Terahertz Electromagnetic Pulses Research Articles

Ultrafast snapshots of terahertz electric potentials across ring-shaped quantum barriers

Abstract Probing the time evolution of the terahertz electric field within subwavelength dimensions plays a crucial role in observing the nanoscale lightwave interactions with fundamental excitations in condensed-matter systems and in artificial structures, such as metamaterials. Here, we propose a novel probing method for measuring terahertz electric potentials across nanogaps using a combination of optical and terahertz pulse excitations. To achieve this, we employ ring-shaped nanogaps that enclose a metallic island, allowing us to capture tunneling charges when subjected to terahertz electromagnetic pulse illumination. By controlling and manipulating the terahertz tunneling charges through a focused optical gate pulse, we can obtain the terahertz potential strength as a function of spatial coordinates and time delays between pulses. To accurately quantify the time evolution of terahertz electric potential across quantum barriers, we carefully calibrate the recorded nonlinear tunneling current. Its on-resonance and off-resonance behaviors are also discussed, providing valuable insights into the antenna’s characteristics and performance.

Terahertz Néel spin-orbit torques drive nonlinear magnon dynamics in antiferromagnetic Mn2Au

Antiferromagnets have large potential for ultrafast coherent switching of magnetic order with minimum heat dissipation. In materials such as Mn2Au and CuMnAs, electric rather than magnetic fields may control antiferromagnetic order by Néel spin-orbit torques (NSOTs). However, these torques have not yet been observed on ultrafast time scales. Here, we excite Mn2Au thin films with phase-locked single-cycle terahertz electromagnetic pulses and monitor the spin response with femtosecond magneto-optic probes. We observe signals whose symmetry, dynamics, terahertz-field scaling and dependence on sample structure are fully consistent with a uniform in-plane antiferromagnetic magnon driven by field-like terahertz NSOTs with a torkance of (150 ± 50) cm2 A−1 s−1. At incident terahertz electric fields above 500 kV cm−1, we find pronounced nonlinear dynamics with massive Néel-vector deflections by as much as 30°. Our data are in excellent agreement with a micromagnetic model. It indicates that fully coherent Néel-vector switching by 90° within 1 ps is within close reach.

Ultrafast and terahertz spintronics: Guest editorial

Spin-based electronics (spintronics) aims at extending electronic functionalities, which rely on the electron charge as information carrier, by the spin of the electron. To make spintronics competitive and compatible with other information carriers like photons and electrons, their speed needs to be pushed to femtosecond time scales and, thus, terahertz frequencies. In ultrafast and terahertz spintronics, femtosecond optical and terahertz electromagnetic pulses are used to induce spin torque and spin transport and to monitor the subsequent time evolution. The two approaches, sometimes referred to as femto-magnetism and terahertz magnetism, have provided new, surprising, and relevant insight as well as applications for spintronics. Examples include the ultrafast optical switching of magnetic order and the generation of broadband terahertz electromagnetic fields. This APL Special Topic Collection is dedicated to provide a platform for the newest developments and future trends in the very active, dynamic, and exciting research field of ultrafast and terahertz spintronics.

Strong-field THz radiation-induced curing of composite resin materials in dentistry.

Dental caries is listed as one of the three major non-communicable diseases by the World Health Organization, and its main treatment method is to restore it by filling it with resin. At present, the visible light-cure method has the problems of non-uniform curing and low penetration efficiency, which makes the bonding area easy to develop marginal leakages, thus leading to secondary caries and requiring repeated treatment. In this work, through the strong terahertz (THz) irradiation-weak THz detection technique, it is found that the strong THz electromagnetic pulses can accelerate the curing process of the resin, and the weak-field THz spectroscopy can be used to monitor this dynamic change in real time, which will greatly promote the potential application of THz technology in dentistry.

High-order nonlinear terahertz probing of the two-band superconductor MgB2 : Third- and fifth-order harmonic generation

We report on high-order harmonic generation in the two-band superconductor ${\mathrm{MgB}}_{2}$ driven by intense terahertz electromagnetic pulses. Third- and fifth-order harmonics are resolved in time domain and investigated as a function of temperature and in applied magnetic fields crossing the superconducting phase boundary. The high-order harmonics in the superconducting phase reflects nonequilibrium dynamics of the superconducting order parameter in ${\mathrm{MgB}}_{2}$, which is probed via nonlinear coupling to the terahertz field. The observed temperature and field dependence of the nonlinear response allows to establish the superconducting phase diagram.

