the sample growing), film thickness, and new structure design. In the end, a short conclusion and future perspective for this research direction are given briefly.
2020, 69 (20): 208501.
doi: 10.7498/aps.69.20200866
Abstract +
After more than 20 years of development, semiconductor spintronics has become an important and interdisciplinary research filed of spin-based physics, materials and phenomenon. Spin light emitting diode (spin LED) is one of the fascinating topics in semiconductor spintronic, and it is also one of devices in which the radiative recombination of spin-polarized carriers results in luminescence exhibiting a net circular polarization. The research of spin LED involves the studies of materials, structures, and spin based physics in spin injector and active region. The spin injection, spin transport, and spin detection are key factors for understanding the spin based physics in spin LED. Here in this paper, we comprehensively review the current research status and the latest results. Finally, we also discuss the future research trend.
2020, 69 (20): 208705.
doi: 10.7498/aps.69.20200757
Abstract +
Spintronic terahertz (THz) emitter, which is based on ultrafast spin-to-charge current conversion in ferromagnetic/nonmagnetic heterostructures, provides excellent advantages such as ultra-broadband, tunable polarization, and ultra-thin structure, thereby attracting increasing interests recently. In this review article, we first introduce the fundamental concepts of THz wave, THz spintronics and spintronic THz emitter. Next, we focus on the recent progress of spintronic THz emitter by closely looking at the performances, manipulations and applications. Performance improvement is presented based on the three fundamental processes: optical excitation, ultrafast spin transport, and THz emission. The active manipulation of polarization and spectral response, as well as the relevant applications such as ultra broadband measurements, magnetic structure detection and imaging, and THz near-field microscopy, are reviewed comprehensively. Finally, a brief summary and outlook are given.
2020, 69 (20): 200702.
doi: 10.7498/aps.69.20200526
Abstract +
We systematically investigate the influence of annealing effect on terahertz (THz) generation from CoFeB/heavy metal heterostructures driven by femtosecond laser pulses. The THz yield is achieved to increase triply in W/CoFeB through annealing effect, and doubly in Pt/CoFeB. The annealing effect originates from both the decrease of synthetic effect of THz absorption and the increase of hot electron mean free path induced by crystallization, with the latter being dominant, which is experimentally corroborated by THz transmission measurement of time-domain spectrum and four-probe resistivity t. Our observations not only deepen understand the spintronic THz radiation mechanism but also provide a novel platform for high speed spintronic opto-electronic devices.
2020, 69 (20): 200705.
doi: 10.7498/aps.69.20200634
Abstract +
High-performance terahertz emitters, which convert the femtosecond laser pulses into terahertz pulses, are essential for terahertz spectroscopy technology and terahertz wireless communication. Spintronic terahertz emitters based on ferromagnet/nonmagnet bilayers have attracted tremendous attention due to their high efficiency, ultra-broadband, low cost and high flexibility. Here, we systematically investigate the terahertz emission from polycrystalline topological insulator Bi2Te3/ferromagnetic CoFeB heterostructure grown by magnetron sputtering. The Bi2Te3/CoFeB heterostructure exhibits high efficiency of terahertz emission, and the polarization of terahertz waves can be controlled by the external magnetic field direction. The performance of Bi2Te3/CoFeB heterostructure is almost comparable to that of the Pt/CoFeB bilayer. In contrast, no terahertz emission is observed in the pure Bi2Te3 or CoFeB film driven by femtosecond laser pulses, probably because the Bi2Te3 prepared by sputtering is polycrystalline and the thickness of CoFeB is too thin. We also compare the performances of Bi2Te3/CoFeB grown on MgO, glass and high-resistivity silicon substrates, and find that the samples grown on MgO substrates exhibit the best emission performances. The glass substrate absorbs more terahertz waves than MgO substrate, resulting in a slightly weaker terahertz signal emitted from the Bi2Te3/CoFeB grown on the glass substrate. Although the absorption coefficient of high-resistivity silicon to terahertz waves is very small, the residual pump light excites the high-resistivity silicon to generate the photo-generated carriers, which change the conductivity of the high-resistivity silicon and reduce the transmittance of terahertz wave. We attribute the mechanism of the terahertz emission to the spin-charge conversion at the interface of Bi2Te3/CoFeB. The terahertz emission efficiency of our sample is expected to be able to be further improved by optimizing the samples. Moreover, with the sputtering method, it is possible to fabricate large area samples at low cost, which is critical for commercial applications.
