Significant progress has been made in answering fundamental questions about how and, more importantly, on what time scales interactions between electrons, spins, and phonons occur in solid state materials. These complex interactions are leading to the first real applications of terahertz (THz) spintronics: THz emitters that can compete with traditional THz sources and provide additional functionalities enabled by the spin degree of freedom. This tutorial article is intended to provide the background necessary to understand, use, and improve THz spintronic emitters. A particular focus is the introduction of the physical effects that underlie the operation of spintronic THz emitters. These effects were, for the most part, first discovered through traditional spin-transport and spintronic studies. We therefore begin with a review of the historical background and current theoretical understanding of ultrafast spin physics that has been developed over the past twenty-five years. We then discuss standard experimental techniques for the characterization of spintronic THz emitters and - more broadly - ultrafast magnetic phenomena. We next present the principles and methods of the synthesis and fabrication of various types of spintronic THz emitters. Finally, we review recent developments in this exciting field including the integration of novel material platforms such as topological insulators as well as antiferromagnets and materials with unconventional spin textures.
THz polarization control upon generation is a crucially missing functionality. THz spintronic emitters based on the inverse spin Hall effect allow for this by the strict implicit orthogonality between their magnetization state and the emitted polarization. This control was up till now only demonstrated using cumbersome external magnetic field biasing to impose a polarization direction. We present here for the first time an efficient voltage control of the polarization state of terahertz spintronic emitters. Using a ferromagnetic spin pumping multilayer exhibiting simultaneously strong uniaxial…
THz pulses are generated from femtosecond pulse-excited ferromagnetic/nonmagnetic spintronic heterostructures via inverse spin Hall effect. The contribution from ultrafast demagnetization/remagnetization is extremely weak, in the comparison. The highest possible THz signal strength from spintronic THz emitters is limited by the optical damage threshold of the corresponding heterostructures. The THz generation efficiency does not saturate with the excitation fluence even up till the damage threshold. Bilayer (Fe, CoFeB)/(Pt, Ta) based FM/NM spintronic heterostructures have been studied for an o…
We report a THz emitter with excellent performances based on nonmagnetic (NM) and ferromagnetic (FM) heterostructures. The spin currents are first excited by the femtosecond laser beam in the NM/FM bilayer, and then transient charge currents are generated by inverse spin Hall effect, leading to THz emission out of the structure. The broadband THz waves emitted from our film stacks have a peak intensity exceeding 500 um thick ZnTe crystals (standard THz emitters). Our device is insensitive to the polarization of an incident laser beam which indicates the noise resistive feature. In contrast, th…
Recent developments in nanomagnetism and spintronics have enabled the use of ultrafast spin physics for terahertz (THz) emission. Spintronic THz emitters, consisting of ferromagnetic FM / non-magnetic (NM) thin film heterostructures, have demonstrated impressive properties for the use in THz spectroscopy and have great potential in scientific and industrial applications. In this work, we focus on the impact of the FM/NM interface on the THz emission by investigating Fe/Pt bilayers with engineered interfaces. In particular, we intentionally modify the Fe/Pt interface by inserting an ordered L1$…
Spin-based terahertz (THz) emitters, utilizing the inverse spin Hall effect, are ultra-modern sources for the generation of THz electromagnetic radiation. To make a powerful emitter having large THz amplitude and bandwidth, fundamental understanding in terms of microscopic models is essential. This study reveals important factors to engineer the THz emission amplitude and bandwidth in epitaxial Pt/Fe emitters grown on MgO and MgAl$_2$O$_4$ (MAO) substrates, where the choice of the substrate plays an important role. The THz amplitude and bandwidth are affected by the induced strain in the Fe sp…
In this work, we report our study on the THz emission in Fe/Pt magnetic heterostructures. We have carried out a comprehensive investigation of THz emission from Fe/Pt magnetic heterostructures, employing time-domain THz spectroscopy. We reveal that by properly tuning the thickness of Fe or Pt layer, THz emission can be greatly improved in this type of heterostructure. We further demonstrate that the THz field strength emitted from a newly designed multilayer (Pt/Fe/MgO)$_n$ with n=3 can reach a value of ~1.6 kV/cm, which is comparable to the values from the conventional GaAs antenna with a bia…
We demonstrate here an efficient THz source with low electrical power consumption. We have increased the maximum THz radiation power emitted from SI-GaAs based photoconductive emitters by two orders of magnitude. By irradiating the SI-GaAs substrate with Carbon-ions up to 2 micron deep, we have created lot of defects and decreased the life time of photo-excited carriers inside the substrate. Depending on the irradiation dose we find 1 to 2 orders of magnitude decrease in total current flowing in the substrate, resulting in subsequent decrease of heat dissipation in the antenna. This has result…
