Segmented Terahertz Electron Accelerator and Manipulator (STEAM)

Zeeman torque sampling of intense terahertz magnetic field in CoFe

Free-space strong-field terahertz (THz) electromagnetic pulses have emerged as a potent tool for non-equilibrium quantum state control. However, these applications predominantly rely on electric field components, with limited utilization of magnetic field components. A traditional electro-optical sampling technique may encounter constraints due to the field strength saturation effect when detecting strong-field THz pulses. Here we have identified intense THz-induced Zeeman torque signals in CoFe and successfully detected the THz magnetic field components. Through variations in the type of ferromagnetic materials and the thickness of ferromagnetic films, we further refined the detection of THz magnetic field components. Our research revealed that a 15-nm CoFe thin film is more responsive in measuring magnetic field components.

Terahertz control and timing correlations in a transmission electron microscope

Ultrafast electron microscopy provides a movie-like access to structural dynamics of materials in space and time, but fundamental atomic motions or electron dynamics are, so far, too quick to be resolved. Here, we report the all-optical control, compression, and characterization of electron pulses in a transmission electron microscope by the single optical cycles of laser-generated terahertz light. This concept provides isolated electron pulses and merges the spatial resolution of a transmission electron microscope with the temporal resolution that is offered by a single cycle of laser light. We also report the all-optical control of multi-electron states and find a substantial two-electron and three-electron anticorrelation in the time domain. These results open up the possibility to visualize atomic and electronic motions together with their quantum correlations on fundamental dimensions in space and time.

Easily variable and scalable terahertz pulse source based on tilted-pulse-front pumped semiconductors

A new type of terahertz source containing only two optical elements - a volume phase holographic grating, and a semiconductor nonlinear slab - is proposed. The setup does not require any microstructuring, has only one diffraction order, and can be scaled to large pump sizes without any principal limitations. Furthermore, it can be easily adapted to different pump wavelengths and THz phase-matching frequencies. The Fresnel loss at the boundary of the materials can be significant at conventional pump polarizations (s-pol), but a single-layer anti-reflection (AR) coating can reduce it. Pumping such a setup with polarization in the dispersion plane (p-pol, TM mode) can reduce the effective nonlinear polarization and consequently the terahertz generation efficiency. However, in the absence of AR coating, this reduction is overcompensated by the reduced Fresnel loss.

Time-resolved THz Stark spectroscopy of molecules in solution

For decades, it was considered all but impossible to perform Stark spectroscopy on molecules in a liquid solution, because their concomitant orientation to the applied electric field results in overwhelming background signals. A way out was to immobilize the solute molecules by freezing the solvent. While mitigating solute orientation, freezing removes the possibility to study molecules in liquid environments at ambient conditions. Here we demonstrate time-resolved THz Stark spectroscopy, utilizing intense single-cycle terahertz pulses as electric field source. At THz frequencies, solute molecules have no time to orient their dipole moments. Hence, dynamic Stark spectroscopy on the time scales of molecular vibrations or rotations in both non-polar and polar solvents at arbitrary temperatures is now possible. We verify THz Stark spectroscopy for two judiciously selected molecular systems and compare the results to conventional Stark spectroscopy and first principle calculations.

Tunable and switchable multifunctional terahertz meta-mirror based on graphene and vanadium dioxide

We design a multifunctional THz polarization modulation meta-mirror integrated with polarization conversion and dichroism functions switched by temperature and voltage. The meta-mirror is composed of two-layered graphene metasurfaces and a layer of vanadium dioxide (VO2) on a gold film substrate. Linear-to-linear polarization conversion and linear dichroism (LD) can be switched by temperature control in the VO2 film and Fermi level adjustments in the graphene metasurfaces, where the polarization conversion ratio (PCR) is higher than 0.9 in the range of 2.89 THz to 4.02 THz, LD value reached a maximum of 0.6 at 3.84 THz, and linear-to-circular polarization conversion and circular dichroism (CD) can also be tuned with ellipticity higher than 0.9 in the range of 2.32 THz to 2.69 THz and CD value as high as 0.71 at 2.45 THz. The proposed meta-mirror is the first THz metamaterial device integrating four switchable functions, including linear-to-linear polarization conversion, linear-to-circular polarization conversion, linear dichroism and circular dichroism. The meta-mirror is a promising design for compact system integration in THz imaging, sensing and biological detection applications.

