基于超快电子自旋动力学的太赫兹辐射研究进展

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.

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.

In this paper, the effects of planar magnetostatic wiggler and s-parameter on the terahertz (THz) radiation generation through rippled plasma have been investigated. Efficient THz radiation generation by photo-mixing of tophat lasers for rippled density plasma in the presence of the wiggler field has been presented. Fundamental equations for the analysis of the non-linear current density and THz radiation generation by wiggler magnetostatic field have been derived. It is shown that for the higher order of the tophat lasers, the values of THz amplitude are greater. In fact, the higher order of the tophat lasers has a sharp gradient in the intensity of lasers, which leads to a stronger nonlinear ponderomotive force and, consequently, a stronger current density. In addition, it is seen that by increasing s-parameter, the normalized transverse profile becomes more focused near the axis of y. Furthermore, it is observed that the normalized laser efficiency has a decreasing trend with increasing normalized THz frequency for different values of the wiggler field. Also, it is shown that by employing a greater order of the tophat lasers and a stronger wiggler field, the efficiency of order of 30% can be achieved. Moreover, it is found that we can control focus and intensity of THz radiation emitted in rippled plasma by choosing the appropriate order of the tophat lasers and tuning of the wiggler field.

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.

Owing to the unique characteristics of terahertz (THz) waves and their interaction with matter, THz imaging has significant potential in diverse fields, including life sciences, non-destructive quality control, and security. However, today’s THz technology faces major obstacles in providing the cost, compactness, and functional scope that is required to facilitate a wide-reaching use outside the laboratory. Therefore, the general aim of this thesis is to increase the societal impact of THz technology by exploring novel THz imaging modalities with integrated circuits (ICs) based on conventional silicon technolgy. Previous research in THz ICs mostly focused on building compact integrated source and detector components for far-field imaging systems that exploit the ability of THz waves to penetrate through diverse dielectric materials. One part of this thesis joins this research and demonstrates low-cost volumetric THz imaging based on computed tomography with silicon components for the first time. To this end, a high-power 0.43 THz source in 0.13 μm silicon-germanium heterojunction bipolar transistor (SiGe HBT) technology is developed in this thesis. However, the resolution of THz far-field imaging is limited by diffraction to the millimeter range, whereas some of the most promising THz applications in life sciences, such as intraoperative imaging of cancerous tissue, are in need for a microscopic resolution to resolve material properties on the cellular level. This thesis presents the realization of microscopic THz imaging with a silicon-based sensor system. The central contribution of this work is the development and analysis of a 128-pixel THz near-field sensor System-on-a-Chip (SoC) operating at 0.55 THz and showing a spatial resolution around 10 μm. The sensor exploits the capacitive near-field interaction between split-ring-resonator probes and imaging objects, giving an imaging contrast based on the dielectric permittivity. To simultaneously enable real-time image acquisition, high sensor sensitivity, and an integration level that is comparable to conventional consumer electronics, the full integration capabilities of a high-speed 0.13 μm SiGe bipolar CMOS (SiGe-BiCMOS) technology are exploited. In particular, the presented SoC employs cointegration of a chip-scale one-dimensional THz sensor front-end, analog signal processing, and digital circuitry for controlling the chip and external communication. The achieved results in terms of imaging speed, system cost, and integration level reach well beyond the state of the art in THz near-field imaging. The ability to rapidly acquire THz images with micrometer-scale resolution will be of benefit to fundamental research into material properties in the THz range and lays the foundation for the exploration of THz bioimaging applications on the cellular level. In particular, the sensor enables the conduction of large-scale clinical studies on the relevance of microscopic THz imaging to ex vivo tumor margin assessment in breast cancer surgeries for the first time.

We report the efficient broadband terahertz (THz) radiation in the metallic ferromagnetic W/CoFeB/Pt trilayer heterostructures, upon irradiation of a femtosecond laser pulse, which can be interpreted by a non-local spin current pulses based on inverse spin Hall effect (ISHE).

