Efficient terahertz radiation based on inverse spin Hall effect in W/CoFeB/Pt heterostructures

Spin Hall effect and its reciprocal, the inverse spin Hall effect (ISHE), provide the means for conversion between spin and charge currents [1, 2]. In particular, the ISHE transforms a pure spin current into a transverse charge current due to spin-orbit coupling. Recently, the decisive role of the ISHE effect on extending the field of spintronics into the terahertz (THz) regime was revealed [3, 4]. Here, we demonstrate the generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses of Fe (2 nm) / Pt (3 nm) were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer [5]. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on optical rectification in nonlinear crystals. Furthermore, we correlate the interface structural properties with the THz-E-field amplitude and the bandwidth of the THz radiation. By allowing the Pt layers to grow along different crystallographic directions on top of Fe we modify the THz emission characteristics of the optimized emitters. We present a theoretical model which correlates the loss of energy of the hot electrons and the electron-phonon/defect scattering lifetime at the Fe/Pt interface with the ISHE current that causes the THz emission. By taking into account the response function of the THz detector we describe the influence of the crystal structure of Pt onto the THz signal shape and spectrum. The demonstration of the role of the individual layer thicknesses and of the interface quality in the THz emission spectra paves the way for more efficient spintronic THz emitters.

In this work, we reveal unconventional, polarization-dependent anisotropic terahertz (THz) emission from a sputtered Co/Mo bilayer attributed to competing mechanisms of THz generation. Unlike conventional spintronic THz emitters that operate solely via the inverse spin Hall effect (ISHE), the present system generates THz radiation through the combined action of ISHE and second-order nonlinear-optical processes. THz magnetometry shows pronounced asymmetries in the THz hysteresis loops compared with planar magneto-optical Kerr effect magnetometry. Systematic experiments on azimuthal rotation of the sample and controlled variation of the pump pulse polarization (linear and elliptical) enable a full decomposition of the THz field. Roughly 70% of the amplitude is an odd-in-field contribution generated by the ISHE in Mo, whereas the remaining 30% is an even-in-field component whose angular behavior points to a second-order interface nonlinearity governed by a frozen in-plane interfacial magnetization <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mi>M</mml:mi> <mml:mi>′</mml:mi> </mml:msup> </mml:mrow> </mml:math> that breaks inversion symmetry. These results demonstrate that the Co/Mo interface functions as a THz emitter whose amplitude is strongly sensitive to both the orientation of magnetization and the polarization of the incident light. The interplay between interfacial spin–orbit coupling and broken inversion symmetry gives rise to a distinct emission mechanism, which in turn results in the observed behavior not commonly found in conventional spintronic emitters. The symmetry-resolved separation of ISHE and optical rectification mechanisms provides clear design rules for tailoring interface-driven THz emitters based on ultrathin heterostructures.

Recent research progress of terahertz (THz) radiation generation based on the ultrafast spin dynamics is reviewed. The transient spin-charge conversion based on the inverse spin Hall effect and the Rashba-Edelstein effect is introduced, and it is pointed out that the ferromagnetic/non-magnetic heterostructure has been used to design a low-cost and high-efficiency THz radiation source. The efficiency and bandwidth of a spintronics-based THz emitter can be improved by optimizing layer thickness, growth conditions, substrates, and construction. The applications of the THz emission spectrum are outlined in the study of the ultrafast formation dynamics of the spin Seebeck effect. © 2019, Chinese Lasers Press. All right reserved.

Dependence of the inverse spin Hall effect in Sr(Nb[formula omitted]Ti[formula omitted])O[formula omitted] on the Nb concentration

For nonlinear optical materials to be applicable as efficient broadband terahertz (THz) wave generators, low absorption with wide transparency in the THz frequency range is highly important. In this study, we report efficient organic THz wave generators, 2-(4-hydroxystyryl)-1-methylquinolinium 4-bromobenzenesulfonate (OHQ-BBS) single crystals. Interestingly, the OHQ-BBS crystals exhibit a wide molecular vibrational mode-free range in the THz frequency region from 1.7 to 5.1 THz with an absorption coefficient of &amp;lt;20 mm−1. By optical rectification employing 130 fs pump pulses at 1300 nm wavelength, the OHQ-BBS crystals generate extremely broad, dimple-free THz waves in the range of 1.2–5.5 THz. Additionally, a THz electric field that is 20 times higher than the field generated from the widely used ZnTe inorganic crystal is achieved. Therefore, the OHQ-BBS single crystals are highly promising materials for diverse THz photonic applications.