Millijoule Terahertz Radiation from Laser Wakefields in Nonuniform Plasmas

We report the experimental measurement of millijoule terahertz (THz) radiation emitted in the backward direction from laser wakefields driven by a femtosecond laser pulse of few joules interacting with a gas target. By utilizing frequency-resolved energy measurement, it is found that the THz spectrum exhibits two peaks located at about 4.5 and 9.0 THz, respectively. In particular, the high frequency component emerges when the drive laser energy exceeds 1.26 J, at which electron acceleration in the forward direction is detected simultaneously. Theoretical analysis and particle-in-cell simulations indicate that the THz radiation is generated via mode conversion from the laser wakefields excited in plasma with an up-ramp profile, where radiations both at the local electron plasma frequency and its harmonics are produced. Such intense THz sources may find many applications in ultrafast science, e.g., manipulating the transient states of matter.

Hollow metal tubes for efficient electron manipulation using terahertz surface waves

Compact electron sources have been instrumental in multidiscipline sciences including fundamental physics, oncology treatments, and advanced industries. Of particular interest is the terahertz-driven electron manipulation that holds great promise for an efficient high gradient of multi-GeV/m inside a regular dielectric-lined waveguide (DLW). The recent study relying on terahertz surface waves has demonstrated both high terahertz energy and improved coupling efficiency with the DLW. However, the large energy spread pertaining to the laser-induced electron pulse impedes the practical use of the system. Here, we propose a scheme for extending the idea of surface-wave-driven electron manipulation to mature electron sources such as commercial direct-current and radio-frequency electron guns. By using a simple hollow cylinder tube for electron transmission, we show that the electron energy modulation can reach up to 860 keV, or compress the electron pulse width to 15 fs using a 2.9 mJ single-cycle terahertz pulse. The trafficability of the hollow tube also allows for a cascade of the system, which is expected to pave the way for compact and highly efficient THz-driven electron sources.

Free-electron crystals for enhanced X-ray radiation

Bremsstrahlung-the spontaneous emission of broadband radiation from free electrons that are deflected by atomic nuclei-contributes to the majority of X-rays emitted from X-ray tubes and used in applications ranging from medical imaging to semiconductor chip inspection. Here, we show that the bremsstrahlung intensity can be enhanced significantly-by more than three orders of magnitude-through shaping the electron wavefunction to periodically overlap with atoms in crystalline materials. Furthermore, we show how to shape the bremsstrahlung X-ray emission pattern into arbitrary angular emission profiles for purposes such as unidirectionality and multi-directionality. Importantly, we find that these enhancements and shaped emission profiles cannot be attributed solely to the spatial overlap between the electron probability distribution and the atomic centers, as predicted by the paraxial and non-recoil theory for free electron light emission. Our work highlights an unprecedented regime of free electron light emission where electron waveshaping provides multi-dimensional control over practical radiation processes like bremsstrahlung. Our results pave the way towards greater versatility in table-top X-ray sources and improved fundamental understanding of quantum electron-light interactions.

Sub-terahertz nearfields for electron-pulse compression

The advent of ultrafast science with pulsed electron beams raised the need to control the temporal features of the electron pulses. One promising suggestion is the nano-selective quantum optics with multi-electrons, which scales quadratically with the number of electrons within the coherence time of the quantum system. Terahertz (THz) radiation from optical nonlinear crystals is an attractive methodology to generate the rapidly varying electric fields necessary for electron compression, with the advantage of an inherent temporal locking to laser-triggered electrons, such as in ultrafast electron microscopes. Longer (picosecond-) pulses require a sub-THz field for their compression. However, the generation of such low frequencies requires pumping with energetic optical pulses and their focusability is fundamentally limited by their mm-wavelength. This work proposes electron-pulse compression with sub-THz fields directly in the vicinity of their dipolar origin, thereby avoiding mediation through radiation. We analyze the merits of nearfields for compression of slow electrons, particularly in challenging regimes for THz radiation, such as small numerical apertures, micro-joule-level optical pump pulses, and low frequencies. This scheme can be implemented within the tight constraints of electron microscopes and reach fields of a few kV/cm below 0.1 THz at high repetition rates. Our paradigm offers a realistic approach for controlling electron pulses spatially and temporally in many experiments, opening the path of flexible multi-electron manipulation for analytic and quantum sciences.

Numerical investigation of sequential phase-locked optical gating of free electrons

Recent progress in coherent quantum interactions between free-electron pulses and laser-induced near-field light have revolutionized electron wavepacket shaping. Building on these advancements, we numerically explore the potential of sequential interactions between slow electrons and localized dipolar plasmons in a sequential phase-locked interaction scheme. Taking advantage of the prolonged interaction time between slow electrons and optical near-fields, we aim to explore the effect of plasmon dynamics on the free-electron wavepacket modulation. Our results demonstrate that the initial optical phase of the localized dipolar plasmon at the starting point of the interaction, along with the phase offset between the interaction zones, can serve as control parameters in manipulating the transverse and longitudinal recoil of the electron wavefunction. Moreover, it is shown that the incident angle of the laser light is an additional control knop for tailoring the longitudinal and transverse recoils. We show that a sequential phase-locking method can be employed to precisely manipulate the longitudinal and transverse recoil of the electron wavepacket, leading to selective acceleration or deceleration of the electron energy along specific diffraction angles. These findings have important implications for developing novel techniques for ultrafast electron-light interferometry, shaping the electron wavepacket, and quantum information processing.