2020, 69 (20): 204205.
doi: 10.7498/aps.69.20201188
Abstract +
As a typical phase transition material, vanadium dioxide has attracted much attention in the study of metal-insulator transition behavior since its phase transition temperature is close to room temperature. The experimental results of various modulation provide important clues to studying the vanadium dioxide phase transition mechanism. These experiments not only deepen the understanding of the strong correlation between electrons with different spins in various transition metal oxides, but also make an opportunity for exploring their potential practical applications. Although the phase transition mechanism of vanadium dioxide is still controversial, one has already made tremendous efforts to understand the mechanism of metal-insulation phase transition in the past few decades, which is stimulated from various experiments on vanadium dioxide modulation. Here in this work, the single crystal and polycrystalline vanadium dioxide are investigated. Their modulation mechanisms are studied by using the continuous laser pumping-terahertz probe technique, and it is found that the absorption behaviors of terahertz pulses at the same pump fluence are obviously different. After systematically discussing the representative phase transition mechanism, it is found that the phase transition of single crystal vanadium dioxide is attributed to the Mott-type phase transition dominated by the electronic structure, and that the polycrystalline vanadium dioxide originates from the Peierls-type phase transition occurring during the lattice distortion. In the past, most of the optical modulation was implemented under the condition of femtosecond laser pumping. The new optical modulation method given in this work, is a supplement to previous all-optical modulation experiment and more likely to be conducive to a more in-depth understanding of the modulation mechanism of vanadium dioxides.
2020, 69 (20): 204206.
doi: 10.7498/aps.69.20201545
Abstract +
In polar materials, the transition of electrons in momentum space will change the spontaneous polarization. When excited by femtosecond pulse laser, the transient modulation of the electric polarization will radiate electromagnetic wave at terahertz frequency. In a magnetic ordered system, the coherent excited spin wave radiates electromagnetic waves of the same frequency in the process of precession and relaxation. The investigation of the terahertz emission spectra of these materials not only helps us to understand the ferroelectric and magnetic ordered dynamic processes of materials, but also provides a reference for searching for new terahertz sources. We study the terahertz emission spectrum of the polar antiferromagnet Fe2Mo3O8. Under the pumping of 800 nm laser, electrons in the material are excited across the band gap leading the electric polarization to be ultra-fast modulated. The broadband terahertz excitation spectrum from 0.1 to 3.5 THz is observed, and the direction of the terahertz electric field is along the inherent electric polarization direction of the material. After entering into the magnetic order state, two new single-frequency terahertz oscillations are observed, located at 1.25 THz and 2.7 THz respectively, which correspond to the excitation of the two antiferromagnetic spin waves of Fe2Mo3O8.
2020, 69 (20): 207302.
doi: 10.7498/aps.69.20200682
Abstract +
Quasi-two-dimensional van der Waals ferromagnetic semiconductor CrSiTe3 with wide potential applications in optoelectronics and nanospintronics has aroused the immense interest of researchers due to the coexistence of intrinsic magnetism and semiconductivity. By combining untrafast femtosecond laser and terahertz spectroscopy, including terahertz time-domain spectroscopy, optical pump-terahertz probe spectroscopy and terahertz emission spectroscopy, we carry out systematic investigation into the van der Waals ferromagnetic semiconductor CrSiTe3 crystal. The experimental results indicate that the conductivity of the sample is robust against the temperature change and isotropic terahertz transmission in the ab-plane. Moreover, it is also observed that the photocarriers induced by 800 nm optical pump exhibit a relaxation in the biexponential form and the complex photoconductivity can be well reproduced by the Drude-Smith model. The main relaxation channel of photocarriers is the recombination of electron-hole pairs. With femtosecond pulse illuminating the surface of sample, a strong terahertz radiation signal with a broad band of 0–2 THz is observed. The present study provides the responses of CrSiTe3 to optical and terahertz frequency and offers crucial information for the future design of CrSiTe3-based electronic and optoelectronic devices.
