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Funder: Alexander von Humboldt Foundation, Funder: Experienced Researcher Fellowship, Funder: DFG Collaborative, Funder: PAPIIT, Funder: Danish National Research Foundation, Funder: JSPS, Funder: STFC Centre for Instrumentation, Funder: UKSA Centre for Earth, Funder: Leverhulme Trust; doi: http://dx.doi.org/10.13039/501100000275, Funder: NRF, Funder: Royal Society; doi: http://dx.doi.org/10.13039/501100000288, Funder: Australian Government; doi: http://dx.doi.org/10.13039/100015539, Funder: Cancer Research UK; doi: http://dx.doi.org/10.13039/501100000289, Funder: University of Warwick; doi: http://dx.doi.org/10.13039/501100000741, 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.

Light-induced ferroelectricity in quantum paraelectrics is a new avenue of achieving dynamic stabilization of hidden orders in quantum materials. In this work, we explore the possibility of driving transient ferroelectric phase in the quantum paraelectric KTaO$_3$ via intense THz excitation of the soft mode. We observe a long-lived relaxation in the THz-driven second harmonic generation signal (SHG) that lasts up to 20 ps at 10 K which may be attributed to light-induced ferroelectricity. Through analyzing the THz-induced coherent soft-mode oscillation and finding its hardening with fluence well described by a single well potential, we demonstrate intense THz pulses up to 500 kV/cm cannot drive a global ferroelectric phase in KTaO$_3$. Instead, we find the unusual long-lived relaxation of SHG comes from a THz-driven moderate dipolar correlation between the defect-induced local polar structures. We discuss the impact of our findings on current investigations of the THz-induced ferroelectric phase in quantum paraelectrics., Comment: Accepted for publication in Physical Review Letters

How does a small change in the structure of a phospholipid affect its supramolecular assembly? In aqueous suspensions, the substitution of one ester linkage in DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) by an ether linkage alters its phase behaviour completely. To unravel the effect of replacing a phospholipid's ester linkage by an ether linkage in lipid monolayers, we characterized pure monolayers of the model lipid DPPC and its sn-2 ether analogue PHPC (1-palmitoyl-2-O-hexadecyl-sn-glycero-3-phosphocholine) as well as mixtures of both by measurements of surface pressure-molecular area (π-Amol) isotherms. In addition, we used infrared reflection absorption spectroscopy (IRRAS) to study lipid condensation, lipid chain orientation, headgroup hydration, and lipid miscibility in all samples. Mixed monolayers consisting of DPPC and PHPC were studied further using epifluorescence microscopy. Our results indicate a strong influence of the sn-2 ether linkage on headgroup hydration and ordering effects in the regions of the apolar chains and the headgroups. Both effects could originate from changes in glycerol conformation. Furthermore, we observed a second plateau in the π-Amol isotherms of DPPC/PHPC mixtures and analysis of the mixed π-Amol isotherms reveals a non-ideal mixing behaviour of both lipids which may be caused by conformational differences in their headgroups.

In this study, we characterized monolayers of an azide-modified lipid at the air-water interface, pure and in its mixtures with the model lipid DPPC, with the aim of proving its potential to be applied for photo-cross-linking with other molecules. We chose a phospholipid bearing a terminal azide group in one of its hydrophobic tails to study its monolayer characteristics with the Langmuir film balance technique. Furthermore, we performed infrared reflection absorption spectroscopy (IRRAS) to get detailed insights into the organization of those monolayers as well as high-resolution mass spectrometry (HRMS) to see the effects of UV-irradiation on the lipids' chemical structure and organization. Our results suggest that in expanded monolayers of pure azide-modified membrane lipids, the azido-terminated chain folds back toward the air-water interface. Above the LE/LC (liquid-expanded/liquid-condensed) phase transition, the chains stretched, and thus, the azide group detaches from the interface. From temperature-dependent monolayer compressions, we evaluated all relevant thermodynamic parameters of the monolayers, such as the phase transition pressure, the critical temperature, and the triple point, and compare them to those of model lipids. For future applications, we studied the miscibility of the azide-modified lipid with DPPC in monolayers and found at least a certain miscibility over all investigated mixing ratios ranging from 10 to 75% of the azidolipid. Finally, we irradiated the azidolipid monolayer with UV light at 305 nm and measured photodissociation of the azide, leading to chemical cross-linking with other lipids, which shows the potential to be used as a cross-linking agent within self-assembled lipid or lipid/protein layers.

