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Antiferromagnets have large potential for ultrafast coherent switching of magnetic order with minimum heat dissipation. In novel materials such as Mn$_2$Au and CuMnAs, electric rather than magnetic fields may control antiferromagnetic order by N\'eel spin-orbit torques (NSOTs), which have, however, not been observed on ultrafast time scales yet. Here, we excite Mn$_2$Au 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$\pm$50) cm$^2$/A s. At incident terahertz electric fields above 500 kV/cm, we find pronounced nonlinear dynamics with massive N\'eel-vector deflections by as much as 30{\deg}. Our data are in excellent agreement with a micromagnetic model which indicates that fully coherent N\'eel-vector switching by 90{\deg} within 1 ps is within close reach., Comment: 16 pages, 4 figures

Spintronic terahertz emitters promise terahertz sources with an unmatched broad frequency bandwidth that are easy to fabricate and operate, and therefore easy to scale at low cost. However, current experiments and proofs of concept rely on free-space ultrafast pump lasers and rather complex benchtop setups. This contrasts with the requirements of widespread industrial applications, where robust, compact, and safe designs are needed. To meet these requirements, we present a novel fiber-tip spintronic terahertz emitter solution that allows spintronic terahertz systems to be fully fiber-coupled. Using single-mode fiber waveguiding, the newly developed solution naturally leads to a simple and straightforward terahertz near-field imaging system with a 90%-10% knife-edge-response spatial resolution of 30 ${\mu}m$.

Ferromagnet/heavy metal bilayers represent a central building block for spintronic devices where the magnetization of the ferromagnet can be controlled by spin currents generated in the heavy metal. The efficiency of spin current generation is paramount. Equally important is the efficient transfer of this spin current across the ferromagnet/heavy metal interface. Here, we show theoretically and experimentally that for Ta as heavy metal the interface only partially transmits the spin current while this effect is absent when Pt is used as heavy metal. This is due to magnetic moment reduction at the interface caused by 3d–5d hybridization effects. We show that this effect can be avoided by atomically thin interlayers. On the basis of our theoretical model we conclude that this is a general effect and occurs for all 5d metals with less than half-filled 5d shell.

Lead halide perovskites (LHPs) have emerged as an excellent class of semiconductors for next-generation solar cells and optoelectronic devices. Tailoring physical properties by fine-tuning the lattice structures has been explored in these materials by chemical composition or morphology. Nevertheless, its dynamic counterpart, phonon-driven ultrafast material control, as contemporarily harnessed for oxide perovskites, has not been established yet. Here we employ intense THz electric fields to obtain direct lattice control via nonlinear excitation of coherent octahedral twist modes in hybrid CH3NH3PbBr3 and all-inorganic CsPbBr3 perovskites. These Raman-active phonons at 0.9 - 1.3 THz are found to govern the ultrafast THz-induced Kerr effect in the low-temperature orthorhombic phase and thus dominate the phonon-modulated polarizability with potential implications for dynamic charge carrier screening beyond the Froehlich polaron. Our work opens the door to selective control of LHP's vibrational degrees of freedom governing phase transitions and dynamic disorder., Comment: 11 pages, 5 figures

The emerging field of orbitronics exploits the electron orbital momentum $\textit{L}$. Compared to spin-polarized electrons, $\textit{L}$ may allow magnetic-information transfera with significantly higher density over longer distances in more materials. However, direct experimental observation of $\textit{L}$ currents, their extended propagation lengths and their conversion into charge currents has remained challenging. Here, we optically trigger ultrafast angular-momentum transport in Ni|W|SiO$_2$ thin-film stacks. The resulting terahertz charge-current bursts exhibit a marked delay and width that grow linearly with W thickness. We consistently ascribe these observations to a ballistic $\textit{L}$ current from Ni through W with giant decay length (~80 nm) and low velocity (~0.1 nm/fs). At the W/SiO$_2$ interface, the $\textit{L}$ flow is efficiently converted into a charge current by the inverse orbital Rashba-Edelstein effect, consistent with ab-initio calculations. Our findings establish orbitronic materials with long-distance ballistic $\textit{L}$ transport as possible candidates for future ultrafast devices and an approach to discriminate Hall- and Rashba-Edelstein-like conversion processes.

Antiferromagnets have large potential for ultrafast coherent switching of magnetic order with minimum heat dissipation. In novel materials such as Mn$_2$Au and CuMnAs, electric rather than magnetic fields may control antiferromagnetic order by Néel spin-orbit torques (NSOTs), which have, however, not been observed on ultrafast time scales yet. Here, we excite Mn$_2$Au 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$\pm$50) cm$^2$/A s. At incident terahertz electric fields above 500 kV/cm, 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 which indicates that fully coherent Néel-vector switching by 90° within 1 ps is within close reach., 16 pages, 4 figures

Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz-∼30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a ‘snapshot’ introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation.