Tilted pulse front pumping techniques for efficient terahertz pulse generation

Optical rectification of femtosecond laser pulses has emerged as the dominant technique for generating single- and few-cycle terahertz (THz) pulses. The advent of the tilted pulse front pumping (TPFP) velocity matching technique, proposed and implemented two decades ago, has ushered in significant advancements of these THz sources, which are pivotal in the realm of THz pump-probe and material control experiments, which need THz pulses with microjoule energies and several hundred kV/cm electric field strengths. Furthermore, these THz sources are poised to play a crucial role in the realization of THz-driven particle accelerators, necessitating millijoule-level pulses with tens of MV/cm electric field strengths. TPFP has enabled the efficient velocity matching in lithium niobate crystals renowned for their extraordinary high nonlinear coefficient. Moreover, its adaptation to semiconductor THz sources has resulted in a two-hundred-times enhancement in conversion efficiency. In this comprehensive review, we present the seminal achievements of the past two decades. We expound on the conventional TPFP setup, delineate its scaling limits, and elucidate the novel generation TPFP configurations proposed to surmount these constraints, accompanied by their preliminary outcomes. Additionally, we provide an in-depth analysis of the THz absorption, refractive index, and nonlinear coefficient spectra of lithium niobate and widely used semiconductors employed as THz generators, which dictate their suitability as THz sources. We underscore the far-reaching advantages of tilted pulse front pumping, not only for LN and semiconductor-based THz sources but also for selected organic crystal-based sources and Yb-laser-pumped GaP sources, previously regarded as velocity-matched in the literature.

Subcycle surface electron emission driven by strong-field terahertz waveforms

The advent of intense terahertz (THz) sources opened a new era when the demonstration of the acceleration and manipulation of free electrons by THz pulses became within reach. THz-field-driven electron emission was predicted to be confined to a single burst due to the single-cycle waveform. Here we demonstrate the confinement of single-cycle THz-waveform-driven electron emission to one of the two half cycles from a solid surface emitter. Either the leading or the trailing half cycle was active, controlled by reversing the field polarity. THz-driven single-burst surface electron emission sources, which do not rely on field-enhancement structures, will impact the development of THz-powered electron acceleration and manipulation devices, all-THz compact electron sources, THz waveguides and telecommunication, THz-field-based measurement techniques and solid-state devices.

Lossless Monochromator in an Ultrafast Electron Microscope Using Near-Field THz Radiation

The ability to form monoenergetic electron beams is vital for high-resolution electron spectroscopy and imaging. Such capabilities are commonly achieved using an electron monochromator, which energy filters a dispersed electron beam, thus reducing the electron flux to yield down to meV energy resolution. This reduction in flux hinders the use of monochromators in many applications, such as ultrafast transmission electron microscopes (UTEMs). Here, we develop and demonstrate a mechanism for electron energy monochromation that does not reduce the flux-a lossless monochromator. The mechanism is based on the interaction of free-electron pulses with single-cycle THz near fields, created by nonlinear conversion of an optical laser pulse near the electron beam path inside a UTEM. Our experiment reduces the electron energy spread by a factor of up to 2.9 without compromising the beam flux. Moreover, as the electron-THz interaction takes place over an extended region of many tens of microns in free space, the realized technique is highly robust-granting uniform monochromation over a wide area, larger than the electron beam diameter. We further demonstrate the wide tunability of our method by monochromating the electron beam at multiple primary electron energies from 60 to 200 keV, studying the effect of various electron and THz parameters on its performance. Our findings have direct applications in the fast-growing field of ultrafast electron microscopy, allowing time- and energy-resolved studies of exciton physics, phononic vibrational resonances, charge transport effects, and optical excitations in the mid IR to the far IR.

Wideband dispersion-free THz waveguide platform

We present a versatile THz waveguide platform for frequencies between 0.1 THz and 1.5 THz, designed to exhibit vacuum-like dispersion and electric as well as magnetic field enhancement. While linear THz spectroscopy benefits from the extended interaction length in combination with moderate losses, nonlinear THz spectroscopy profits from the field enhancement and zero dispersion, with the associated reshaping-free propagation of broadband single- to few-cycle THz pulses. Moreover, the vacuum-like dispersion allows for velocity matching in mixed THz and visible to infrared pump-probe experiments. The platform is based on the motif of a metallic double ridged waveguide. We experimentally characterize essential waveguide properties, for instance, propagation and bending losses, but also demonstrate a junction and an interferometer, essentially because those elements are prerequisites for THz waveform synthesis, and hence, for coherently controlled linear and nonlinear THz interactions.