Visualizing ultrafast dynamics at the atomic scale requires time-resolved characterization with femtosecond temporal resolution. For fully relativistic electron bunch probes, existing techniques for single-shot ultrafast electron diffraction (UED) are limited by the achievable electron probe bunch length, charge, and timing jitter. We present the first experimental demonstration of dual-fed THz-driven compression and time-stamping that enables electron probes with improved temporal resolution. This technique utilizes two counter-propagating quasi-single-cycle THz pulses generated from two OH-1 organic crystals coupled into an optimized THz compressor structure. We demonstrate electron bunch compression and time-of-arrival jitter suppression by a factor of 3 paving the way toward unique opportunities for UED time-resolved measurements.

We demonstrate a technique for robust spatial and temporal characterization of single cycle strong field Terahertz pulses in the near-field of a LiNbO3 source using electro-optic sampling.

We observed structural evolution into the excited state pericyclic minimum during photochemical ring-opening of α-terpinene by ultrafast electron diffraction and ab-initio multiple spawning wavepacket simulation.

Strong Terahertz (THz) pulses provide a unique opportunity to excite low frequency modes of complex materials. Ultrafast structural probes such as femtosecond x-ray or ultrafast electron diffraction can then be used to investigate structural changes of the lattice in response to the excitation. Since the probe pulses can potentially be much shorter than a cycle of the THz excitation waveform, it is possible to analyze the instantaneous response of the lattice to the known excitation field.

With this study we aim at comparing the well-known lipid membrane model system of liposomes and polymer-encapsulated nanodiscs regarding their lipid properties. Using differential scanning calorimetry (DSC) and continuous-wave electron paramagnetic resonance (CW EPR) spectroscopy, we characterize the temperature-dependent lipid behavior within 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes and nanodiscs made from such liposomes by application of various polymers based on styrene-co-maleic acid (SMA), diisobutylene-alt-maleic acid (DIBMA), and styrene-co-maleic amide sulfobetaine (SMA-SB), a new SMA-derived copolymer containing sulfobetaine side chains. By incorporating a spin label doxyl moiety into the lipid bilayer in position 16 or 5 we were able to study the micropolarity as well as rotational restrictions onto the lipids in the apolar bilayer center and the chain region adjacent to the carbonyl groups, respectively. Our results suggest that all polymers broaden the main melting transition of DMPC, change the water accessibility within the lipid bilayer, and exhibit additional constraints onto the lipids. Independent of the used polymer, the rotational mobility of both spin-labeled lipids decreased with DIBMA exerting less restraints probably due to its aliphatic side chains. Our findings imply that the choice of the solubilizing polymer has to be considered an important step to form lipid nanodiscs which should be included into research of lipid membranes and membrane proteins in the future.

The collective dynamics of topological structures1-6 are of interest from both fundamental and applied perspectives. For example, studies of dynamical properties of magnetic vortices and skyrmions3,4 have not only deepened our understanding of many-body physics but also offered potential applications in data processing and storage7. Topological structures constructed from electrical polarization, rather than electron spin, have recently been realized in ferroelectric superlattices5,6, and these are promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics underlying the functionality of such complex extended nanostructures. Here, using terahertz-field excitation and femtosecond X-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders-of-magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices3. A previously unseen tunable mode, hereafter referred to as a vortexon, emerges in the form of transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond timescales. Its frequency is considerably reduced (softened) at a critical strain, indicating a condensation (freezing) of structural dynamics. We use first-principles-based atomistic calculations and phase-field modelling to reveal the microscopic atomic arrangements and corroborate the frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density.