Transition-metal dichalcogenides (TMDCs) are an aspiring class of materials with unique electronic and optical properties and potential applications in spin-based electronics. Here, we use terahertz emission spectroscopy to study spin-to-charge current conversion (S2C) in the TMDC NbSe$_2$ in ultra-high-vacuum-grown F|NbSe$_2$ thin-film stacks, where F is a layer of ferromagnetic Fe or Ni. Ultrafast laser excitation triggers an ultrafast spin current that is converted into an in-plane charge current and, thus, a measurable THz electromagnetic pulse. The THz signal amplitude as a function of the NbSe$_2$ thickness shows that the measured signals are fully consistent with an ultrafast optically driven injection of an in-plane-polarized spin current into NbSe$_2$. Modeling of the spin-current dynamics reveals that a sizable fraction of the total S2C originates from the bulk of NbSe$_2$ with the same, negative, sign as the spin Hall angle of pure Nb. By quantitative comparison of the emitted THz radiation from F|NbSe$_2$ to F|Pt reference samples and the results of ab-initio calculations, we estimate that the spin Hall angle of NbSe$_2$ for an in-plane polarized spin current lies between -0.2% and -1.1%, while the THz spin-current relaxation length is of the order of a few nanometers.

An important vision of modern magnetic research is to use antiferromagnets as controllable and active ultrafast components in spintronic devices. Hematite is a promising model material in this respect because its pronounced Dzyaloshinskii-Moriya interaction leads to the coexistence of antiferromagnetism and weak ferromagnetism. Here, we use femtosecond laser pulses to drive terahertz spin currents from hematite into an adjacent Pt layer. We find two contributions to the generation of the spin current with distinctly different dynamics: the impulsive stimulated Raman scatting that relies on the antiferromagnetic order and the ultrafast spin Seebeck effect that relies on the net magnetization. The total THz spin current dynamics can thus be manipulated by a medium-strength magnetic field. The controllability of the THz spin current achieved in hematite opens the pathway toward controlling the exact spin current dynamics from ultrafast antiferromagnetic spin sources.

Transition-metal dichalcogenides (TMDCs) are an aspiring class of materials with unique electronic and optical properties and potential applications in spin-based electronics. Here, terahertz emission spectroscopy is used to study spin-to-charge current conversion (S2C) in the TMDC NbSe2 in ultra-high-vacuum-grown F|NbSe2 thin-film stacks, where F is a layer of ferromagnetic Fe or Ni. Ultrafast laser excitation triggers an ultrafast spin current that is converted into an in-plane charge current and, thus, a measurable THz electromagnetic pulse. The THz signal amplitude as a function of the NbSe2 thickness shows that the measured signals are fully consistent with an ultrafast optically driven injection of an in-plane-polarized spin current into NbSe2. Modeling of the spin-current dynamics reveals that a sizable fraction of the total S2C originates from the bulk of NbSe2 with the opposite, negative sign of the spin Hall angle as compared to Pt. By a quantitative comparison of the emitted THz radiation from F|NbSe2 to F|Pt reference samples and the results of ab initio calculations, it is estimated that the spin Hall angle of NbSe2 for an in-plane polarized spin current lies between -0.2% and -1.1%, while the THz spin-current relaxation length is of the order of a few nanometers.

Laser-induced terahertz spin transport (TST) and ultrafast demagnetization (UDM) are central but so far disconnected phenomena in femtomagnetism and terahertz spintronics. Here, we use broadband terahertz emission spectroscopy to reliably measure both processes in one setup. We find that the rate of UDM of a single ferromagnetic metal film F has the same time evolution as the flux of TST from F into an adjacent normal-metal layer N. This remarkable agreement shows that UDM and TST are driven by the same force, which is fully determined by the state of the ferromagnet. An analytical model consistently and quantitatively explains our observations. It reveals that both UDM in F and TST in the F|N stack arise from a generalized spin voltage, which is defined for arbitrary, nonthermal electron distributions. We also conclude that contributions due to a possible temperature difference between F and N are minor and that the spin-current amplitude can, in principle, be increased by one order of magnitude. In general, our findings allow one to apply the vast knowledge of UDM to TST, thereby opening up new pathways toward large-amplitude terahertz spin currents and, thus, energy-efficient ultrafast spintronic devices., Comment: 23 pages

Metallic spintronic THz emitters have become well-established for offering ultra-broadband, gap-less THz emission in a variety of excitation regimes, in combination with reliable fabrication and excellent scalability. However, so far, their potential for high-average-power excitation to reach strong THz fields at high repetition rates has not been thoroughly investigated. In this article, we explore the power scaling behavior of tri-layer spintronic emitters using an Yb-fiber excitation source, delivering an average power of 18.5 W at 400 kHz repetition rate, temporally compressed to a pulse duration of 27 fs. We confirm that the reflection geometry with back-side cooling is ideally suited for these emitters in the high-average-power excitation regime. In order to understand limiting mechanisms, we disentangle the effects on THz power generation by average power and pulse energy, by varying the repetition rate of the laser. Our results show that the conversion efficiency remains mostly dependent on the incident fluence in this high-average-power, high-repetition-rate excitation regime if the emitters are efficiently cooled. Using these findings, we optimize the conversion efficiency to reach 5e-6 at highest excitation powers in the back-cooled reflection geometry. Our findings provide guidelines for scaling the power of THz radiation emitted by spintronic emitters to the mW-level by using state-of-the-art femtosecond sources with multi-hundred-Watt average power to reach ultra-broadband, strong-field THz sources with high repetition rate., The following article has been submitted to Optics Express. 18 pages, 8 figures

he 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 phenomena at terahertz (THz) frequencies. Here, it is revealed 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: DyCo5, Co32Fe68, and Gd27Fe73. The frequency-dependent conductivity-tensor elements ?xx and ?yx?are?measured, and good agreement with DC measurements is found. The experimental findings are fully consistent with ab initio calculations of ?yx for CoFe and highlight the role of the large Drude scattering rate (?100 THz) of metal thin films, which smears out any sharp spectral features of the THz AHE. Finally, it is found that the intrinsic contribution to the THz AHE dominates over the extrinsic mechanisms for the Co32Fe68 sample. The results imply that the AHE and related effects such as the spin Hall effect are highly promising ingredients of future THz spintronic devices reliably operating from DC to 40 THz and beyond.