The relativistic inverse spin Hall effect (ISHE) plays a pivotal role in terahertz (THz) emission in magnet/heavy metal bilayers. Here, THz emission from RuO2/Py bilayers is reported due to not only the relativistic ISHE but also the nonrelativistic inverse altermagnetic spin splitting effect (IASSE) in RuO2. For the (100)‐oriented RuO2/Py bilayers, the THz emission is greatly enhanced owing to IASSE once the polarization direction of the spin current (from the Py layer induced by pump laser) is parallel to the Néel vector of RuO2, producing the anisotropic THz emission with twofold symmetry. In contrast, the RuO2(110)/Py bilayer only exhibits isotropic THz emission because of the absence of IASSE‐induced spin‐charge conversion in the (110)‐oriented RuO2 films. Apart from the fundamental significance of exploring the IASSE‐induced spin‐to‐charge conversion, the finding also enriches the physical mechanism and modulation method of THz emission in spintronic systems.

The ever-growing number of interconnected devices and the emergence of high-bandwidth technologies such as holographic communications in mixed reality are driving an exponential growth in the demand for terabits-per-second (Tbps) data rates with sub-milliseconds latency. For this reason, the sixth-generation (6G) wireless communications are moving towards the terahertz (THz) frequency band because of its wider spectral bandwidth availability to enable higher channel capacity and data rates according to the Shannon-Hartley theorem. However, the efficiency of wireless THz signal transmission is reduced by the increased path loss and atmospheric attenuation due to water vapor absorption at THz frequencies, which significantly decreases the signal-to-noise ratio and the spectral efficiency of THz wireless communications. The realization of THz band communications is further impeded by the limited power output of existing compact THz sources due to the extremely low conversion efficiencies. Therefore, it is crucial to develop broadband THz integrated circuits with waveguiding and beamforming functionalities for supporting high data-rate on-chip communications and to provide sufficient gains to counteract the path loss and limited power of THz wireless transmissions. This thesis aims to improve the spectral efficiency of 6G wireless communications via the spatiotemporal shaping of THz waves using silicon photonics. Spatiotemporal light shaping is the basis of nearly all THz photonic devices, ranging from THz wavefront manipulation using metasurfaces to THz planar waveguiding using photonic crystal waveguides. Silicon photonics is a promising material platform for THz wave shaping due to the low-loss propagation of THz waves in high-resistivity silicon and the prospect of a monolithic integrated circuit due to the complementary metal-oxide semiconductor (CMOS) fabrication process compatibility. Silicon metasurfaces allow the wavefront modulation of incident THz radiation by exploiting the generalized Snell’s law of refraction and reflection to beamform highly directional THz waves in a dynamic multi-user communication environment for enhancing the spectral efficiency. Likewise, broadband silicon photonic crystal waveguides allow the in-plane and lossless transport of THz waves to support on-chip high data-rate THz communications that can also seamlessly integrate multiple THz photonic devices in a THz integrated circuit. In addition, silicon electron emitters and silicon gratings potentially allow the on-chip generation of highly intense and broadband THz radiation to counteract the significant THz attenuation. In this thesis, spatiotemporal wave shaping techniques using silicon photonics for enhancing the spectral efficiency of THz band communications is presented. First, a self-adaptive and deep reinforcement learning model for THz beamforming is proposed, which can beamform multiple highly directional THz radiation beam in real-time to serve a dynamically changing multi-user communication environment and improve the signal-to-noise ratio of THz wireless transmission while potentially reducing the communication latency. Second, a broadband THz topological silicon waveguide designed based on valley photonic crystals is proposed for the robust and lossless transport of broadband THz waves for the construction of THz integrated circuits with broadband channel capacity for enabling higher data-rate on-chip communication. Last, the versatility of free-electron radiation from silicon gratings is investigated for the generation of highly intense spatiotemporal wave packets with user-designed group velocities and tunable intensity profile, with the potential to realize integrated high-power THz sources. The deep learning beamforming model, the broadband topological waveguide designs, and the free-electron radiation intensity shaping techniques presented in this thesis extends beyond the THz regime and are scalable across the entire electromagnetic spectrum. The CMOS process compatibility of silicon metasurfaces, silicon planar waveguides, along with silicon electron emitters and silicon gratings offers the opportunity to realize a broadband THz photonic integrated circuit with waveguiding, wave generation and beamforming functionalities for enhancing the spectral efficiency of THz communications in 6G wireless networks.