This Conceptual Design Report describes LUXE (Laser Und XFEL Experiment), an experimental campaign that aims to combine the high-quality and high-energy electron beam of the European XFEL with a powerful laser to explore the uncharted terrain of quantum electrodynamics characterised by both high energy and high intensity. We will reach this hitherto inaccessible regime of quantum physics by analysing high-energy electron-photon and photon-photon interactions in the extreme environment provided by an intense laser focus. The physics background and its relevance are presented in the science case which in turn leads to, and justifies, the ensuing plan for all aspects of the experiment: Our choice of experimental parameters allows (i) field strengths to be probed where the coupling to charges becomes non-perturbative and (ii) a precision to be achieved that permits a detailed comparison of the measured data with calculations. In addition, the high photon flux predicted will enable a sensitive search for new physics beyond the Standard Model. The initial phase of the experiment will employ an existing 40 TW laser, whereas the second phase will utilise an upgraded laser power of 350 TW. All expectations regarding the performance of the experimental set-up as well as the expected physics results are based on detailed numerical simulations throughout.

How does a small change in the structure of a phospholipid affect its supramolecular assembly? In aqueous suspensions, the substitution of one ester linkage in DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) by an ether linkage alters its phase behaviour completely. To unravel the effect of replacing a phospholipid’s ester linkage by an ether linkage in lipid monolayers, we characterized pure monolayers of the model lipid DPPC and its sn-2 ether analogue PHPC (1-palmitoyl-2-O-hexadecyl-sn-glycero-3-phosphocholine) as well as mixtures of both by measurements of surface pressure – molecular area (π–A_mol) isotherms. In addition, we used infrared reflection absorption spectroscopy (IRRAS) and epifluorescence microscopy to study lipid condensation, lipid chain orientation, headgroup hydration, and lipid miscibility in all samples.

We present experimental results for THz-driven bunch compression of a relativistic electron beam performed at multiple beam energies. Compression is achieved using a shorted parallel plate structure to establish energy exchange between the electron beam and orthogonally propagating THz pulse. We discuss the interaction dynamics and trends in minimum bunch length as a function of beam energy.

A collaboration between DESY, PSI and CERN has developed and built an advanced modular $X$-band transverse deflection structure (TDS) system with the new feature of providing variable polarization of the deflecting force. The prototype of the novel $X$-band TDS, the polarizable $X$-band (PolariX) TDS, was fabricated at PSI following the high-precision tuning-free production process developed for the C-band Linac of the SwissFEL project. Bead-pull rf measurements were also performed at PSI to verify, inparticular, that the polarization of the dipole fields does not have any rotation along the structure. The high-power test was performed at CERN and now the TDS is at DESY and has been installed in theFLASHForward beamline, where the first streaking experience with beam has been accomplished. We summarize in this paper the rf design of the TDS and its key components, such as the $X$-band pulse compressor, E-rotator, and phase shifter, the results of the bead-pull measurements and the high power test and finally the rf setup at DESY.

Intense Terahertz (THz) pulses offer a unique way to manipulate matter on a sub-picosecond timescale. Their strong electric and magnetic field components efficiently couple to low-energy degrees of freedom like optical phonons or magnons while we can simultaneously use ultrafast probe techniques to monitor changes in the sample with femtosecond resolution. This allows us to study novel nonequilibrium phases and the response of matter to strong transient fields. I will give a brief overview over THz experiments at the LCLS and how we can probe changes in properties like ferroelectricity, magnetism or orbital order with femtosecond x-ray pulses. I will specifically focus on nonlinear phonon dynamics and discuss recent results on SrTiO3 where we observed strong nonlinearity of the soft phonon mode and the transfer of energy to higher frequency vibrational modes.

In the mixed-valence manganites, a near-infrared laser typically melts the orbital and spin order simultaneously, corresponding to the photoinduced d1d0→d0d1 excitations in the Mott-Hubbard bands of manganese. Here, we use ultrafast methods-both femtosecond resonant X-ray diffraction and optical reflectivity-to demonstrate that the orbital response in the layered manganite Nd1-xSr1+xMnO4(x=2/3) does not follow this scheme. At the photoexcitation saturation fluence, the orbital order is only diminished by a few percent in the transient state. Instead of the typical d1d0→d0d1 transition, a near-infrared pump in this compound promotes a fundamentally distinct mechanism of charge transfer, the d0→d1L, where L denotes a hole in the oxygen band. This finding may pave a different avenue for selectively manipulating specific types of order in complex materials of this class.