Accurate simulation of THz generation with finite-element time domain methods

We investigate the accurate full broadband simulation of complex nonlinear optical processes. A mathematical model and numerical simulation techniques in the time domain are developed to simulate complex nonlinear optical processes without the usual used slowly varying envelope approximation. We illustrate the accuracy by numerical simulations. Furthermore, they are used to elucidate THz generation in periodically poled Lithium Niobate (PPLN) including optical harmonic generation.

Optimized terahertz pulse generation with chirped pump pulses from an echelon-based tilted-pulse-front (TPF) scheme

We successfully demonstrated the generation of single-cycle terahertz (THz) pulses through tilted-pulse-front (TPF) pumping using a reflective echelon in a lithium niobate crystal. By optimizing the pump pulse duration using a chirp, we achieved a maximum pump-to-THz conversion efficiency of 0.39%. However, we observed that the saturation behavior began at a relatively low pump energy (0.37 mJ), corresponding to a pump intensity of 22 GW/cm2. To elucidate this behavior, we measured the near- and far-field THz beam profiles and found variations in their beam characteristics, such as the beam size, location, and divergence angle in the plane of the tilted pulse direction, with the pump energy (intensity). This nonlinear behavior is attributed to the reduced effective interaction length, which ultimately leads to the saturation of THz generation. The results obtained from our study suggest that it is feasible to develop an effective THz source using echelon-based TPF pumping while also considering the impact of nonlinear saturation effects.

Demonstration of an imaging-free terahertz generation setup using segmented tilted-pulse-front excitation

A novel, to the best of our knowledge, compact, imaging-free, tilted-pulse-front (TPF) pumped terahertz (THz) source based on a LiNbO₃ slab with a small wedge angle (< 8°) and with an echelon microstructure on its input surface has been demonstrated. Single-cycle pulses of more than 40-µJ energy and 0.28-THz central frequency have been generated by 100-mJ, 400-fs pump pulses with 4.1 × 10^-4 efficiency and excellent focusability. The peak electric field value focused by a single parabolic mirror was 540 kV/cm. Using 200-fs-long pump pulses, the efficiency increased to 1.0 × 10^-3, which is in qualitative agreement with the measured increased diffraction efficiency in the velocity matched diffraction order. A further ∼8x increase in efficiency is expected by pumping a cryogenically cooled wedged echelon with appropriate step sizes, better microstructured surface quality, and antireflection coating on both the input and the output sides. THz generation efficiency maxima were found at ∼2.7-mm crystal thickness for both pump pulse durations. The focused THz beam was diffraction limited within 5% accuracy. Compared to conventional THz sources, this setup is very compact, easy to align, can be pumped by larger beam sizes maintaining the high THz generation efficiency, and produces THz pulses with superior focusability.

Towards precise diagnosis time profile of ultrafast electron bunch trains using orthogonal terahertz streak camera

Ultrafast electron microbunch trains have broad applications in which the individual bunch length and the bunch-to-bunch interval are critical parameters that need to be precisely diagnosed. However, directly measuring these parameters remains challenging. This paper presents an all-optical method that simultaneously measures the individual bunch length and the bunch-to-bunch spacing through an orthogonal THz-driven streak camera. For a 3 MeV electron bunch train, the simulation indicates that the temporal resolution of individual bunch length and the bunch-to-bunch spacing is 2.5 fs and 1 fs, respectively. Through this method, we expect to open a new chapter in the temporal diagnostic of electron bunch trains.

Generation of 13.9-mJ Terahertz Radiation from Lithium Niobate Materials

Extremely strong-field terahertz (THz) radiation in free space has compelling applications in nonequilibrium condensed matter state regulation, all-optical THz electron acceleration and manipulation, THz biological effects, etc. However, these practical applications are constrained by the absence of high-intensity, high-efficiency, high-beam-quality, and stable solid-state THz light sources. Here, the generation of single-cycle 13.9-mJ extreme THz pulses from cryogenically cooled lithium niobate crystals and a 1.2% energy conversion efficiency from 800 nm to THz are demonstrated experimentally using the tilted pulse-front technique driven by a home-built 30-fs, 1.2-Joule Ti:sapphire laser amplifier. The focused peak electric field strength is estimated to be 7.5 MV cm^-1 . A record of 1.1-mJ THz single-pulse energy at a 450 mJ pump at room temperature is produced and observed that the self-phase modulation of the optical pump can induce THz saturation behavior from the crystals in the substantially nonlinear pump regime. This study lays the foundation for the generation of sub-Joule THz radiation from lithium niobate crystals and will inspire more innovations in extreme THz science and applications.