Ultrafast light-induced THz switching in exchange-biased Fe/Pt spintronic heterostructure

The ultrafast optical control of magnetization in spintronic structures enables one to access to the high-speed information processing, approaching the realm of terahertz (THz). Femtosecond visible/near-infrared laser-driven ferromagnetic/nonmagnetic metallic spintronic heterostructures-based THz emitters combine the aspects from the ultrafast photo-induced dynamics and spin-charge inter-conversion mechanisms through the generation of THz electromagnetic pulses. In this Letter, we demonstrate photoexcitation density-dependent induced exchange-bias tunability and THz switching in an annealed Fe/Pt thin-film heterostructure, which otherwise is a widely used conventional spintronic THz emitter. By combining the exchange-bias effect along with THz emission, the photo-induced THz switching is observed without any applied magnetic field. These results pave the way for an all-optical ultrafast mechanism to exchange-bias tuning in spintronic devices for high-density storage, read/write magnetic memory applications.

Terahertz-wave decoding of femtosecond extreme-ultraviolet light pulses

In recent years, femtosecond extreme-ultraviolet (XUV) and x-ray pulses from free-electron lasers have developed into important probes to monitor processes and dynamics in matter on femtosecond-time and angstrom-length scales. With the rapid progress of versatile ultrafast x-ray spectroscopy techniques and more sophisticated data analysis tools, accurate single-pulse information on the arrival time, duration, and shape of the probing x-ray and XUV pulses becomes essential. Here, we demonstrate that XUV pulses can be converted into terahertz electromagnetic pulses using a spintronic terahertz emitter. We observe that the duration, arrival time, and energy of each individual XUV pulse is encoded in the waveform of the associated terahertz pulses, and thus can be readily deduced from single-shot terahertz time-domain detection.

Terahertz-field-induced second optical harmonic generation from Si(111) surface

We study, both experimentally and theoretically, the second-harmonic (SH) generation (SHG) of optical radiation assisted by intense terahertz electromagnetic pulses from the surface of high-resistivity Si(111). The study was performed within the framework of the analysis of the superposition of different SH sources, including the surface-dipole, bulk-quadruple, and electric-field-induced (EFI) bulk-dipole (arising from internal built-in and external terahertz fields) contributions. Azimuthal rotation anisotropy of SHG for an arbitrary mutual polarization of the terahertz, fundamental, and SH optical fields was calculated and experimentally verified. The strongest impact of the terahertz field on SHG was observed in the case where the polarizations of the terahertz and SH fields coincide. In these polarization configurations, the terahertz field added mainly an isotropic term to the SH azimuthal dependences. It has been demonstrated that the application of the terahertz field can be helpful for distinguishing the SHG mechanism. For the sample under study, the contribution to the SH energy from the built-in field-induced source was found to be >4 times greater than the contribution from the surface and quadrupole nonlinear sources. The dominance of the EFISHG allowed us to find the ratio between the components of a silicon third-order nonlinear tensor ${\ensuremath{\chi}}_{}^{(3)}(2\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},0)$: ${\ensuremath{\chi}}_{1122}={\ensuremath{\chi}}_{1212}\ensuremath{\approx}0.51{\ensuremath{\chi}}_{1111}$ and ${\ensuremath{\chi}}_{1212}\ensuremath{\approx}{\ensuremath{\chi}}_{1221}$ for the fundamental optical wavelength 790 nm.

Low-Loss Polytetrafluoroethylene Hexagonal Porous Fiber for Terahertz Pulse Transmission in the 6G Mobile Communication Window

Hexagonal porous optical fiber without a central defect is proposed and numerically analyzed with the finite element method (FEM) for transmitting terahertz (THz) electromagnetic wave pulse. In experiments, the transmission characteristics of polytetrafluoroethylene (PTFE) hexagonal porous optical fibers were measured using a THz time-domain spectroscopy (THz-TDS) system. To precisely estimate the effective material loss (EML), we measured the refractive index and absorption coefficient of PTFE in the THz range to use them in FEM analyses, and the EML of the porous fiber was estimated to be lower than that of a bulk rod as large as by a factor of 2 in the frequency range from 0.1 to 0.33 THz. In experiments, we measured the transmission characteristics of both the porous fibers and the bulk rod, to confirm a significant improvement in THz wave transmission nearly by an order of magnitude in the 6G telecommunication window, showing a better performance than theoretical estimations.

Temperature‐Dependent Terahertz Emission from Co/Mn2Au Spintronic Bilayers

Recently, ferromagnetic/nonmagnetic heavy metal heterostructures have been intensively investigated as terahertz (THz) emitters. The interconversion of spin‐to‐charge dynamics plays a central role for efficient emission of THz electromagnetic pulses. However, a direct observation of spin–charge interconversion in antiferromagnetic (AFM) materials occurring on the sub‐picosecond time scale remains a challenge. Herein, the magnetic‐field‐, pump‐fluence‐, and polarization‐dependent THz emission behaviors by a femtosecond optical pump in cobalt (Co)/Mn2Au nanometer heterostructure are experimentally investigated. The Co/Mn2Au bilayer generates sizable THz signals, whereas the Mn2Au/Pt bilayer does not show any THz emission. In addition, the thickness‐ and temperature‐dependent THz emission measurements indicate a direct relation between the THz amplitude and the conductivity of AFM Mn2Au layer. The results obtained will not only promote the fundamental understanding of ultrafast spin–charge interconversion in Co/Mn2Au heterostructures, but also provide a possibility of spectroscopic‐based spin current detector at THz‐frequency range.