A two-bunch scheme to generate high-power, high-repetition-rate THz radiation for pump–probe experiments at LCLS II is studied. The two-bunch beam dynamics, the THz wiggler and radiation are described, as well as the transport system bringing the THz pulses from the wiggler to the X-ray experimental hall., Experiments using a THz pump and an X-ray probe at an X-ray free-electron laser (XFEL) facility like the Linac Coherent Light Source II (LCLS II) require frequency-tunable (3 to 20 THz), narrow bandwidth (∼10%), carrier-envelope-phase-stable THz pulses that produce high fields (>1 MV cm−1) at the repetition rate of the X-rays and are well synchronized with them. In this paper, a two-bunch scheme to generate THz radiation at LCLS II is studied: the first bunch produces THz radiation in an electromagnet wiggler immediately following the LCLS II undulator that produces X-rays from the second bunch. The initial time delay between the two bunches is optimized to compensate for the path difference in THz transport. The two-bunch beam dynamics, the THz wiggler and radiation are described, as well as the transport system bringing the THz pulses from the wiggler to the experimental hall.

Manipulating the atomic and electronic structure of matter with strong terahertz (THz) fields while probing the response with ultrafast pulses at x-ray free electron lasers (FELs) has offered unique insights into a multitude of physical phenomena in solid state and atomic physics. Recent upgrades of x-ray FEL facilities are pushing to much higher repetition rates, enabling unprecedented signal to noise for pump probe experiments. This requires the development of suitable THz pump sources that are able to deliver intense pulses at compatible repetition rates. Here we present a high power laser-driven THz source based on optical rectification in LiNbO3 using tilted pulse front pumping. Our source is driven by a kilowatt-level Yb:YAG amplifier system operating at 100 kHz repetition rate and employing nonlinear spectral broadening and recompression to achieve sub-100 fs pulses at 1030 nm wavelength. We demonstrate a maximum of 144 mW average THz power (1.44 uJ pulse energy), consisting of single-cycle pulses centered at 0.6 THz with a peak electric field strength exceeding 150 kV/cm. These high field pulses open up a range of possibilities for nonlinear time-resolved experiments with x-ray probing at unprecedented rates., 17 pages, 10 figures

We present the first demonstration of THz-driven bunch compression and timing stabilization of a few-fC relativistic electron beam with kinetic energy of 2.5 MeV using quasi-single-cycle strong field THz radiation in a shorted parallel-plate structure. Compression by nearly a factor of 3 produced a 39 fs rms bunch length and a reduction in timing jitter by more than a factor of 2, to 31 fs rms, offering a significant improvement to beam performance for applications like ultrafast electron diffraction. This THz-driven technique provides a critical step towards unprecedented timing resolution in ultrafast sciences and other accelerator applications using femtosecond-scale electron beams., Comment: 6 pages, 5 figures

Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. As the relaxation occurs primarily via Auger emission, excited state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained elusive at XFELs due to inherent timing and phase jitter, which can be orders of magnitude larger than the timescale of Auger decay. Here, we develop a new approach termed self-referenced attosecond streaking, based upon simultaneous measurements of streaked photo- and Auger electrons. Our technique enables sub-femtosecond resolution in spite of jitter. We exploit this method to make the first XFEL time-domain measurement of the Auger decay lifetime in atomic neon, and, by using a fully quantum-mechanical description, retrieve a lifetime of $2.2^{ + 0.2}_{ - 0.3}$ fs for the KLL decay channel. Importantly, our technique can be generalised to permit the extension of attosecond time-resolved experiments to all current and future FEL facilities., Comment: Main text: 20 pages, 3 figures. Supplementary information: 17 pages, 6 figures

We present the initial characterization of a THz-based dual-fed electron bunch compressor. Intense single-cycle THz pulses, generated by optical rectification in organic crystals OH1, are coupled into a parallel-plate waveguide and characterized using electro-optic sampling.