Terahertz (THz) radiation sources are increasingly significant for many scientific frontiers, while the generation of THz radiation with high-power at wide-tunable frequencies is still a limitation for most existing methods. In this paper, a compact accelerator-based light source is proposed to produce coherent THz radiation with high pulse energy and tunable frequency from 0.1 THz to 60 THz. By using a frequency beating laser-modulated electron beam and undulator taper, intense coherent THz radiation can be generated through undulators. Theoretical analysis and numerical simulations demonstrate that the proposed technique can generate narrow-bandwidth THz radiation with a pulse energy up to 6.3 millijoule (mJ) and the three-dimensional effects of beam has limited influence on its performance. The proposed technique will open up new opportunities for THz spectroscopic and time-resolved experiments.

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.

This study demonstrates an analytic framework for generating terahertz (THz) radiation through the nonlinear interaction of frequency-varying lasers with a longitudinally nonuniform plasma. When two lasers beat the plasma density gradient at an angle, it imparts an oscillatory velocity to the plasma electrons. Variation in laser amplitude creates a non-linear ponderomotive force that oscillates at the beating frequency of the laser, typically within the THz range. This force drives a non-linear current density in the plasma, with its transverse component contributing to the generation of THz radiation. Using a frequency-varying laser prolongs the interaction time between the laser and the plasma, leading to the production of broadband THz radiation. Optimizing key factors, such as the angle of the laser relative to the plasma, its time-dependent frequency, and its intensity, allows the control of the THz radiation power. These optimized conditions are essential for developing efficient THz sources for various practical applications.

Tera-Hertz (THz) transmission can offer several attractive applications, yet developing low-cost energy-efficient THz devices is at an early stage. The most promising low-cost THz transmitter architecture in the literature is the so-called frequency-multiplier-last architecture but it is incapable of transmitting quadrature amplitude modulation (QAM) due to the architecture's inherent nonlinear distortions. We study such nonlinear THz communication systems by incorporating the nonlinearity aspects of the low- cost THz devices into the signal model. Then we propose novel pre-compensation schemes to compensate the nonlinearity effect, thus enabling QAM-capable frequency-multiplier-last architecture for THz systems. Numerical results show that the proposed pre-compensation schemes overcome the prominent problems experienced in the existing scheme, namely severe nonlinear distortions of the modulation symbols as well as spectral spreading and/or large spectrum sidelobes.

Studies on the generation of terahertz (THz) radiation in p-n-heterostructures based on a-Si:H/c-Si upon their photoexcitation by a femtosecond titanium-sapphire laser with a wavelength of 800 nm are presented. The properties of observed THz radiation allow to explain its nature by excitation of fast photocurrent of nonequilibrium charge carriers created in the region of the potential barrier under femtosecond interband photoexcitation of the structure. The fast photocurrent, in turn, emits THz electromagnetic waves. The waveforms and amplitude spectra of the observed THz radiation reflect the dynamics of photoexcited charge carriers in the structures. The intensity of THz radiation observed in the studied p-n-heterostructures based on a-Si:H/c-Si is comparable to that generated in n-InAs crystals, which are widely used as emitters in systems of THz time-domain spectroscopy. Therefore, a-Si:H/c-Si p-n-heterostructures can be used as THz emitters for need of THz spectroscopy. Keywords: femtosecond laser photoexcitation, heterostructures, fast photocurrent, terahertz electromagnetic radiation.

The paper examines theoretically the generation of terahertz (THz) radiation as a result of wakefields generated by skew-cosh-Gaussian (skew-chG) laser beam propagating in collisional plasma under an undulator magnetic field. Analytical and non-linear equations have been derived to describe the generation of THz radiation by skew-chG laser beam. Using equations of motion, continuity equations, and perturbation technique, the transverse and longitudinal wakefield has also been derived in the presence of an undulator magnetic field. It is shown that the collisional plasma carries a higher longitudinal wakefield amplitude while the undulator amplitudes are lower. In addition, it is found that smaller collision frequencies make larger wakefield amplitudes that generate terahertz radiation. However, when the collision frequency becomes equal to the plasma frequency, the wakefield amplitude is greatly reduced. Moreover, it is observed that the THz field decreases with the increase of the beam waist of the skew-chG laser beam. It is also shown that the THz field exhibits an increasing behavior with increasing s parameter and undulator amplitudes despite the presence of significant collisions.