Spintronic terahertz (THz) emitters (STE) based on the inverse spin Hall effect in ferromagnetic/heavy metal (FM/HM) heterostructures have become important sources for THz pulse generation. The design, materials and control of these interfaces at the nanometer level has become vital to engineer their THz emission properties.In this work, we present studies of the optimization of such structures through a multi-pronged approach, taking advantage of material and interface engineering to enhance the THz spintronic emission. This includes: the application of multi-stacks of HM/FM junctions and the…
Terahertz (THz) sciences and technologies have contributed to a rapid development of a wide range of applications and expanded the frontiers in fundamental science. Spintronic terahertz emitters offer conceptual advantages since the spin orientation in the magnetic layer can be easily controlled either by the externally applied magnetic field or by the internal magnetic field distribution determined by the specific shape of the magnetic elements. Here, we report a switchable terahertz source based on micropatterned magnetic heterostructures driven by femtosecond laser pulses. We show that the …
Magnetic Field-Controlled THz Modulation in Uniaxial Anisotropic Spin-Valves Emitters
MIREA – Russian Technological University（莫斯科无线电工程、电子和自动化国立大学）；Institute for Physics of Microstructures RAS（俄罗斯科学院微观物理研究所）；Lobachevsky State University（下诺夫哥罗德国立罗巴切夫斯基大学）
Recent intense electrical and optical studies of graphene have pushed the material to the forefront of optoelectronic research. Of particular interest is the few terahertz (THz) frequency regime where efficient light sources and highly sensitive detectors are very challenging to make. Here we present THz sources and detectors made with graphene field effect transistors (GFETs) enhanced by a double-patch antenna and an on-chip silicon lens. We report the first experimental observation of 1-3 THz radiation from graphene, as well as four orders of magnitude performance improvements in a GFET ther…
We formulate and justify several proposals utilizing the unique electronic properties of different types of carbon nanotubes for a broad range of applications to THz optoelectronics, including THz generation by hot electrons in quasi-metallic nanotubes, frequency multiplication in chiral-nanotube-based superlattices controlled by a transverse electric field, and THz radiation detection and emission by armchair nanotubes in a strong magnetic field.
Femtosecond laser-induced photoexcitation of ferromagnet (FM)/heavy metal (HM) heterostructures have attracted attention by emitting broadband terahertz frequencies. The phenomenon relies on the formation of ultrafast spin current, which is largely attributed to the direct photoexcitation of the FM layer. However, we reveal that during the process, the FM layer also experiences a secondary excitation led by the hot electrons from the HM layer that travel across the FM/HM interface and transfer additional energy in the FM. Thus, the generated secondary spins enhance the total spin current forma…
Spintronic terahertz (THz) emitter provides the advantages such as apparently broader spectrum, significantly lower cost, and more flexibility in compared with the commercial THz emitters, and thus attracts great interests recently. In past few years, efforts have been made in optimizing the material composition and structure geometry, and the conversion efficiency has been improved close to that of ZnTe crystal. One of the drawbacks of the current designs is the rather limited laser absorption - more than 50% energy is wasted and the conversion efficiency is thus limited. Here, we theoretical…
Terahertz electromagnetic radiation is extremely useful for numerous applications such as imaging and spectroscopy. Therefore, it is highly desirable to have an efficient table-top emitter covering the 1-to-30-THz window whilst being driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source which relies on tailored fundamental spintronic and photonic phenomena in…
Strategies and concepts for the design of THz emitters based on the quantum cascade scheme are analyzed and modeled in terms of a fully three-dimensional Monte Carlo approach; this allows for a proper inclusion of both carrier-carrier and carrier-phonon scattering mechanisms. Starting from the simulation of previously published far-infrared emitters, where no population inversion is achieved, two innovative designs are proposed. The first one follows the well-established chirped-superlattice scheme whereas the second one employs a double-quantum well superlattice to allow energy relaxation thr…
Spintronic terahertz emitters are novel, broadband and efficient sources of terahertz radiation, which emerged at the intersection of ultrafast spintronics and terahertz photonics. They are based on efficient spin-current generation, spin-to-charge-current and current-to-field conversion at terahertz rates. In this review, we address the recent developments and applications, the current understanding of the physical processes as well as the future challenges and perspectives of broadband spintronic terahertz emitters.