Two-dimensional transition metal dichalcogenides (TMDs) draw strong interest in materials science, with applications in optoelectronics and many other fields. Good performance requires high carrier concentrations and long lifetimes. However, high concentrations accelerate energy exchange between charged particles by Auger-type processes, especially in TMDs where many-body interactions are strong, thus facilitating carrier trapping. We report time-resolved optical pump-THz probe measurements of carrier lifetimes as a function of carrier density. Surprisingly, the lifetime reduction with increased density is very weak. It decreases only by 20% when we increase the pump fluence 100 times. This unexpected feature of the Auger process is rationalized by our time-domain

We demonstrate the electromagnetic performance of waveguides for femtosecond electron beam bunch manipulation and compression with strong-field terahertz (THz) pulses. The compressor structure is a dispersion-free exponentially-tapered parallel-plate waveguide (PPWG) that can focus single-cycle THz pulses along one dimension. We show test results of the tapered PPWG structure using electro-optic sampling (EOS) at the interaction region with peak fields of at least 300 kV/cm given 0.9 uJ of incoming THz energy. We also present a modified shorted design of the tapered PPWG for better beam manipulation and reduced magnetic field as an alternative to a dual-feed approach. As an example, we demonstrate that with 5 uJ of THz energy, the PPWG compresses a 2.5 MeV electron bunch by a compression factor of more than 4 achieving a bunch length of about 18 fs., 6 pages, 5 figures

In this study, we focus on analyzing the non-covalent interaction patterns between three lipid-nanodisc-forming polymers and nitroxide radicals which are used as small organic tracer molecules. Besides the negatively charged polymers DIBMA and SMA(2:1), we characterize the solvation behavior of a new zwitterionic styrene/maleic amide sulfobetaine copolymer named SMA-SB. The used nitroxide spin probes vary in their respective chemical structure, allowing the report of different local micropolarities and nanoscopic regions by recording temperature-dependent continuous-wave electron paramagnetic resonance (CW EPR) spectra. In combination with dynamic light scattering (DLS) and electrophoretic light scattering (ELS) experiments, we are able to provide a nanoscopic interpretation of the dominant interaction patterns between the polymer and the chosen guest molecule.

The interactions between electrons and lattice vibrations are fundamental to materials behaviour. In the case of group IV–VI, V and related materials, these interactions are strong, and the materials exist near electronic and structural phase transitions. The prototypical example is PbTe whose incipient ferroelectric behaviour has been recently associated with large phonon anharmonicity and thermoelectricity. Here we show that it is primarily electron-phonon coupling involving electron states near the band edges that leads to the ferroelectric instability in PbTe. Using a combination of nonequilibrium lattice dynamics measurements and first principles calculations, we find that photoexcitation reduces the Peierls-like electronic instability and reinforces the paraelectric state. This weakens the long-range forces along the cubic direction tied to resonant bonding and low lattice thermal conductivity. Our results demonstrate how free-electron-laser-based ultrafast X-ray scattering can be utilized to shed light on the microscopic mechanisms that determine materials properties., Group IV–VI materials often exist in a state near an electronic or structural phase transition. Here, the authors use ultrafast X-ray scattering to show that coupling of band-edge electrons and phonons causes the ferroelectric instability observed in lead telluride.

Complex systems, which consist of a large number of interacting constituents, often exhibit universal behavior near a phase transition. A slowdown of certain dynamical observables is one such recurring feature found in a vast array of contexts. This phenomenon, known as critical slowing down, is well studied mostly in thermodynamic phase transitions. However, it is less understood in highly nonequilibrium settings, where the time it takes to traverse the phase boundary becomes comparable to the timescale of dynamical fluctuations. Using transient optical spectroscopy and femtosecond electron diffraction, we studied a photo-induced transition of a model charge-density-wave (CDW) compound, LaTe$_3$. We observed that it takes the longest time to suppress the order parameter at the threshold photoexcitation density, where the CDW transiently vanishes. This finding can be quantitatively captured by generalizing the time-dependent Landau theory to a system far from equilibrium. The experimental observation and theoretical understanding of dynamical slowing down may offer insight into other general principles behind nonequilibrium phase transitions in many-body systems.