Modulating the polarization of broadband terahertz pulses from a spintronic emitter at rates up to 10 kHz

Reliable modulation of terahertz electromagnetic waveforms is important for many applications. Here, we rapidly modulate the direction of the electric field of linearly polarized terahertz electromagnetic pulses with 1–30 THz bandwidth by applying time-dependent magnetic fields to a spintronic terahertz emitter. Polarity modulation of the terahertz field with more than 99% contrast at a rate of 10 kHz is achieved using a harmonic magnetic field. By adding a static magnetic field, we modulate the direction of the terahertz field between angles of, for instance, −53° and 53° at kilohertz rates. We believe our approach makes spintronic terahertz emitters a promising source for low-noise modulation spectroscopy and polarization-sensitive techniques such as ellipsometry at 1–30 THz.

On the progress of ultrafast time-resolved THz scanning tunneling microscopy

Scanning tunneling microscopy combined with terahertz (THz) electromagnetic pulses and its related technologies have developed remarkably. This technology has atomic-level spatial resolution in an ultrahigh vacuum and low-temperature environment, and it measures the electrical dynamical behavior of a sample’s surface with femtosecond temporal resolution. In particular, it has been used to image the diffusion and relaxation dynamics of electrons in real time and real space and even instantaneously control molecular motions. In this Perspective, we focus on recent progress in research and development of ultrafast time-resolved THz scanning tunneling microscopy and its application to materials research.

Broadband Terahertz Probes of Anisotropic Magnetoresistance Disentangle Extrinsic and Intrinsic Contributions

Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of magnetic order in contemporary spintronics research. Its origins are usually ascribed to extrinsic effects (i.e. spin-dependent electron scattering), whereas intrinsic (i.e. scattering-independent) contributions are neglected. Here, we measure AMR of polycrystalline thin films of the standard ferromagnets Co, Ni, Ni81Fe19 and Ni50Fe50 over the frequency range from DC to 28 THz. The large bandwidth covers the regimes of both diffusive and ballistic intraband electron transport and, thus, allows us to separate extrinsic and intrinsic AMR components. Analysis of the THz response based on Boltzmann transport theory reveals that the AMR of the Ni, Ni81Fe19 and Ni50Fe50 samples is of predominantly extrinsic nature. However, the Co thin film exhibits a sizeable intrinsic AMR contribution, which is constant up to 28 THz and amounts to more than 2/3 of the DC AMR contrast of 1%. These features are attributed to the hexagonal structure of the Co crystallites. They are interesting for applications in terahertz spintronics and terahertz photonics. Our results show that broadband terahertz electromagnetic pulses provide new and contact-free insights into magneto-transport phenomena of standard magnetic thin films on ultrafast time scales.

Nondestructive measurement of mill-scale thickness on steel by terahertz time-of-flight tomography

We measure in a nondestructive and noncontact fashion the thicknesses of three scale films with thicknesses 28.5 ± 1.4 μm, 13.4± 0.9 μm, and 5.1 ± 0.3 μm on steel substrates employing terahertz time-of-flight tomography combined with advanced signal-processing techniques. Wüstite is the dominant phase in the scale films, though magnetite and hematite are also present. Because wüstite is electrically insulating, the incident terahertz electromagnetic pulses largely penetrate into the scale film; however, the pulses are entirely reflected by the underlying electrically conductive steel substrate. Because the film layers are thin, in some cases optically thin, the distinct pulses reflected at the air/scale and scale/steel interfaces overlap in time and thus are not visually evident in the reflected terahertz signal, necessitating the use of deconvolution techniques to recover the sample structure. We compare the merits of three deconvolution techniques, one unsuccessful (frequency-wavelet domain deconvolution) and two successful (sparse deconvolution and autoregressive extrapolation), to characterize the thicknesses of these scale films.

Temporal and spectral fingerprints of ultrafast all-coherent spin switching.