Ultrafast spin dynamics is the study of the evolution of spin degrees of freedom on a time scale from picoseconds to attoseconds after being excited by an external field. With the development of laser technology, ultrafast spin dynamics has presented new opportunities for realizing ultrafast spintronic devices since 1996. However, despite decades of development, many aspects of femtosecond magnetism remain unclear. Understanding the parameters of these ultrafast spin dynamics processes requires experiments on an even faster timescale. Attosecond magnetism and the interaction of attosecond laser pulses with magnetic materials can reveal spin dynamics on a sub-femtosecond to attosecond time scale. In this review, we first introduce the significant research progress, including the mechanisms of ultrafast demagnetization, all-optical switching, ultrafast spin currents, and terahertz waves. Secondly, we analyze the problems in ultrafast spin dynamics, such as the unclear physical mechanisms of ultrafast demagnetization, the uncertain relationship between magnetic damping and ultrafast demagnetization time, and the unexplored anisotropic ultrafast demagnetization. Thirdly, we discuss the opportunities and challenges in attosecond magnetism. Finally, we analyze and discuss the future development and prospects of ultrafast spin dynamics.

In the present paper, a scheme for generation of terahertz (THz) radiation in electron-neutral collisional plasma based on beating of two Gaussian laser array beams has been proposed. It is shown that the efficiency of THz radiation based on the Gaussian laser array beams can be enhanced drastically in comparison with the efficiency of THz radiation based on the Gaussian one. Furthermore, the producing THz radiation by the Gaussian laser array beams, which has an exclusive field profile, is affected by some array structure parameters. It can also be used to overcome the negative consequences of electron neutral collisions in plasma, which may be occurring in the THz radiation generation process. Optimizing the collisional plasma, laser beams and array structure parameters, THz radiation efficiency up to 0.07% can be obtained in our scheme which is about three times greater than the maximum efficiency obtained for standard (single) Gaussian laser beam. Also, considering the electrostatic energy channel in solving the THz wave equation, and reduction of THz radiation efficiency to 0.054%, in this assumption, the ratio between the efficiency of Gaussian laser array beams and standard Gaussian laser beam remained unchanged.

Second-order nonlinear optical wavelength-conversion has been attractive for generating terahertz (THz) wave with high peak-power and for THz-wave detection with high sensitivity. Over 50 kilowatt peak-power THz-wave radiation and sensitive THz-wave detection down to several tens of atto-joule using LiNbO 3 or 4-dimethylamino- N’ -methyl-4’- stilbazolium tosylate (DAST) crystals have been demonstrated. LiNbO 3 crystal is an eminent nonlinear crystal for converting wavelengths between THz wave and near infrared (NIR) at frequency range from 1 to 3 THz. Mixing THz wave with an intense NIR pump beam in the LiNbO 3 provides generation of a signal light at a different frequency because of efficient figure of merit. Additionally, sensitivity of up-conversion detection working at room temperature is more than that of cryogenically cooled THz detector. Here, we report on a sensitive THz-wave detection based on novel design using a slant-stripe-type periodically poled Mg doped lithium niobate (PPMgLN) for practical use. The efficient scheme that two optical waves, the pump and up-conversion signal beams, propagate collinearly in the PPMgLN to achieve effective parametric amplification for the signal beam was designed. Minimum THz-wave detection was achieved down to energy about 100 aJ at the frequency of 1.6 THz. The result leads to a novel THz detector based on fiber and integrated optics with high sensitivity, robustness, and easy handling. The nonlinear optical up-conversion detection is promising and broadening THz horizons.