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-speedspintronic 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: DyCo5, Co32Fe68 and Gd27Fe73. We measure the frequency-dependent conductivity-tensor elements σxx and σyx and find good agreement with DCmeasurements. The experimental findings are fully consistent with ab-initio calculations of σyx for CoFe and highlight the role of the large Drude scattering rate (~100 THz) of metal thin films, which smears out any sharp spectral features of the THz AHE. Finally, we find that the intrinsic contribution to the THz AHE dominates over the extrinsic mechanisms for our samples. Our results imply that the AHE and related effects such as the spinHall effect are highly promising ingredients of future THz spintronic devices reliably operating from DC to 40 THz and beyond.

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 phenomena at terahertz (THz) frequencies. Here, it is revealed 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: DyCo5, Co32Fe68, and Gd27Fe73. The frequency‐dependent conductivity‐tensor elements σxx and σyx are measured, and good agreement with DC measurements is found. The experimental findings are fully consistent with ab initio calculations of σyx for CoFe and highlight the role of the large Drude scattering rate (≈100 THz) of metal thin films, which smears out any sharp spectral features of the THz AHE. Finally, it is found that the intrinsic contribution to the THz AHE dominates over the extrinsic mechanisms for the Co32Fe68 sample. The results imply that the AHE and related effects such as the spin Hall effect are highly promising ingredients of future THz spintronic devices reliably operating from DC to 40 THz and beyond.

We show scalable and complete suppression of the recently reported terahertz-pulse-induced switching between different resistance states of antiferromagnetic CuMnAs thin films by ultrafast gating. The gating functionality is achieved by an optically generated transiently conductive parallel channel in the semiconducting substrate underneath the metallic layer. The photocarrier lifetime determines the timescale of the suppression. As we do not observe a direct impact of the optical pulse on the state of CuMnAs, all observed effects are primarily mediated by the substrate. The sample region of suppressed resistance switching is given by the optical spot size, thereby making our scheme potentially applicable for transient low-power masking of structured areas with feature sizes of about 200 nm and even smaller.

We investigate the THz emission characteristics of ferromagnetic/non-magnetic metallic heterostructures, focusing on thin Fe/Pt bilayers. In particular, we report on the impact of optimized crystal growth of the epitaxial Fe layers on the THz emission amplitude and spectral bandwidth. We demonstrate an enhancement of the emitted intensity along with an expansion of the emission bandwidth. Both are related to reduced spin scattering and higher interface transmission. Our work provides a pathway for devicing optimal spintronic THz emitters based on epitaxial Fe. It also highlights how THz emission measurements can be utilized to characterize the changes in out-of-equilibrium spin current dynamics in metallic heterostructures, driven by subtle structural refinement., 5 pages, 3 figures Complementary, new verification measurements, did not confirm in the same way the reported results in this work

We measure the inverse spin Hall effect of Cu$_{1-x}$Ir$_{x}$ thin films on yttrium iron garnet over a wide range of Ir concentrations ($0.05 \leqslant x \leqslant 0.7$). Spin currents are triggered through the spin Seebeck effect, either by a DC temperature gradient or by ultrafast optical heating of the metal layer. The spin Hall current is detected by, respectively, electrical contacts or measurement of the emitted THz radiation. With both approaches, we reveal the same Ir concentration dependence that follows a novel complex, non-monotonous behavior as compared to previous studies. For small Ir concentrations a signal minimum is observed, while a pronounced maximum appears near the equiatomic composition. We identify this behavior as originating from the interplay of different spin Hall mechanisms as well as a concentration-dependent variation of the integrated spin current density in Cu$_{1-x}$Ir$_{x}$. The coinciding results obtained for DC and ultrafast stimuli show that the studied material allows for efficient spin-to-charge conversion even on ultrafast timescales, thus enabling a transfer of established spintronic measurement schemes into the terahertz regime., 12 pages, 4 figures

Optically heterodyned detected terahertz Kerr effect of gases of polar molecules is reported. Strikingly, the birefringence signal from fluoroform is found to have opposite polarity compared to water and acetonitrile. This behavior is a hallmark of the opposite sign of the polarizability anisotropy of these molecules. Resonant excitation of the rotational degrees of freedom of the molecules aligns their permanent dipoles along the terahertz electric field. This alignment is translated into an optical birefringence through the polarizability anisotropy of each molecule. Therefore, the resulting net signal scales with the polarizability anisotropy, whose sign is imprinted faithfully onto the transient birefringence signal.