Narrowband, Angle-Tunable, Helicity-Dependent Terahertz Emission from Nanowires of the Topological Dirac Semimetal Cd3As2

All-optical control of terahertz pulses is essential for the development of optoelectronic devices for next-generation quantum technologies. Despite substantial research in THz generation methods, polarization control remains difficult. Here, we demonstrate that by exploiting band structure topology, both helicity-dependent and helicity-independent THz emission can be generated from nanowires of the topological Dirac semimetal Cd₃As₂. We show that narrowband THz pulses can be generated at oblique incidence by driving the system with optical (1.55 eV) pulses with circular polarization. Varying the incident angle also provides control of the peak emission frequency, with peak frequencies spanning 0.21-1.40 THz as the angle is tuned from 15 to 45°. We therefore present Cd₃As₂ nanowires as a promising novel material platform for controllable terahertz emission.

Cascade bunch focusing on chip using terahertz pulses to drive prism arrays

The dielectric laser accelerator (DLA) is a promising candidate for next-generation table-top and even on-chip particle accelerators. Long-range focusing of a tiny-size electron bunch on chip is crucial for the practical applications of DLA, which has been a challenge. Here we propose a bunch focusing scheme, which uses a pair of readily available few-cycle terahertz (THz) pulses to drive an array of millimeter-scale prisms via the inverse Cherenkov effect. The THz pulses are reflected and refracted multiple times through the prism arrays, synchronizing with and periodically focusing the electron bunch along the bunch channel. Cascade bunch-focusing is realized by making the electromagnetic field phase experienced by electrons in each stage of the array, that is, the synchronous phase, in the focusing phase region. The focusing strength can be adjusted via changing the synchronous phase and THz field intensity, optimization of which will sustain the stable bunch transportation in a tiny-size bunch channel on chip. This bunch-focusing scheme sets a base for developing a long-acceleration-range and high-gain DLA.

Attosecond electron-beam technology: a review of recent progress

Electron microscopy and diffraction with ultrashort pulsed electron beams are capable of imaging transient phenomena with the combined ultrafast temporal and atomic-scale spatial resolutions. The emerging field of optical electron beam control allowed the manipulation of relativistic and sub-relativistic electron beams at the level of optical cycles. Specifically, it enabled the generation of electron beams in the form of attosecond pulse trains and individual attosecond pulses. In this review, we describe the basics of the attosecond electron beam control and overview the recent experimental progress. High-energy electron pulses of attosecond sub-optical cycle duration open up novel opportunities for space-time-resolved imaging of ultrafast chemical and physical processes, coherent photon generation, free electron quantum optics, electron-atom scattering with shaped wave packets and laser-driven particle acceleration. Graphical Abstract.

Large-area periodically-poled lithium niobate wafer stacks optimized for high-energy narrowband terahertz generation

Periodically-poled lithium niobate (PPLN) sources consisting of custom-built stacks of large-area wafers provide a unique opportunity to systematically study the multi-cycle terahertz (THz) generation mechanism as they are assembled layer-by-layer. Here we investigate and optimize the THz emission from PPLN wafer stacks as a function of wafer number, pump fluence, pulse duration and chirp, wafer separation, and pump focusing. Using 135 µm-thick, 2"-diameter wafers we generate high-energy, narrowband THz pulses with central frequencies up to 0.39 THz, directly suitable for THz-driven particle acceleration applications. We explore the multi-cycle pulse build-up with increasing wafer numbers using electro-optic sampling measurements, achieving THz conversion efficiencies up to 0.17%, while demonstrating unique control over the pulse length and bandwidth these sources offer. Guided by simulations, observed frequency-dependence on both stack-mounting and pump focusing conditions have been attributed to inter-wafer etalon and Gouy phase-shifts respectively, revealing subtle features that are critical to the understanding and performance of PPLN wafer-stack sources for optimal narrowband THz generation.

One-joule 500-Hz cryogenic Yb:YAG laser driver of composite thin-disk design

We present results on the development of a cryogenic Yb:YAG multi-pass laser amplifier based on a composite thin-disk design and demonstrate one-joule, diffraction limited, chirped 234-ps pulses with 50% optical-to-optical efficiency. High beam quality was obtained for repetition rates up to 400 Hz. The hardware was disassembled and thoroughly inspected after accumulating 80 hours of use at repetition rates from 100 to 500 Hz and exhibited no signs of damage. This laser driver is now commissioned to a dedicated laboratory where a grating compressor is producing 5.2-ps pulses used in the development of a compact x ray source based on inverse Compton scattering.