2020, 69 (20): 207501.
doi: 10.7498/aps.69.20201507
Abstract +
As a typical helimagnet, ZnCr2Se4 possesses fascinating effects including magnetoelectric coupling, magnetostriction, negative thermal expansion, as well as possible diversity in quantum ground states. Here in this work, we investigate magnetic excitation arising from spiral spin structure in ZnCr2Se4 single crystal by using terahertz (THz) time domain spectroscopy (THz-TDS) under magnetic fields up to 10 T and at low temperatures. The magnetic resonance absorption is observed in a sub-THz region as the applied magnetic field is above 4 T, featuring the blue shift with magnetic field increasing. As the THz wave vector ( k ) is vertical to the external magnetic field (H), the single resonance frequency conforms well with the linear Larmor relation, corresponding to a spin structure transformation from helical to ferromagnetic state with magnetic field increasing in ZnCr2Se4. However, in the geometry in which both k and H are along the $ \langle 111\rangle $ direction of crystal, a well-defined resonance splitting emerges when H > 7 T. Especially, the high-frequency absorption shows pronouncedly nonlinear magnetic field dependence. It is suggested that such anisotropic spin dynamics below Néel temperature be linked with the field-driven quantum criticality unveiled in recent work.
2020, 69 (20): 207801.
doi: 10.7498/aps.69.20200031
Abstract +
Ultrafast spectroscopy is a powerful method to generate and control topological phase transitions and spin-polarized electrical currents in topological quantum materials. These light-induced novel physical properties originate from the topologically nontrivial states of Dirac and Weyl fermions. The topological semimetal molybdenum phosphide (MoP) exhibits double and triple degenerate points in the momentum space. We present the preliminary results of spin-polarized electrical currents and optical response investigations of MoP. We design and construct an experimental setup to perform the photocurrent generation and control by circularly polarized light in topological insulator Bi2Se3. The results compare well with those reported, which confirms the validity and reliability of our experimental setup. Further, we conduct the photocurrent experiment on MoP by using 400 nm laser pulses for excitation and successfully detect the current signals at different sample positions. We attribute the observed currents to photo-induced thermal currents (not the photo current associated with the triple degenerate topological properties), which facilitates generating and controlling photocurrents in MoP in the future investigation. Our thermal current investigations are of essence for further exploring the photocurrents in various types of topological quantum materials.
2020, 69 (20): 207802.
doi: 10.7498/aps.69.20201518
Abstract +
In this paper, the effects of magnetic field and nonmagnetic Y3+ doping on spin state and spin reorientation in HoFeO3 single crystal are systematically studied by the self-developed terahertz time-domain spectroscopy (THz-TDS) under magnetic field. By doping nonmagnetic Y3+, we find that the spin reorientation temperature range decreases. Meanwhile, we also find the type of spin reorientation of HoFeO3 does not change with Y3+ doping, indicating that the Y3+ doping can exchange the interaction energy of Ho3+-Fe3+ without introducing any new magnetic structure. Moreover, the resonance frequency of quasi-ferromagnetic mode (q-FM) decreases with temperature increasing in the low temperature range, while the resonance frequency of quasi-antiferromagnetic mode (q-AFM) increases with temperature increasing in the high temperature range in Ho1–xYxFeO3 single crystals. With the external magnetic field ( H DC) applied along the (110) axis, on the one hand the magnetic field can not only tune the resonant frequency of q-FM but also induce the spin reorientation in Ho1–xYxFeO3 single crystals, and on the other hand this magnetic field induced spin reorientation phenomenon can happen more easily if the temperature approaches to the intrinsic spin reorientation temperature range of the single crystals. Besides, the critical magnetic field induced spin reorientation increases with the doping of Y3+ increasing. Our research shows that THz spectroscopy data can be used to detect the doping concentration of Y3+ ions in HoFeO3; in addition, Y3+ doping can make the spin state in HoFeO3 crystal more stable and not easily affected by external magnetic fields. We anticipate that the role of doping and magnetic field in spin reorientation transition will trigger great interest in understanding the mechanism of the spin exchange interaction and the mechanism of external field tuning effect in the vast family of rare earth orthoferrites.