Formation of local molecular structures in liquid water is believed to have marked effect on the bulk properties of water, however, resolving such structural motives in an experiment is challenging. This challenge might be handled if the relevant low-frequency structural motion of the liquid is directly driven with an intense electromagnetic pulse. Here, we resonantly excite diffusive reorientational motions in water with intense terahertz pulses and measure the resulting transient optical birefringence. The observed response is shown to arise from a particular configuration, namely the restricted trans-lational motion of water molecules whose motions are predominantly orthogonal to the dipole moment of the excited neighboring water molecules. Accordingly, we estimate the strength of the anharmonic coupling between the rotational and the restricted translational degrees of freedom of water.

Topological insulators constitute a new and fascinating class of matter with insulating bulk yet metallic surfaces that host highly mobile charge carriers with spin-momentum locking. Remarkably, the direction and magnitude of surface currents can be controlled with tailored light beams, but the underlying mechanisms are not yet well understood. To directly resolve the "birth" of such photocurrents we need to boost the time resolution to the scale of elementary scattering events ($\sim$ 10 fs). Here, we excite and measure photocurrents in the three-dimensional model topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$ with a time resolution as short as 20 fs by sampling the concomitantly emitted broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast surface current response is dominated by a charge transfer along the Se-Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to have orders of magnitude smaller magnitude than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are also of direct relevance for optoelectronic devices based on topological-insulator surface currents.

The ability to actively tune the properties of a nanocavity is crucial for future applications in photonics and quantum information. Two important man-made classes of materials have emerged to mold the flow of electromagnetic waves. Firstly, photonic crystals are dielectric nanostructures that can be used to confine and slow down light and control its emission. They act primarily on the electric component of the light field. More recently, a novel class of metallo-dielectric nanostructures has emerged. These so-called metamaterials enable fascinating phenomena, such as negative refraction, super-focusing and cloaking. This second class of materials realizes light control through effective interactions with both electric and magnetic component. In this work, we combine both concepts to gain an active and reversible control of light trapping on subwavelength length scales. By actuating a nanoscale magnetic coil close to a photonic crystal nanocavity, we interact with the rapidly varying magnetic field and accomplish an unprecedented control of the optical properties of the cavity. We achieve a reversible enhancement of the lifetime of photons in the cavity. By successfully combining photonic crystal and metamaterials concepts, our results open the way for new light control strategies based on interactions which include the magnetic component of light.