Photons, electrons, and their interplay are at the heart of photonic devices and modern instruments for ultrafast science [1-10]. Nowadays, electron beams of the highest intensity and brightness are created by photoemission with short laser pulses, and then accelerated and manipulated using GHz radiofrequency electromagnetic fields. The electron beams are utilized to directly map photoinduced dynamics with ultrafast electron scattering techniques, or further engaged for coherent radiation production at up to hard X-ray wavelengths [11-13]. The push towards improved timing precision between the electron beams and pump optical pulses though, has been stalled at the few tens of femtosecond level, due to technical challenges with synchronizing the high power rf fields with optical sources. Here, we demonstrate attosecond electron metrology using laser-generated single-cycle THz radiation, which is intrinsically phase locked to the optical drive pulses, to manipulate multi-MeV relativistic electron beams. Control and single-shot characterization of bright electron beams at this unprecedented level open up many new opportunities for atomic visualization.

We present recent progress in understanding nonlinear phonon dynamics driven by intense THz field strengths using ultrafast x-ray diffraction. This technique allows us to characterize the motion of the atomic lattice inside the sample on a sub-cycle timescale while the driving field is still present. In the example of strontium titanate (SrTiO3) we observe harmonic generation and phonon upconversion when the fundamental soft mode is driven into the nonlinear regime by a single-cycle THz pulse.

Physical review accelerators and beams 21(9), 092802 (2018). doi:10.1103/PhysRevAccelBeams.21.092802, In this paper, the system identification and feedback controller design for a normal conducting Standing wave radio frequency cavity operated in pulsed mode of a free electron laser is presented. The system identification, essential to achieve high performance control, is based on a grey box model, with a priori knowledge about the physical behavior. The feedback regulation concept is separated into two Basic controller parts. A radio frequency controller design, based on the identified radio frequency model, is followed by a system identification and regulation of the pulse width to overcome the limitation of the underlying temperature regulation. It will be shown by measurements at the free electron laser FLASH at DESY, Germany, that the combination of both cascaded control schemes lead to high radio frequency field performance., Published by American Physical Society, College Park, MD

We present a one-dimensional charge density model for an oscillating “plasmon” in a 200 nm thick silicon membrane. The free carriers are generated using photoexcitation, then a single-cycle THz pump induces an oscillation of those carriers. The plasmonic oscillation is probed using a short relativistic electron bunch which is sensitive to the dipole field produced by the separation of charges. We compare the results of the model with measurements of the local field from ultrafast electron diffraction.

The journal of physical chemistry letters 9(12), 3538 - 3543 (2018). doi:10.1021/acs.jpclett.8b01429, Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time-resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal–ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. π-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences σ-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes., Published by ACS, Washington, DC

Topological quantum materials exhibit fascinating properties1–3, with important applications for dissipationless electronics and fault-tolerant quantum computers4,5. Manipulating the topological invariants in these materials would allow the development of topological switching applications analogous to switching of transistors6. Lattice strain provides the most natural means of tuning these topological invariants because it directly modifies the electron–ion interactions and potentially alters the underlying crystalline symmetry on which the topological properties depend7–9. However, conventional means of applying strain through heteroepitaxial lattice mismatch10 and dislocations11 are not extendable to controllable time-varying protocols, which are required in transistors. Integration into a functional device requires the ability to go beyond the robust, topologically protected properties of materials and to manipulate the topology at high speeds. Here we use crystallographic measurements by relativistic electron diffraction to demonstrate that terahertz light pulses can be used to induce terahertz-frequency interlayer shear strain with large strain amplitude in the Weyl semimetal WTe2, leading to a topologically distinct metastable phase. Separate nonlinear optical measurements indicate that this transition is associated with a symmetry change to a centrosymmetric, topologically trivial phase. We further show that such shear strain provides an ultrafast, energy-efficient way of inducing robust, well separated Weyl points or of annihilating all Weyl points of opposite chirality. This work demonstrates possibilities for ultrafast manipulation of the topological properties of solids and for the development of a topological switch operating at terahertz frequencies. Terahertz light pulses induce transitions between a topological and a trivial phase in the Weyl semimetal WTe2 through an interlayer shear strain.