The anomalous Hall effect (AHE) is a fundamental spintronic charge-to-charge-current conversion phenomenon and closely related to spin-to-charge-current conversion by the spin Hall effect. Future high-speed spintronic devices will crucially rely on such conversion effects at terahertz (THz) frequencies. Here, we reveal that the AHE remains operative from DC up to 40 THz with a flat frequency response in thin films of three technologically relevant magnetic materials: DyCo$_{5}$, Co$_{32}$Fe$_{68}$ and Gd$_{27}$Fe$_{73}$. We measure the frequency-dependent conductivity-tensor elements $σ_{xx}$ …
Phase-stable electromagnetic pulses in the THz frequency range offer several unique capabilities in time-resolved spectroscopy. However, the diversity of their application is limited by the covered spectral bandwidth. In particular, the upper frequency limit of photoconductive emitters - the most widespread technique in THz spectroscopy - reaches only up to 7 THz in regular transmission mode due to the absorption by infrared-active optical phonons. Here, we present ultra-broadband (extending up to 70 THz) THz emission from Au implanted Ge emitter which is compatible with a fibre laser operatin…
To explore the capabilities of metallic spintronic thin-film stacks as a source of intense and broadband terahertz electromagnetic fields, we excite a W/CoFeB/Pt trilayer on a large-area glass substrate (diameter of 7.5 cm) by a femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, wavelength 800 nm). After focusing, the emitted terahertz pulse is measured to have a duration of 230 fs, a peak field of 300 kV cm$^{-1}$ and an energy of 5 nJ. In particular, the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of amplitude maximum, thereby facilitating nonlinear control ov…
To achieve a large terahertz (THz) amplitude from a spintronic THz emitter (STE), materials with 100\% spin polarisation such as Co-based Heusler compounds as the ferromagnetic layer are required. However, these compounds are known to loose their half-metallicity in the ultrathin film regime, as it is difficult to achieve L2$_1$ ordering, which has become a bottleneck for the film growth. Here, the successful deposition using room temperature DC sputtering of the L2$_1$ and B2 ordered phases of the Co$_2$FeAl full Heusler compound is reported. Co$_2$FeAl is used as ferromagnetic layer together…
Thickness-Dependent Spintronic Terahertz Emission in MBE-Grown PtTe$_2$: From Semiconductor to Type-II Dirac Semimetal
Univ. Grenoble Alpes, CEA, CNRS, IRIG-SPINTEC（格勒诺布尔阿尔卑斯大学、法国原子能和替代能源委员会、法国国家科学研究中心、IRIG自旋技术中心）；Centre for Nanodiagnostics of Materials, Faculty of Materials Science and Technology, Slovak University of Technology（材料纳米诊断中心、材料与科技学院、斯洛伐克科技大学）
Spin-to-charge conversion (SCC) involving topological surface states (TSS) is one of the most promising routes for highly efficient spintronic devices for terahertz (THz) emission. Here, the THz generation generally occurs mainly via SCC consisting in efficient dynamical spin injection into spin-locked TSS. In this work, we demonstrate sizable THz emission from a nanometric thick topological insultator (TI)/ferromagnetic junction - SnBi$_2$Te$_4$/Co - specifically designed to avoid bulk band crossing with the TSS at the Fermi level, unlike its parent material Bi$_2$Te$_3$. THz emission time do…
THz magnetization dynamics is a key property of antiferromagnets as well as ferrimagnets that could harness the THz forefront and spintronics. While most of the present THz measurement techniques are for bulk materials whose sensitivities rely on the volume of the material, measurement techniques suitable for thin films are quite limited. In this study, we explored and demonstrated electrical detection of the antiferromagnetic dynamics in ferrimagnetic Gd-Co thin films by using a 154 GHz gyrotron, a high-power electromagnetic wave source. Captured resonant modes allow us to characterize the pe…
Antiferromagnetic spin motion at terahertz (THz) frequencies attracts growing interests for fast spintronics, however their smaller responses to external field inhibit device application. Recently the noncollinear antiferromagnet Mn$_3$Sn, a Weyl semimetal candidate, was reported to show large anomalous Hall effect (AHE) at room temperature comparable to ferromagnets. Dynamical aspect of such large responses is an important issue to be clarified for future THz data processing. Here the THz anomalous Hall conductivity in Mn$_3$Sn thin films is investigated by polarization-resolved spectroscopy.…