A 5.8 GHz Fabry–Perot resonator antenna is proposed to improve the performance of the spin rectification effect and inverse spin Hall effect in the far field region. A planar inverse F-shaped antenna and a metamaterial superstrate are used to increase the magnitude of the electromagnetic field. The microwave electric field is enhanced by ∼10-fold, while the magnetic field is enhanced by ∼80-fold. Experimental results prove that the dc signal from the spin rectification effect and inverse spin Hall effect is significantly improved. This letter paves the way for the use of spintronic devices in wireless excitation applications.

THz spintronics emitters represent today novel sources for broadband emission using nanometer-scaled materials. We present and model the THz spintronic emission based on the spin Hall effect in 3d metals and on the inverse Rashba-Edelstein effect in the optimized topological insulator Bi1-xSbx.1. IntroductionTerahertz (THz) spintronic emitters are promising candidates for covering the so-called THz gap as they offer broadband emission going up to 30 THz [1]. Conventionally, they consist in nanometer-thin heterostructures composed by a ferromagnetic layer and a heavy metal from the 3d-5d family which, owing to their high spin-orbit coupling (SOC), allow spin-to-charge conversion (SCC) from a pure transient spin-current coming from the ferromagnetic layer into a transverse transient charge current. Efforts have been made recently to increase the output emission power of such devices by layer thickness engineering in order to optimize spin-to-charge interconversion and to reduce both THz absorption in metallic layers and spin current relaxation in thick ferromagnets.In this study, we evidence the processes timescales at stake in the emission properties of metallic THz spintronic emitters using Finite Domain Time Domain (FDTD) simulations. We evidence the main figure of merit which will permit to optimize the next generation of THz emitters. We then turn towards an alternative type of THz spintronic emitters based on topological insulators. We discuss the role of surface states in the conversion mechanisms and highlight promising THz emission in topological insulator Bi1-xSbx.2. Optimization of THz metallic spintronic emitters based on FDTD modellingConventional THz spintronic emitters are based on ferromagnet/heavy metal heterostructures such as Co/Pt, W/CoFeB/Pt or Fe/Pt. Shining infrared femtosecond laser pulses on the heterostructure induces a transient spin current diffusing from the ferromagnet to the heavy metal. Through spin-to-charge conversion due to the inverse spin Hall effect (ISHE), a THz wave is generated. The direction of the THz emission depends on the direction of the magnetization: the polarization can thus be easily tuned by small external magnetic field. The amplitude of the THz emission depends strongly on the amplitude of the interconversion mechanism, which is related to skew-scattering and side-jump effects in heavy metals. In first approximation, the amplitude of the emission is thus given by the spin Hall angle θSHE = jc/js, ratio between the incoming spin current and the generated charge current given by jc ∝ θSHE (js x σ) where σ is the spin accumulation orientation [2,3].Here, we provide a comprehensive understanding of the THz emission processes using FDTD modelling, including spin-dependent timescales such as spin-flip rates in heterostructures. We obtain a good agreement between experiments obtained by THz time-domain spectroscopy (THz-TDS) and performed simulations as shown in Fig. 1. We also evidence the crucial role of the interface quality between the heavy metal and the ferromagnet, as spin backflow or interfacial spin-flip may decrease the conversion. We thus propose a figure of merit η ∝ g↑↓σSHElsfHM which defines the THz emission strength in metallic heterostructures. This product includes the spin-mixing conductance g↑↓ illustrating interface quality, spin-Hall conductivity σSHE linked to interconversion strength and spin diffusion length in the conversion material lsfHM proportional to spin-current relaxation. We believe that our results will help optimizing the next generation of THz spintronic emitters.3. Towards surface-states mediated THz emission using topological insulatorsAlongside ISHE-based emitters, topological insulators (TI) are studied here as an alternative source for THz emitters. They present conductive surface-states which allow interfacial interconversion via inverse Rashba-Edelstein effect (IREE). In these systems, strong spin-to-charge conversion is expected owing to i) the Fermi velocity of interfacial carriers and ii) the insulating behaviour of the material bulk, reducing possible THz absorption in the heterostructure. Such systems have been proven to show emission in the THz range, for example with topological insulator Bi2Se3 [4].We report in Fig. 2 emission features from Bi1-xSbx/Co interface. Experimental data are taken with standard electro optic sampling (EOS) using ZnTe(111) 500 μm thickness. Emission performance of Bi0.79Sb0.21(15)/Co(4) is about the same order of magnitude as Co(2)/Pt(4) single-ISHE conversion state-of-the-art emitter. As recovered on IREE-based emitter, THz emission phase is reversed as applied magnetic field direction is reversed. Topological insulators here illustrated by Bi1-xSbx are thus suitable candidates for strong output THz emitters. **