Scalable microstructured semiconductor THz pulse sources

In recent years several microstructured lithium niobate THz pulse source were suggested for high-energy applications. Two types of those, the reflective and the transmissive nonlinear slab are adopted here for semiconductors. These new sources are scalable both in THz energy and size. Furthermore, they can outperform the already demonstrated contact grating source in diffraction and THz generation efficiency. Compared to the lithium niobate sources, they are more feasible, thanks to the easier manufacturing and the longer pump wavelength. They can produce intense, nearly single-cycle THz pulses at higher frequencies. With 20 mJ pumping at 1.8 µm wavelength, 45 µJ THz energy, and 17 MV/cm focused peak electric field can be expected at 3 THz phase matching frequency from the transmissive nonlinear echelon slab setup consisting of a 4 mm thick structured plan-parallel gallium phosphide crystal.

Field-resolved THz-pump laser-probe measurements with CEP-unstable THz light sources

Radiation sources with a stable carrier-envelope phase (CEP) are highly demanded tools for field-resolved studies of light-matter interaction, providing access both to the amplitude and phase information of dynamical processes. At the same time, many coherent light sources, including those with outstanding power and spectral characteristics lack CEP stability, and so far could not be used for this type of research. In this work, we present a method enabling linear and non-linear phase-resolved terahertz (THz) -pump laser-probe experiments with CEP-unstable THz sources. THz CEP information for each pulse is extracted using a specially designed electro-optical detection scheme. The method correlates the extracted CEP value for each pulse with the THz-induced response in the parallel pump-probe experiment to obtain an absolute phase-resolved response after proper sorting and averaging. As a proof-of-concept, we demonstrate experimentally field-resolved THz time-domain spectroscopy with sub-cycle temporal resolution using the pulsed radiation of a CEP-unstable infrared free-electron laser (IR-FEL) operating at 13 MHz repetition rate. In spite of the long history of IR-FELs and their unique operational characteristics, no successful realization of CEP-stable operation has been demonstrated yet. Being CEP-unstable, IR-FEL radiation has so far only been used in non-coherent measurements without phase resolution. The technique demonstrated here is robust, operates easily at high-repetition rates and for short THz pulses, and enables common sequential field-resolved time-domain experiments. The implementation of such a technique at IR-FEL user end-stations will facilitate a new class of linear and non-linear experiments for studying coherent light-driven phenomena with increased signal-to-noise ratio.

Antiferromagnetic-Ferromagnetic Heterostructure-Based Field-Free Terahertz Emitters

Recently, ferromagnetic-heterostructure spintronic terahertz (THz) emitters have been recognized as one of the most promising candidates for next-generation THz sources, owing to their peculiarities of high efficiency, high stability, low cost, ultrabroad bandwidth, controllable polarization, and high scalability. Despite the substantial efforts, they rely on external magnetic fields to initiate the spin-to-charge conversion, which hitherto greatly limits their proliferation as practical devices. Here, a unique antiferromagnetic-ferromagnetic (IrMn₃ |Co₂₀ Fe₆₀ B₂₀ ) heterostructure is innovated, and it is demonstrated that it can efficiently generate THz radiation without any external magnetic field. It is assigned to the exchange bias or interfacial exchange coupling effect and enhanced anisotropy. By precisely balancing the exchange bias effect and enhanced THz radiation efficiency, an optimized 5.6 nm-thick IrMn₃ |Co₂₀ Fe₆₀ B₂₀ |W trilayer heterostructure is successfully realized, yielding an intensity surpassing that of Pt|Co₂₀ Fe₆₀ B₂₀ |W. Moreover, the intensity of THz emission is further boosted by togethering the trilayer sample and bilayer sample. Besides, the THz polarization may be flexibly controlled by rotating the sample azimuthal angle, manifesting sophisticated active THz field manipulation capability. The field-free coherent THz emission that is demonstrated here shines light on the development of spintronic THz optoelectronic devices.

Spatiotemporal modeling of direct acceleration with high-field terahertz pulses

We present an improved model for electron acceleration in vacuum with high-energy THz pulses that includes spatiotemporal effects. In our calculations, we examined the acceleration with 300 GHz and 3.0 THz central frequency THz pulses with properties corresponding to common sources, and compared the Gaussian and Poisson spectral amplitudes and the associated time profiles of the electric fields. Our calculation takes into account both the longitudinal field and the spatio-spectral evolution around the focus. These aspects of the model are necessary due to the tight focusing and the duration towards a single-cycle of the THz pulses, respectively. The carrier-to-envelope phase (CEP) and the tilting angle of the coincident few- or single-cycle THz pulses must be tuned in all cases in order to optimize the acceleration scheme. We reveal additionally that electron beams with different final energies and different divergences can be generated based on simulated THz pulses having different Porras factors, describing the frequency dependence of the spatiotemporal amplitude profile, which may depend strongly on the method used to generate the single-cycle THz pulses.