This thesis deals with the dynamics of charge carriers in optically excited solids and gases. The manner in which the excited electrons relax their energy and their average velocity is of particular importance in terms of technical applications. For example, graphite and, in particular, carbon nanotubes are potential alternatives to traditional semiconductors as base material for smaller and faster electronic devices. Therefore, electrons in these materials should relax their energy as slowly as possible and maintain an electronic current as long as possible when the driving electric field is switched off. The opposite behavior is required for insulating gases in high-voltage devices: Quasifree electrons should decay quickly and exhibit a low mobility in order to prevent short-circuits. The conductivity of a material is determined by the electrons with the lowest excitation energies, for instance the electrons around the Fermi energy of a metal. Therefore, THz radiation is particularly sensitive to these charge carriers due to its very low photon energy. In this thesis, time-resolved THz spectroscopy is the method of choice in which an ultrashort visible laser pulse excites the charge carriers in the sample. After a variable temporal delay, they are probed by a THz pulse. Two THz spectrometers were built in the course of this work. One of them is driven by a laser oscillator delivering 10-fs laser pulses with a 10-nJ pulse energy. It is used for the measurements of graphite and carbon nanotubes. However, the optical ionization of gas molecules requires much higher intensities and therefore another spectrometer driven by 20-fs laser pulses with a 1-mJ pulse energy. The THz pulses obtained have a duration of about 100fs and cover the spectral range from 10 to 25THz. It should be emphasized that the electric field of the THz pulse is detected which allows to determine the instantaneous dielectric function of the excited sample. The extraction of the dielectric function in a pump-probe experiment has to take the THz propagation through the excited sample into account. Corresponding relations have been derived in this work. In the semimetal graphite, the analysis of the transient dielectric function is based on linear-response theory and yields the temporal evolution of 3 important observables of the excited system: The electronic temperature, the plasma frequency, and the Drude scattering rate. Our data exhibit a twofold dynamics: Within the first 500fs, the electrons thermalize and lose more than 90% of their initial excess energy. This is an amazing result since the small Fermi surface of graphite implies that only ~1% of all phonon modes can directly dissipate the electronic heat. Our simulations based on the 2-temperature model reveal that strongly coupled optical phonons with quantum energies of up to 0.2eV dominate the energy transfer until they have heated up. The remaining slow decay of the electronic temperature is due to the cooling of these hot phonons by other, cold vibrational modes with a 5.4-ps time constant. In addition, the Drude collision rate of the electrons is found to increase by more than 30% during the first ps after excitation. This is at least one order of magnitude larger than found in comparable experiments on doped semiconductors. Numerical estimates show that the hot phonons can explain a significant part of this rise. Therefore, they might limit the performance of graphite and carbon-nanotube circuits at elevated temperatures and high frequencies. On the other hand, these results show a route to populate certain phonon modes selectively which might be exploited in fields like surface femtochemistry. In our carbon-nanotube sample, the pump pulse excites all types of tubes which can be metallic or exhibit electronic energy gaps of ~20meV or ~1eV. The THz spectra lack a signature of free charge carriers which clearly indicates that strongly bound excitons are the main product of photoexcitation of the tubes with an energy gap of ~1eV. We find a spectral feature of enhanced transmission which is caused by the blocking of optical transitions in tubes with an energy gap of ~20meV. Similar to the dynamics in graphite, the decay of this feature is assigned to the cooling of hot optical phonons by other, cold phonons with a time constant of 1ps. This is significantly faster than in graphite and points to a stronger anharmonic coupling between the phonon modes of the nanotube. Finally, a small and featureless background of increased absorption exhibits a remarkable optical anisotropy. By using simple geometrical arguments, this can be directly traced back to the localization of charge carriers on a length scale of 100nm. We can consistently assign the localized excitations to optical transitions between higher-lying intraexcitonic levels in the tubes with energy gaps of ~1eV. These levels are rapidly depopulated with a time constant of 150fs. These findings may be of great importance for the application of carbon nanotubes in photoconductive and nanoelectronic devices. In the experiments with optically excited gases, the pump pulse is found to ionize about 1% of all molecules. The THz response of the quasifree electrons is well described by the Drude model and yields the temporal evolution of the electron density and the Drude collision rate. The electrons in ionized Ar decay on a time scale of more than 1ns which is an order of magnitude slower than in O2 and due to a lacking dissipation channel for the kinetic and the binding energy of the electron. However, the electron decay can be accelerated enormously by adding the electron scavenger SF6 to Ar. In pure SF6, the free- electron decay occurs with a time constant as short as 12ps which directly demonstrates the ultrafast extinction of free electrons in SF6. It allows for a reliable estimate of the electronic temperature of 17000 to 23000K. The Drude scattering rate was found to increase with the electron density. A model based on the Boltzmann equation which only accounts for collisions between electrons and ions and electrons and neutral particles underestimates the measured collision rates significantly. This and the fact that the electron capture by SF6 does not increase the Drude scattering rate might point to a dominant contribution of electron-electron scattering to the current relaxation. The measurements demonstrate that THz spectroscopy provides new and important information on the dynamics of quasifree electrons in gases., Die vorliegende Arbeit untersucht die ultraschnelle Ladungsträgerdynamik in Graphit, Kohlenstoff-Nanoröhrchen und optisch ionisierten Gasen mit Hilfe der zeitaufgelösten THz- Spektroskopie. Bei dieser sogenannten Anrege-Abfrage- Technik regt ein sichtbarer Laserpuls die Probe an, die nach einer variablen Verzögerungszeit von einem THz-Puls abgefragt wird. Dabei hat man die volle Information über Amplitude und Phase des Abfragepulses und somit Zugriff auf die momentane komplexe dielektrische Funktion der Probe im Spektralbereich von etwa 10 bis 30THz. Im Halbmetall Graphit ergibt die Datenanalyse, daß mehr als 90% der absorbierten Anregungsenergie innerhalb von 500fs vom Elektronensystem ins Kristallgitter transferiert werden. Dies ist erstaunlich, da nur ~1% aller Phononenwellenvektoren an der Elektron-Phonon-Streuung teilnehmen können. Modellrechnungen zeigen, daß der Energietransfer von optischen Phononen mit sehr hohen Quantenergien und starker Kopplung an die Elektronen dominiert wird. Diese wenigen Phononenmoden heizen sich auf und werden auf einer Zeitskala von 5ps durch andere, noch kalte Phononen abgekühlt. Diese Ergebnisse weisen einen Weg, wie man mit Hilfe eines kurzen Laserpulses selektiv wenige Gitterschwingungen eines Halbmetalls aufheizen kann. In der Probe mit den Kohlenstoff-Nanoröhrchen wird der Anregungspuls sowohl von halbleitenden Röhren mit einer Bandlücke von ~1eV als auch "metallischen" Röhren mit einer Bandlücke von 0 oder ~20meV absorbiert. Die gemessenen THz- Spektren weisen keine Signatur freier Ladungsträger auf. Das ist ein klares Zeichen dafür, daß in hableitenden Röhren hauptsächlich stark gebundene Exzitonen anstatt freier Elektron-Loch-Paare erzeugt werden. Spektrale Anteile mit verringerter Absorption werden durch das Blockieren von optischen Übergängen in den "metallischen" Röhren verursacht. Analog zu Graphit wird das Abklingen dieses Signals mit einer Zeitkonstanten von 1ps auf das Kühlen heißer optischer Phononen zurückgeführt. Die spektralen Anteile mit erhöhter Absorption zeigen einen deutlichen Dichroismus, woraus wir auf eine Lokalisierung der zugrundeliegenden Anregungen auf einer Längenskala von 100nm schließen. Intraexzitonische Übergänge werden als Quelle für dieses Signal vorgeschlagen. Seine Abklingzeit von 150fs legt damit die Zeitskala für die Entvölkerung der höherliegenden exzitonischen Niveaus fest. In den Experimenten mit Gasen ionisiert ein intensiver Anregungspuls etwa 1% aller Moleküle. In Ar verschwinden die freien Elektronen auf einer Zeitskala von mehr als 1ns. Dies ist eine Größenordnung langsamer als in O2 und auf fehlende Dissipationskanäle für die kinetische Energie und Bindungsenergie des Elektrons zurückzuführen. Die Rekombinationsrate von Ar kann durch Zugabe des Schutzgases SF6 enorm beschleunigt werden. In reinem S6 relaxiert die Elektronendichte mit einer Zeitkonstanten von nur 12ps bei einer Elektronentemperatur von etwa 20000K. Die elektronische Drude-Streurate steigt monoton mit der Elektronendichte und ist wesentlich größer als von einem Modell vorhergesagt, das auf der Boltzmann-Gleichung und Elektron-Ion-Streuung bei statischer Abschirmung basiert. Dies zeigt, daß weitere Effekte wie etwa Elektron-Elektron-Streuung und dynamische Abschirmung der Coulomb- Wechselwirkung bei der theoretischen Behandlung der Leitfähigkeit von Gasplasmen im THz-Bereich berücksichtigt werden müssen.