Terahertz (THz) wave science and technology have been found countless applications in biomedical imaging, security screening, and non-destructive testing as they approach maturity. However, due to the challenge of high ambient moisture absorption, the development of remote open-air broadband THz spectroscopy technology is lagging behind the compelling need that exists in homeland security, astronomy and environmental monitoring. Furthermore, the underlying physical mechanisms behind the interaction between the THz wave and laserinduced plasma which responds strongly to electromagnetic waves have not been fully understood. This review aims to explain the light-plasma interaction at THz frequencies within a semiclassical framework along with experimental study of the femtosecond-laserinduced nitrogen plasma fluorescence under the illumination of single-cycle THz pulses. The results indicate that THz-radiation-enhanced-emission-of-fluorescence (THz-REEF) is dominated by electron kinetics in the THz field and the electron-impact excitation of gas molecules/ions. The information of the time-dependent THz field can be recovered from the measured time-resolved THz-REEF from single-color laser induced plasma with the help of the bias as local oscillator. The calculations and experimental verification lead to complete understanding of the science behind these effects and push forward to extend their capabilities in related applications such as remote THz sensing, plasma diagnostics and ultrafast photoluminescence modulation. Systematic studies in selected gases including neon, argon, krypton, xenon, methane (CH4), ethane (C2H6), propane (C3H8), and n-butane (C4H10) gases were performed to obtain an improved understanding of the THz-REEF. The dependences of the enhanced fluorescence on the THz field, laser excitation intensity, gas pressure, and intrinsic atomic properties were experimentally characterized. Both narrow line emission and broad continuum emission of the gas plasma were enhanced by the THz field. Their fluorescence enhancement ratios and time-resolved enhanced fluorescence were largely dependent on the scattering cross section and ionization potential of atoms. For the first time, we demonstrated a novel ‘all-optical’ technique of broadband THz wave remote sensing by coherently manipulating the fluorescence emission from asymmetrically ionized gas plasma that interacted with THz waves. By studying the ultrafast electron dynamics under the single cycle THz radiation, we found that the fluorescence emission from laser-induced air plasma was highly dependent on the THz electric field and the symmetry of the electron drift velocity distribution created by two-color laser fields. The time-resolved THz-REEF can be tailored by switching the relative two-color phase and laser polarizations. Owing to the high atmospheric transparency and omni-directional emission pattern of fluorescence, this technique can be used to measure THz pulses at standoff distances with minimal water vapor absorption and unlimited directionality for optical signal collection. The coherent THz wave detection at a distance of 10 m had been demonstrated. The combination of this method and previously demonstrated remote THz generation would eventually make remote THz spectroscopy available. We also introduced a unique plasma diagnostic method utilizing the THz-wave-enhanced fluorescence emission from the excited atoms or molecules. The electron relaxation time and plasma density were deduced through applying the electron impact excitation/ionization and electron-ion recombination processes to the measured time-delay-dependent enhanced fluorescence. The electron collision dynamics of nitrogen plasma excited at different gas pressures and laser pulse energies were systematically investigated. This plasma diagnostic method offers picosecond temporal resolution and is capable of omnidirectional optical signal collection. The ultrafast quenching dynamics of laser-pulse-induced photoluminescence in semiconductors under the radiation of single-cycle THz pulses was studied. It was found that the quenching in both cadmium telluride (CdTe) and gallium arsenide (GaAs) was linearly proportional to the intensity of incident THz waves and reaches up to 17% and 4% respectively at the peak intensity of 13 MW/cm2. The THz-wave-induced heating of the carriers and lattice and the subsequent decreased efficiency of photocarrier generation and recombination were most likely to be responsible for the quenching. This is potentially useful for the applications of a non-invasive ultrafast light modulator for photoluminescence devices with picoseconds switching time in the fields of the light-emitting devices and optical communication.

We have measured the inverse spin Hall effect (ISHE) in \textit{n}-Ge at room temperature. The spin current in germanium was generated by spin pumping from a CoFeB/MgO magnetic tunnel junction in order to prevent the impedance mismatch issue. A clear electromotive force was measured in Ge at the ferromagnetic resonance of CoFeB. The same study was then carried out on several test samples, in particular we have investigated the influence of the MgO tunnel barrier and sample annealing on the ISHE signal. First, the reference CoFeB/MgO bilayer grown on SiO$_{2}$ exhibits a clear electromotive force due to anisotropic magnetoresistance and anomalous Hall effect which is dominated by an asymmetric contribution with respect to the resonance field. We also found that the MgO tunnel barrier is essential to observe ISHE in Ge and that sample annealing systematically lead to an increase of the signal. We propose a theoretical model based on the presence of localized states at the interface between the MgO tunnel barrier and Ge to account for these observations. Finally, all of our results are fully consistent with the observation of ISHE in heavily doped $n$-Ge and we could estimate the spin Hall angle at room temperature to be $\approx$0.001.