Waveguide structure based electron acceleration using terahertz pulses

We have developed a waveguide structure for electron acceleration using a few µJ energy THz pulse. The metallic device focuses the incoming linearly polarized nearly single-cycle THz pulse, hence increasing the peak electric field strength. We experimentally verified the gain and the temporal profile of the electric field in the structure using electro-optic sampling technique. The acceleration of the electron bunch from rest up to 8 keV was predicted using single-cycle THz pulses with µJ-energy level.

Parameter sensitivities in tilted-pulse-front based terahertz setups and their implications for high-energy terahertz source design and optimization

Despite the popularity and ubiquity of the tilted-pulse-front technique for single-cycle terahertz (THz) pulse generation, there is a deficit of experimental studies comprehensively mapping out the dependence of the performance on key setup parameters. The most critical parameters include the pulse-front tilt, the effective length of the pump pulse propagation within the crystal as well as effective length over which the THz beam interacts with the pump before it spatially walks off. Therefore, we investigate the impact of these parameters on the conversion efficiency and the shape of the THz beam via systematically scanning the 5D parameter space spanned by pump fluence, pulse-front-tilt, crystal-position (2D), and the pump size experimentally. We verify predictions so far only made by theory regarding the optimum interaction lengths and map out the impact of cascading on the THz radiation generation process. Furthermore, distortions imposed on the spatial THz beam profile for larger than optimum interaction lengths are observed. Finally, we identify the most sensitive parameters and, based on our findings, propose a robust optimization strategy for tilted-pulse-front THz setups. These findings are relevant for all THz strong-field applications in high demand of robust high-energy table-top single-cycle THz sources such as THz plasmonics, high-harmonic generation in solids as well as novel particle accelerators and beam manipulators.

Efficient multicycle terahertz pulse generation based on the tilted pulse-front technique

Controlling the time-domain oscillation of a terahertz (THz) wave offers promising capabilities for THz-based all-optical particle acceleration and strong-field THz nonlinear physics. However, the lack of highly efficient and frequency-modulable multicycle THz sources is impeding the spread of strong-field THz science and applications. Here, we show that by simply adding an echelon into a single-cycle THz source based on optical rectification in lithium niobate crystals via the tilted pulse-front technique, multicycle THz pulses can be efficiently generated with an 800 nm-to-THz efficiency of 0.1% at room temperature. The radiated THz properties can be engineered by precisely designing the echelon structure. Our proposed multicycle THz generation method has the advantages of high efficiency, ease of operation, and quick switching between single-cycle and multicycle working modes, all of which are important in the application of high-field THz radiation.

Highly efficient generation of GV/m-level terahertz pulses from intense femtosecond laser-foil interactions

The terahertz radiation from ultraintense laser-produced plasmas has aroused increasing attention recently as a promising approach toward strong terahertz sources. Here, we present the highly efficient production of millijoule-level terahertz pulses, from the rear side of a metal foil irradiated by a 10-TW femtosecond laser pulse. By characterizing the terahertz and electron emission in combination with particle-in-cell simulations, the physical reasons behind the efficient terahertz generation are discussed. The resulting focused terahertz electric field strength reaches over 2 GV/m, which is justified by experiments on terahertz strong-field-driven nonlinearity in semiconductors.

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Ultraintense laser-foil interactions generate a 2.1-mJ strong terahertz pulse

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Nearly 1% generation efficiency originates from optimized laser-plasma conditions

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2-GV/m high THz fields induce absorption bleaching and impact ionization

Highly efficient generation of narrowband terahertz radiation driven by a two-spectral-line laser in PPLN

We demonstrate record ∼0.9% efficiencies for optical conversion to narrowband (<1% relative bandwidth) terahertz (THz) radiation by strongly cascaded difference frequency generation. These results are achieved using a novel, to the best of our knowledge, laser source, customized for high efficiencies, with two narrow spectral lines of variable separation and pulse duration (≥250 ps). THz radiation generation in 5% MgO-doped periodically poled lithium niobate (PPLN) crystals of varying poling period was explored at cryogenic and room temperature operation as well as with different crystal lengths. This work addresses an increasing demand for high-field THz radiation pulses which has, up to now, been largely limited by low optical-to-THz radiation conversion efficiencies.