The efficient conversion of spin to charge transport and vice versa is of major relevance for the detection and generation of spin currents in spin‐based electronics. Interfaces of heterostructures are known to have a marked impact on this process. Here, terahertz (THz) emission spectroscopy is used to study ultrafast spin‐to‐charge‐current conversion (S2C) in about 50 prototypical F|N bilayers consisting of a ferromagnetic layer F (e.g., Ni81Fe19, Co, or Fe) and a nonmagnetic layer N with strong (Pt) or weak (Cu and Al) spin‐orbit coupling. Varying the structure of the F/N interface leads to a drastic change in the amplitude and even inversion of the polarity of the THz charge current. Remarkably, when N is a material with small spin Hall angle, a dominant interface contribution to the ultrafast charge current is found. Its magnitude amounts to as much as about 20% of that found in the F|Pt reference sample. Symmetry arguments and first‐principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spin‐polarized electrons at interface imperfections. The results highlight the potential of skew scattering for interfacial S2C and propose a promising route to enhanced S2C by tailored interfaces at all frequencies from DC to terahertz., Advanced Materials, 33 (9), ISSN:0935-9648, ISSN:1521-4095

The terahertz Kerr effect (TKE) of polar molecular vapors is reported. The birefringence signal of fluoroform appears with opposite polarity compared to acetonitrile and water. This behavior is a hallmark of the opposite sign of a new molecular polarizability anisotropy [Formel], with [Formel] being the polarizability along the permanent dipole moment. As the excitation of the rotational states orients the permanent dipoles along the terahertz electric field, the orientation is translated into an optical birefringence proportional to [Formel]. Thus, the sign of [Formel] is imprinted onto the TKE signal, providing novel insights into the polarizability tensor of water.

The efficient conversion of spin to charge transport and vice versa is of major relevance for the detection and generation of spin currents in spin‐based electronics. Interfaces of heterostructures are known to have a marked impact on this process. Here, terahertz (THz) emission spectroscopy is used to study ultrafast spin‐to‐charge‐current conversion (S2C) in about 50 prototypical F|N bilayers consisting of a ferromagnetic layer F (e.g., Ni81Fe19, Co, or Fe) and a nonmagnetic layer N with strong (Pt) or weak (Cu and Al) spin‐orbit coupling. Varying the structure of the F/N interface leads to a drastic change in the amplitude and even inversion of the polarity of the THz charge current. Remarkably, when N is a material with small spin Hall angle, a dominant interface contribution to the ultrafast charge current is found. Its magnitude amounts to as much as about 20% of that found in the F|Pt reference sample. Symmetry arguments and first‐principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spin‐polarized electrons at interface imperfections. The results highlight the potential of skew scattering for interfacial S2C and propose a promising route to enhanced S2C by tailored interfaces at all frequencies from DC to terahertz.

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with

Coupling phase-stable single-cycle terahertz (THz) pulses to scanning tunneling microscope (STM) junctions enables spatiotemporal imaging with femtosecond temporal and Angstrom spatial resolution. The time resolution achieved in such THz-gated STM is ultimately limited by the subcyde temporal variation of the tip-enhanced THz field acting as an ultrafast voltage pulse, and hence by the ability to feed high-frequency, broadband THz pulses into the junction. Here, we report on the coupling of ultra-broadband (1-30 THz) single-cycle THz pulses from a spintronic THz emitter (STE) into a metallic STM junction. We demonstrate broadband phase-resolved detection of the THz voltage transient directly in the STM junction via THz-field-induced modulation of ultrafast photocurrents. Comparison to the unperturbed far-field THz waveform reveals the antenna response of the STM tip. Despite tip-induced low-pass filtering, frequencies up to 15 THz can be detected in the tip-enhanced near-field, resulting in THz transients with a half-cycle period of 115 fs. We further demonstrate simple polarity control of the THz bias via the STE magnetization and show that up to 2 V THz bias at 1 MHz repetition rate can be achieved in the current setup. Finally, we find a nearly constant THz voltage and waveform over a wide range of tip-sample distances, which by comparison to numerical simulations confirms the quasi-static nature of the THz pulses. Our results demonstrate the suitability of spintronic THz emitters for ultrafast THz-STM with unprecedented bandwidth of the THz bias and provide insight into the femtosecond response of defined nanoscale junctions.