Spin pumping in ferromagnets provides a source of pure spin currents. Via the\ninverse spin Hall effect a spin current is converted into a charge current and\na corresponding detectable DC-voltage. The ratio of injected spin current to\nresulting charge current is given by the spin Hall angle. However, the number\nof experiments more or less equals the number of different values for spin Hall\nangles, even for the most studied normal metal platinum. This publication\nprovides a full study of inverse spin Hall effect and anisotropic\nmagnetoresistance for different NiFe(Py) / normal metal bilayers using a\ncoplanar waveguide structure. Angle and frequency dependent measurements\nstrongly suggest that spin pumping and inverse spin Hall effect can be used to\nquantify spin Hall angles only if certain conditions are met. Ruling out the\nanisotropic magnetoresistance as a parasitic voltage generating effect\nmeasurements of the inverse spin Hall effect in Py/Pt and Py/Au yield spin Hall\nangles of 0.09 and 0.008 respectively. Furthermore, DC-voltages at\nferromagnetic resonance for Py/Pt are studied as a function of temperature and\nthe results are compared to theoretical models.\n

Ferrimagnetic materials have garnered significant attention due to their broad range of tunabilities and functionalities in spintronics applications. Among these materials, rare earth-transition metal GdFeCo alloy films have been the subject of intensive investigation due to their spin-dependent transport properties and strong spin–orbit coupling. In this report, we present self-induced spin-to-charge conversion in single-layer GdFeCo films of different thicknesses via an inverse spin Hall effect. A detailed investigation of spin dynamics was carried out using broadband ferromagnetic resonance measurements. The anisotropy constant and the effective g-factor are found to decrease with thickness, and they become nearly constant for thicknesses beyond 25 nm. A remarkably low damping constant of 0.0029 ± 0.0003 is obtained in the 43 nm-thick film, which is the lowest among all previous reports on GdFeCo thin films. Furthermore, we have demonstrated a self-induced inverse spin Hall effect, which has not been reported so far in a single-layer of GdFeCo thin films. Our analysis shows that the inverse spin Hall effect becomes increasingly dominant over the spin rectification effect with increasing film thickness. The in-plane angular-dependent voltage measurement of the 43 nm-thick film reveals a spin pumping voltage of 1.64 μV. The observation of spin-to-charge current conversion could be due to the high spin–orbit coupling element Gd in the film as well as the interface between GeFeCo/Ti and substrate/GdFeCo of the films. Our findings underscore the potential of GdFeCo as a prime ferrimagnetic material for emerging spintronic technologies.

In order to determine the limits of the biologically safe maximum allowable levels of pulsed terahertz (THz) radiation, a prototype of apparatus has been developed for the irradiation of cell cultures immersed in a liquid medium with broad-band THz radiation in the 0.05–1.2 THz frequency range. The action of THz radiation on the functional activity of the cells of a number of biological cultures, including mouse thymocytes and splenocytes, has been investigated. This was evaluated by means of flow-through cytometry, using fluorescence-intensity analysis of DNA-binding stains. It is shown that THz radiation with a mean power density of up to 10 μW/cm2 and a time of action of 1 min has no effect on the variation of the functional activity of the cells. No statistically reliable variations were detected in the relationship of living cells and cells at different stages of apoptosis or in the distribution of cells over the phases of the cellular cycle. It is concluded that THz radiation with such characteristics can be used for medical spectroscopy.

Applications of semiconductors in spintronics are being developed and revolutionized to fabricate nanostructures in which the spin current is injected, detected and manipulated. The manifestation of the spin Hall effect (SHE) in semiconductors with a simple geometry is still a challenge to implement in spintronics. In this work, we demonstrate the distinct electrical detection of the inverse spin Hall effect (ISHE) in WS2 multilayer crystals at temperatures ranging from 4.2 K to 300 K. The novel idea of this study is the electrical detection of the ISHE in WS2 using an in-plane spin-polarized current, which is realized using non-ferromagnetic electrodes on the top and bottom surfaces of the WS2 channel. The flow alteration of spin-polarized current results in a polarity change in the inverse spin Hall signal. By analyzing the magnitude of the ISHE, we estimated a spin diffusion length of ~250 nm, spin Hall angle of ~0.028, spin polarization of ~0.20 and spin lifetime of ~560 ps in WS2-layered crystals at room temperature. The ISHE in layered, 2D materials, including WS2 crystals, provides a new methodology to develop low-power and fast memory devices to write, read and store information.