2020, 69 (20): 208704.
doi: 10.7498/aps.69.20200733
Abstract +
Recently, ferromagnetic/non-magnetic heterostructures have been widely studied for the generation of terahertz (THz) emitter based on spin-to-charge conversion. Actually, thermal spintronics effectively combines thermal transport with magnetism for creating and detecting non-equilibrium spin transport. A spin current or voltage can be induced by a temperature bias applied to a ferromagnetic material, which is called spin Seebeck effect (SSE). In this paper, we present a SSE based THz emission by using the heterostructures made of insulating ferrimagnet yttrium iron garnet (Y3Fe5O12, YIG) and platinum (Pt) with large spin orbit coupling. Upon exciting the Pt layer with a femtosecond laser pulse, a spin Seebeck current arises, applying a temperature gradient to the interface. Based on the inverse spin Hall effect, the spin Seebeck current is converted into a transient charge current and then yields the THz transients, which are detected by electrooptic sampling through using a ZnTe crystal at room temperature. The polarity of the THz pulses is flipped by 180° when the direction of the external magnetic field is reversed. By changing the direction of the pump beam excitation geometry to vary the sign of the temperature gradient at the YIG/Pt interface, the polarity of the THz signal is reversed. Fast Fourier transformation of the THz signals yields the amplitude spectra centered near 0.6 THz with a bandwidth in a range of 0.1–2.5 THz. We systematically investigate the influence of annealing effect on the THz emission from different YIG/Pt heterostructures. It can be found that the THz radiation is achieved to increase ten times in the YIG/Pt grown on a Gd3Ga5O12 (GGG) substrate through high-temperature annealing. The mechanism of annealing effect can be the increase of the spin mixing conductance of the interface between YIG and Pt. Finally, we investigate the pump fluence dependent THz peak-to-peak values for the annealed YIG/Pt grown on the Si substrate. Due to the spin accumulation effect at the interface of the YIG/Pt heterostructure, the THz radiation intensity gradually becomes saturated with the increase of pump fluence. Our results conclude that annealing optimization is of importance for increasing the THz amplitude, and open a new avenue to the future applications of spintronic THz emitters based on ultrafast SSE.
2020, 69 (20): 209501.
doi: 10.7498/aps.69.20200732
Abstract +
Terahertz (THz) transient has become an effective method to study the optical and electronic spin characteristics of the rare earth orthoferrites RFeO3. High-throughput grown crystal sample is sliced at different locations, then the continuously tunable rare earth elements co-doped single crystal SmxPr1–xFeO3 is studied with antiferromagnetic spin mode (qAFM) and crystal field transitions of rare earth ions under zero magnetic fields. Using THz time-domain spectroscopy, the qAFM resonance frequencies of Sm0.2Pr0.8FeO3 and Sm0.4Pr0.6FeO3 single crystals are located on the connection line of the qAFM frequencies of PrFeO3 (0.57 THz) and SmFeO3 (0.42 THz), therefore the frequency of qAFM increases linearly with doping concentration of Sm3+ ion increasing. The Sm0.4Pr0.6FeO3 crystal undergoes a temperature-induced spin reorientation phase transition at about 160 K. When the crystal temperature is lower than 80 K, a wide band absorption peak of about 0.5 THz appears in the absorption spectrum of Sm0.2Pr0.8FeO3 due to the crystal field effect. Our results show that THz spectral data not only allow us to monitor the quality of rare earth orthoferrite crystals prepared by high throughput and analyze the rare earth elements of the sample, but also improve the ability to analyze the physical properties of the co-doped RFeO3.
2022, 71 (9): 090702.
doi: 10.7498/aps.71.20201139
Abstract +
Since the discovery of the ultrafast demagnetization of the ferromagnetic metal, the spin degree of electrons is gradually used to generate terahertz radiation. The terahertz radiation generated by the inverse Rashba-Edelstein effect was confirmed first at the interface of Ag/Bi. However, the spin-to-charge conversion efficiency of the LaAlO3/SrTiO3 interface is one order of magnitude lager than that of the Ag/Bi interface under equilibrium or quasi-equilibrium condition. Whether the LaAlO3/SrTiO3 heterostructures can be used to convert spin current to generate terahertz radiation remains to be systemically studied. In this work, we fabricate the NiFe/LaAlO3//SrTiO3 heterostructures and investigate the generation of terahertz radiation by femtosecond laser pumping and its dependence of the magnetic field direction. We change the thickness of the LaAlO3 to show the applicability of the superdiffusive spin transport model and optical transmission model. We find the multireflections at the LaAlO3/SrTiO3 interface weaken the terahertz radiation intensity. This work provides experimental and theoretical support for further optimizing the generation of terahertz electromagnetic waves.