New journal of physics 20(3), 033008 (2018). doi:10.1088/1367-2630/aab548, Arbitrary manipulation of the temporal and spectral properties of x-ray pulses at free-electron lasers would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the free-electron laser driving electron bunch can be tuned to emit a pair of x-ray pulses with independently variable photon energy and femtosecond delay. However, while accelerator parameters can easily be adjusted to tune the electron bunch phase-space, the final impact of these actuators on the x-ray pulse cannot be predicted with sufficient precision. Furthermore, shot-to-shot instabilities that distort the pulse shape unpredictably cannot be fully suppressed. Therefore, the ability to directly characterize the x-rays is essential to ensure precise and consistent control. In this work, we have generated x-ray pulse pairs via electron bunch shaping and characterized them on a single-shot basis with femtosecond resolution through time-resolved photoelectron streaking spectroscopy. This achievement completes an important step toward future x-ray pulse shaping techniques., Published by IOP, [London]

Terahertz pulses are applied as an all-optical bias to ferroelectric thin-film BiFeO3 while monitoring the time-dependent ferroelectric polarization through its nonlinear optical response. Modulations in the intensity of the second harmonic light generated by the film correspond to on-off ratios of 220× gateable on femtosecond timescales. Polarization modulations comparable to the built-in static polarization are observed.

We demonstrate a terahertz pump-ultrafast electron diffraction probe apparatus. Using the instrument, we show that intense focused THz pulses can characterize ultrashort electron bunches.

We present recent advancements in echelon-based single-shot terahertz spectroscopy for broadband spectroscopic applications. We also apply an echelon-based scheme to study photo-induced changes in correlated electron materials and warm dense matter.

We explore the scaling behavior of THz generation through optical rectification in DSTMS as function of repetition rate from 100 Hz to 200 kHz. We observe a strong drop-off in conversion efficiency above 10 kHz.

We demonstrate a self-referencing method to reduce noise in a single-shot terahertz detection scheme. By splitting a single terahertz pulse and using a reflective echelon, both the signal and reference terahertz time-domain waveforms were measured using one laser pulse. Simultaneous acquisition of these waveforms significantly reduces noise originating from shot-to-shot fluctuations. We show that correlation function based referencing, which is not limited to polarization dependent measurements, can achieve a noise floor that is comparable to state-of-the-art polarization-gated balanced detection. Lastly, we extract the DC conductivity of a 30 nm free-standing gold film using a single THz pulse. The measured value of σ0 = 1.3 ± 0.4 × 107 S m-1 is in good agreement with the value measured by four-point probe, indicating the viability of this method for measuring dynamical changes and small signals.

We demonstrate how to track atomic motion within a unit cell when driving a phonon mode in the prototypical perovskite SrTiO 3 with intense THz pulses. We show that at the highest THz fields nonlinear phonon coupling is observed and energy is transferred coherently from the low frequency soft mode into higher frequency phonon modes.

We use ultrafast X-ray pulses to characterize the lattice response of SrTiO3 when driven by strong terahertz fields. We observe transient changes in the diffraction intensity with a delayed onset with respect to the driving field. Fourier analysis reveals two frequency components corresponding to the two lowest energy zone-center optical modes in SrTiO3. The lower frequency mode exhibits clear softening as the temperature is decreased while the higher frequency mode shows slight temperature dependence.

Approximately 400 participants learned about the latest scientific capabilities at the Stanford Synchrotron Radiation Lightsource (SSRL) and Linac Coherent Light Source (LCLS) during the 2013 Users' Meeting at the SLAC National Accelerator Laboratory, October 1–4, 2013. The Users' Meeting included more than a dozen workshops as well as scientific awards, poster presentations, and talks on the latest scientific trends, challenges, and opportunities.