The spin Hall effect in heavy-metal thin films is routinely used to convert charge currents into transverse spin currents and can be used to exert torque on adjacent ferromagnets. Conversely, the inverse spin Hall effect is frequently used to detect spin currents by charge currents in spintronic devices up to the terahertz frequency range. Numerous techniques to measure the spin Hall effect or its inverse have been introduced, most of which require extensive sample preparation by multistep lithography. To enable rapid screening of materials in terms of charge-to-spin conversion, suitable high-throughput methods for measuring the spin Hall angle are required. Here we compare two lithography-free techniques, terahertz emission spectroscopy and broadband ferromagnetic resonance, with standard harmonic Hall measurements and theoretical predictions using the binary-alloy series AuxPt1−x as a benchmark system. Despite their being highly complementary, we find that all three techniques yield a spin Hall angle with approximately the same x dependence, which is also consistent with first-principles calculations. Quantitative discrepancies are discussed in terms of magnetization orientation and interfacial spin-memory loss.

A review on the recent development of intense laser‐driven terahertz (THz) sources is provided here. The technologies discussed include various types of sources based on optical rectification (OR), spintronic emitters, and laser‐filament‐induced plasma. The emphasis is on OR using pump pulses with tilted intensity front. Illustrative examples of newly emerging applications are briefly discussed, in particular strong‐field THz control of materials and acceleration and manipulation of charged particles.

Terahertz emission spectroscopy (TES) of ultrathin multilayers of magnetic and heavy metals has recently attracted much interest. This method not only provides fundamental insights into photoinduced spin transport and spin–orbit interaction at highest frequencies, but has also paved the way for applications such as efficient and ultrabroadband emitters of terahertz (THz) electromagnetic radiation. So far, predominantly standard ferromagnetic materials have been exploited. Here, by introducing a suitable figure of merit, we systematically compare the strength of THz emission from X/Pt bilayers with X being a complex ferro-, ferri- and antiferromagnetic metal, that is, dysprosium cobalt (DyCo5), gadolinium iron (Gd24Fe76),magnetite (Fe3O4) and iron rhodium (FeRh). We find that the performance in terms of spin-current generation not only depends on the spin polarization of the magnet’s conduction electrons, but also on the specific interface conditions, thereby suggesting TES to be a highly interface-sensitive technique. In general, our results are relevant for all applications that rely on the optical generation of ultrafast spin currents in spintronic metallic multilayers., SPIN, 7 (3)

The electron linear accelerators driving modern X-ray free-electron lasers can emit intense, tunable, quasi-monochromatic terahertz (THz) transients with peak electric fields of V Å−1 and peak magnetic fields in excess of 10 T when a purpose-built, compact, superconducting THz undulator is implemented. New research avenues such as X-ray movies of THz-driven mode-selective chemistry come into reach by making dual use of the ultra-short GeV electron bunches, possible by a rather minor extension of the infrastructure.

Superconducting MgB{sub 2} thin films were fabricated by magnetron rf and dc co-sputtering on heated silicon and diamond substrates. They were annealed ex-situ for one hour at 650 C. The superconducting phase transition was characterized contactless by means of terahertz time-domain spectroscopy in the frequency range from 0.8 to 4 THz. For this purpose, amplitude- and phase-resolved transmission measurements of the MgB{sub 2} films for temperatures between 5 and 50 K were performed to extract the complex conductivity. The data show that both samples are homogeneous on a length scale of millimeters and superconducting below a transition temperature of about 22 K. This paves the way to investigate charge-carrier dynamics in MgB{sub 2} with time-resolved THz spectroscopy.

Energy dissipation in water is very fast and more efficient than in many other liquids. This behavior is commonly attributed to the intermolecular interactions associated with hydrogen bonding. Here, we investigate the dynamic energy flow in the hydrogen bond network of liquid water by a pump-probe experiment. We resonantly excite intermolecular degrees of freedom with ultrashort single-cycle terahertz pulses and monitor its Raman response. By using ultrathin sample cell windows, a background-free bipolar signal whose tail relaxes monoexponentially is obtained. The relaxation is attributed to the molecular translational motions, using complementary experiments, force field, and ab initio molecular dynamics simulations. They reveal an initial coupling of the terahertz electric field to the molecular rotational degrees of freedom whose energy is rapidly transferred, within the excitation pulse duration, to the restricted translational motion of neighboring molecules. This rapid energy transfer may be rationalized by the strong anharmonicity of the intermolecular interactions.