Terahertz (THz) radiation is a powerful tool with widespread applications ranging from imaging, sensing, and broadband communications to spectroscopy and nonlinear control of materials. Future progress in THz technology depends on the development of efficient, structurally simple THz emitters that can be implemented in advanced miniaturized devices. Here we show how the natural electronic anisotropy of layered conducting transition metal oxides enables the generation of intense terahertz radiation via the transverse thermoelectric effect. In thin films grown on offcut substrates, femtosecond l…
Long-range orbital transport and inverse orbital Hall effect in Co/Ru-based terahertz emitters
Shaanxi University of Technology（陕西理工大学）；Fudan University（复旦大学）；Shanghai Research Center for Quantum Sciences（上海量子科学研究中心）；The Chinese University of Hong Kong, Shenzhen（香港中文大学（深圳））
Spintronic structures are extensively investigated for their spin orbit torque properties, required for magnetic commutation functionalities. Current progress in these materials is dependent on the interface engineering for the optimization of spin transmission. Here, we advance the analysis of ultrafast spin-charge conversion phenomena at ferromagnetic-transition metal interfaces due to their inverse spin-Hall effect properties. In particular the intrinsic inverse spin Hall effect of Pt-based systems and extrinsic inverse spin-Hall effect of Au:W and Au:Ta in NiFe/Au:(W,Ta) bilayers are inves…
Development of future sensor, memory, and computing nanodevices based on novel physical concepts is one of the significant research endeavors in solid-state research. The field of spintronics is one such promising area of nanoelectronics which utilizes both the charge and spin of an electron for device operations. The advantage offered by spin systems is in their non-volatility and low-power functionality. This paper reviews emerging spintronic phenomena and the research advancements in diverse spin based applications. Spin devices and systems for logic, memories, emerging computing schemes, f…
Antiferromagnetic transition metal oxides are an established and widely studied materials system in the context of spin-based electronics, commonly used as passive elements in exchange bias-based memory devices. Currently, major interest has resurged due to the recent observation of long-distance spin transport, current-induced switching, and THz emission. As a result, insulating transition metal oxides are now considered to be attractive candidates for active elements in novel spintronic devices. Here, we discuss some of the most promising materials systems and highlight recent advances in re…
Although the development of spintronic devices has advanced significantly over the past decade with the use of ferromagnetic materials, the extensive implementation of such devices has been limited by the notable drawbacks of these materials. Antiferromagnets claim to resolve many of these shortcomings leading to faster, smaller, more energy-efficient, and more robust electronics. Antiferromagnets exhibit many desirable properties including zero net magnetization, imperviousness to external magnetic fields, intrinsic high-frequency dynamics with a characteristic precession frequency on the ord…
Optimizing spin-based terahertz emission from magnetic heterostructures
Uppsala University（乌普萨拉大学）；VTT Technical Research Center（VTT技术研究中心）；Laboratoire Albert Fert, CNRS, Thales, Université Paris-Saclay（阿尔伯特·费尔特实验室，法国国家科学研究中心，泰雷兹集团，巴黎萨克雷大学）；Laboratoire de Physique de l’Ecole Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité（巴黎高等师范学院物理实验室，巴黎高等师范学院，巴黎文理研究大学，法国国家科学研究中心，索邦大学，巴黎西岱大学）
Topologically-protected surface states present rich physics and promising spintronic, optoelectronic and photonic applications that require a proper understanding of their ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of bulk carriers by combining photoexcitation with below-bandgap terahertz (THz) photons with TI samples with varying Fermi level, including one sample with the Fermi level located within the bandgap. W…
Ultrafast THz radiation generation from ferromagnetic/nonmagnetic bilayer heterostructure-based spintronic emitters generally exploits the conversion from spin- to charge-current within the nonmagnetic layer and its interface with the ferromagnetic layer. Various possible sub-contributions to the underlying mechanism of inverse spin Hall effect for the THz emission from such structures, need to be exploited for not only investigating the intricacies at the fundamental level in the material properties themselves but also for improving their performance for broadband and high-power THz emission.…