Future information technology demands ever-faster, low-loss quantum control. Intense light fields have facilitated milestones along this way, including the induction of novel states of matter1-3, ballistic acceleration of electrons4-7 and coherent flipping of the valley pseudospin8. These dynamics leave unique 'fingerprints', such as characteristic bandgaps or high-order harmonic radiation. The fastest and least dissipative way of switching the technologically most important quantum attribute-the spin-between two states separated by a potential barrier is to trigger an all-coherent precession. Experimental and theoretical studies withpicosecond electric and magnetic fields have suggested this possibility9-11, yet observing the actual spindynamics has remained out of reach. Here we show that terahertz electromagnetic pulses allow coherent steering of spins over a potential barrier, and we report the corresponding temporal and spectral fingerprints. This goal is achieved by coupling spins in antiferromagnetic TmFeO3 (thulium orthoferrite) with the locally enhanced terahertz electric field of custom-tailored antennas. Within their duration of one picosecond, the intense terahertz pulses abruptly change the magnetic anisotropy and trigger a large-amplitude ballistic spin motion. A characteristic phase flip, an asymmetric splitting of the collective spinresonance and a long-lived offset of the Faraday signal are hallmarks of coherent spin switching into adjacent potential minima, in agreement with numerical simulations. The switchable states can be selected by an external magnetic bias. The low dissipation and the antenna's subwavelength spatial definition could facilitate scalable spin devices operating at terahertz rates.

Excitation of Nonlinear Spin Oscillations in an Antiferromagnet under the Action of Pulsed Terahertz Pumping

We present a model of excitation of nonlinear spin oscillations in an antiferromagnet under the action of terahertz pulses of an electromagnetic field. It is shown that, with increasing pumping pulse amplitude, the spin system response increases linearly in the fundamental (resonant) quasi-ferromagnetic mode and quadratically in the second harmonic. We have compared the theoretically obtained dependences of the intensities of the sample response amplitudes for the fundamental mode and the second harmonic with the experimental ones. The comparison demonstrates the applicability of the developed method of analyzing dynamic processes in antiferromagnets occurring under the action of terahertz electromagnetic pulses.

Second order nonlinear optical processes in [111] cubic crystals for terahertz optoelectronics

In this paper, we give an overview of emission and detection of terahertz electromagnetic pulses using, respectively, optical rectification and electro-optic effect in [111] zinc blende crystals. This crystal orientation allows us to generate and read any polarization state of the THz beam only by controlling the polarization state of the laser beam that excites or probes the emitting and receiving crystals. This technique is very useful for polarimetric terahertz spectroscopic studies.

Возбуждение нелинейных спиновых колебаний в антиферромагнетике под действием терагерцевых импульсов накачки

AbstractWe present a model of excitation of nonlinear spin oscillations in an antiferromagnet under the action of terahertz pulses of an electromagnetic field. It is shown that, with increasing pumping pulse amplitude, the spin system response increases linearly in the fundamental (resonant) quasi-ferromagnetic mode and quadratically in the second harmonic. We have compared the theoretically obtained dependences of the intensities of the sample response amplitudes for the fundamental mode and the second harmonic with the experimental ones. The comparison demonstrates the applicability of the developed method of analyzing dynamic processes in antiferromagnets occurring under the action of terahertz electromagnetic pulses.

Ultrafast Zero-Bias Photocurrent in GeS Nanosheets: Promise for Photovoltaics

Ferroelectric semiconductors have been predicted to exhibit strong zero-bias shift current, spurring the search for ferroelectric semiconductors with band gaps in the visible range as candidates for so-called shift current photovoltaics with efficiencies not constrained by the Schockley–Queisser limit. Recent theoretical works have predicted that two-dimensional IV–VI monochalcogenides are multiferroic and capable of generating significant shift currents. Here we present experimental validation of this prediction, observing ultrafast shift currents by detecting terahertz electromagnetic pulses emitted by the photoexcited GeS nanosheets without external bias. We explore excitation fluence, orientation, and excitation polarization dependence of the terahertz emission to confirm that shift currents are indeed responsible for the observed emission. Experimental observation of zero-bias photocurrents puts GeS nanosheets forth as a promising candidate material for applications in third-generation photovoltaics ...

Terahertz-field-induced optical birefringence in common window and substrate materials.

We apply intense terahertz (THz) electromagnetic pulses with field strengths exceeding 2 MV cm(-1) at ~1 THz to window and substrate materials commonly used in THz spectroscopy and determine the induced optical birefringence. Materials studied are diamond, sapphire, magnesium oxide (MgO), polymethylpentene (TPX), low-density polyethylene (LDPE), silicon nitride membrane (SiN) and crystalline quartz. We observe a Kerr-effect-type transient birefringence in all samples, except in quartz and Si, where, respectively, a linear electrooptic signal and a response beyond the perturbative regime are found. We extract the nonlinear refractive indices and the electrooptic coefficient (in the case of quartz) of these materials and discuss implications for their use as windows or substrates in THz pump-optical probe spectroscopy.