This thesis is dedicated to improving the understanding of nonlinear electromagnetic probes of solid-state properties in the ultrafast temporal domain. Nonlinear methods are able to provide an unparalleled sensitivity in the search for new physics in solid-state science. They can yield insight into the material symmetry and potentially induced changes thereof, which enables e.g. to track the inversion of polarization in a ferroelectric material 1. The latter is of great interest in the search for novel and fast electronic gates and storage devices. More fundamentally, nonlinear probes can reveal coupling between various degrees of freedom and hence can give insight into the interplay and causal relations between electronic, magnetic and lattice excitations of a material. As switching processes are inherently time-dependent, it is crucial to fully comprehend the mechanism that generates such a nonlinear signal in the space and time-domain in order to make definite statements about the time-evolution of ordering based on these techniques. Here, we present two major studies focusing on improving the understanding of ultrafast nonlinear probes in the optical and the terahertz (THz) range of frequencies. The first part of this thesis examines previous observations made in the prototypical anti-ferromagnet nickel(II) oxide (NiO) with the time-resolved nonlinear method of magnetic-dipole sensitive second-harmonic generation (SHG). These findings suggested that the material provides a coherent gateway to a non-thermal state of magnetic ordering via an ultrafast change in magnetic anisotropy. Using a complementary direct probe of antiferromagnetic order, i.e. picosecond-time-resolved non-resonant magnetic x-ray diffraction, we study the sublattice magnetism after ultrafast optical excitation and our findings suggest that the previously observed dynamics in the second-harmonic response of NiO do not directly reflect dynamics of the antiferromagnetic order parameter. Our re-investigation of the experimental observations using SHG supplement the prior experiments and reveal inconsistencies with the current explanation for the ultrafast response of SHG in NiO which can instead be reconciled with an acoustic origin of the dynamics. The simulations of this process are likely to present the first treatment of second-order stimulated Brillouin scattering in non-ideally phase-matched crystals. Consistent with the experimental observations, the simulation gives rise to two signature frequencies ω +,− = Re(2k ω ± k 2ω )v s leading to oscillations at frequencies that are different by orders of magnitude. The higher of these frequencies resembles linear stimulated Brillouin scattering but is shifted by phase mismatch. In the second part of this work, we investigate the novel method of broadband 2D THz-spectroscopy. The technique combines the process of driving intrinsic infrared active material excitations, while at the same time probing the very same and associated, i.e. coupled, modes. Even in an excitation regime far away from driving permanent changes, the method can provide information about the curvature and symmetry of the energy landscape in which the modes reside. To improve the understanding of this means of spectroscopy, we perform reflective 2D THz measurements accompanied by time-domain simulations on low band gap III-V semiconductors. These materials represent well-understood polaron systems with an early onset of electric field- nonlinearity. Experimentally, we observe in indium antimonide and indium arsenide on timescales on the order of 7.5 ps inter and intraband changes of carrier population. Interestingly, for excitation fields below 80 kV/cm we find that the nonlinear response observed at very early times is composed of a multitude of combination tones of the material’s intrinsic excitations, i.e. the plasmon and phonon modes, in the 2D frequency spectrum. Observing the full phase information and having control over the polarity of the excitation fields allowed us to further differentiate between the parity of these spectral features. To understand this coherent response in more detail, we simulate field-driven ballistic carrier transport using realistic band structures in the time domain. For this, we solve Maxwell’s equations in the presence of a polaronic medium using Yee’s algorithm. The calculated ballistic transport leads the mean conduction band population into regions distant from the equilibrium position at the Γ-point by as far as 3% of the zone-boundary. The simulations are able to reproduce the approximate magnitude as well as all observed spectral features and their correct parity. The nonlinear 2D spectra, including features at phonon frequencies, can thus be assigned to be solely based on plasmonic anharmonicity and their explanation does not require additional coupling between the material’s intrinsic excitations. Our studies show that nonlinear probes can lead to rather complex results when used under conditions of low absorption and non-ideal phase matching. The latter can be transiently altered by pulsed laser excitation, which is here discussed for the case of acoustic perturbation of SHG, and hence for extended sample volumes the assignment of the detected intensity to the nonlinear susceptibility cannot readily be made on ultrafast timescales. In addition, we find that 2D THz-spectroscopy provides a sensitive tool to study electric field nonlinearities, in which e.g. purely plasmonic excitations can lead to a manifold of spectral features observed in the nonlinear response. Nonetheless, we can show that these spectra can be qualitatively and quantitatively understood using suitable simulation techniques. In the light of the findings presented in this thesis, we propose that evaluation of nonlinear probes should always be accompanied by considerations of the signal’s generation process with the implications of inhomogeneous ultrafast excitation. It appears essential to develop comprehensive time-dependent analysis tools in order to achieve a solid understanding of these signals.

Coherent radiation with frequencies ranging from 0.3 to 30 THz has recently become accessible by femtosecond laser technology. Terahertz (THz) waves have already found many applications in spectroscopy and imaging, and they can be manipulated using static optical elements such as lenses, polarizers, and filters. However, ultrafast modulation of THz radiation is required as well, for instance in short-range wireless communication or for preparing shaped THz transients for coherent control of numerous material excitations. Here, we demonstrate an all-optically created transient metamaterial that permits to manipulate the polarization of THz waveforms with sub-cycle precision. The polarization-modulated pulses are potentially interesting for controlling elementary motions such as vibrations of crystal lattices, rotations of molecules, and the precession of spins., Comment: 14 pages, 7 Figures

The terahertz (THz) conductivity of photoinduced charge carriers in two common polytypes of silicon carbide, 3C-SiC and 6H-SiC, is studied on picosecond time scales using an optical-pump THz-probe technique. We find that the conductivity, measured from 0.7 to 3 THz, is well described by the Drude model, and obtain a velocity relaxation time of 75 fs, independent of sample and charge-carrier density. In contrast, the carrier relaxation rates in the two polytypes differ by orders of magnitude: in 6H- and 3C-SiC, recombination proceeds on a time scale of few picoseconds and beyond nanoseconds, respectively.

We present a method for determining the dielectric function of opaque materials precisely and reproducibly in the frequency range from 8 to 30 THz and higher. Our approach is based on measuring the polarization- and phase-resolved THz electrical transients reflected by the sample. This mid-infrared time-domain ellipsometry is applied to pure and Nb-doped strontium titanate SrTiO3, which allows us to infer the longitudinal and transverse optical phonon frequencies and the free-carrier plasma frequency as a function of the charge carrier concentration. We extract and discuss the value of the effective mass of the charge carriers. VC