Generation of sub-100 fs electron pulses for time-resolved electron diffraction using a direct synchronization method

To investigate photoinduced phenomena in various materials and molecules, ultrashort pulsed x-ray and electron sources with high brightness and high repetition rates are required. The x-ray and electron's typical and de Broglie wavelengths are shorter than lattice constants of materials and molecules. Therefore, photoinduced structural dynamics on the femtosecond to picosecond timescales can be directly observed in a diffraction manner by using these pulses. This research created a tabletop ultrashort pulsed electron diffraction setup that used a femtosecond laser and electron pulse compression cavity that was directly synchronized to the microwave master oscillator (∼3 GHz). A compressed electron pulse with a 1 kHz repetition rate contained 228 000 electrons. The electron pulse duration was estimated to be less than 100 fs at the sample position by using photoinduced immediate lattice changes in an ultrathin silicon film (50 nm). The newly developed time-resolved electron diffraction setup has a pulse duration that is comparable to femtosecond laser pulse widths (35-100 fs). The pulse duration, in particular, fits within the timescale of photoinduced phenomena in quantum materials. Our developed ultrafast time-resolved electron diffraction setup with a sub-100 fs temporal resolution would be a powerful tool in material science with a combination of optical pump-probe, time-resolved photoemission spectroscopic, and pulsed x-ray measurements.

Experimental observation of optically generated unipolar electromagnetic precursors

It was recently predicted [Phys. Rev. A95(6), 063817 (2017) 10.1103/PhysRevA.95.063817] that an intense femtosecond laser pulse propagating in an electro-optic crystal and producing free carriers via multiphoton ionization can generate a unipolar electromagnetic precursor propagating ahead of the laser pulse. Here we report the experimental observation of this phenomenon in a GaP crystal excited by an amplified Ti:sapphire laser.

Spintronic terahertz emission with manipulated polarization (STEMP)

Highly efficient generation and arbitrary manipulation of spin-polarized terahertz (THz) radiation will enable chiral lightwave driven quantum nonequilibrium state regulation, induce new electronic structures, consequently provide a powerful experimental tool for investigation of nonlinear THz optics and extreme THz science and applications. THz circular dichromic spectroscopy, ultrafast electron bunch manipulation, as well as THz imaging, sensing, and telecommunication, also need chiral THz waves. Here we review optical generation of circularly-polarized THz radiation but focus on recently emerged polarization tunable spintronic THz emission techniques, which possess many advantages of ultra-broadband, high efficiency, low cost, easy for integration and so on. We believe that chiral THz sources based on the combination of electron spin, ultrafast optical techniques and material structure engineering will accelerate the development of THz science and applications.

High efficiency and collimated terahertz pulse from ultra-short intense laser and cone target

We propose a new, to the best of our knowledge, method to radiate a high-efficiency and collimated terahertz (THz) pulse from a relativistic femtosecond laser and cone target. Particle-in-cell simulations demonstrate that a THz source of 40 mJ, pointing at an angle of ∼20 ^∘, can be generated from a laser pulse of 1.9 J by using a cone target whose open angle is 10 ^∘. The peak power of the THz pulse is 10¹¹ W. This method, which manipulates the divergence angle and the energy conversion efficiency of the THz source, should promote THz science into the extra strong region with a compact laser system.

Uniformly scalable lithium niobate THz pulse source in transmission geometry

A novel THz source, based on optical rectification in LiNbO₃ using the tilted-pulse-front technique, is proposed and experimentally demonstrated. The pulse-front tilt is introduced by a volume phase holographic grating, efficiently used at perpendicular incidence in transmission, and the THz pulses are produced in a LiNbO₃ plane-parallel nonlinear echelon slab, arranged parallel to the grating. As a unique feature, the entire setup has a plane-parallel, transmission-type configuration, which straightforwardly enables distortion-free scaling to large sizes, high pulse energies and high THz field strengths. The possibility of operating the setup at cryogenic temperature for increased THz generation efficiency is also investigated. Calculations predict efficiencies of 95% for diffraction and 2% for THz generation at room temperature with a refractive-index-matching liquid between the grating and the echelon slab.

Electron Pulse Compression with Optical Beat Note

Compressing electron pulses is important in many applications of electron beam systems. In this study, we propose to use optical beat notes to compress electron pulses. The beat frequency is chosen to match the initial electron pulse duration, which enables the compression of electron pulses with a wide range of durations. This functionality extends the optical control of electron beams, which is important in compact electron beam systems such as dielectric laser accelerators. We also find that the dominant frequency of the electron charge density changes continuously along its drift trajectory, which may open up new opportunities in coherent interaction between free electrons and quantum or classical systems.

THz-driven dielectric particle accelerator on chip

Recently, terahertz (THz)-driven particle accelerators have drawn increasing attention. The development of high-energy-gain THz accelerators on chip has been a challenge. Here we propose a concept of an on-chip THz-driven particle accelerator that uses few-cycle THz pulses to drive dielectric prisms. It avoids the serious waveguide dispersion of previous THz linacs based on dielectric lined waveguides and enhances the electron-energy gain. In addition, we propose to use prism stacks to overcome the asynchronization effect when accelerating low-energy particles, by which a longer acceleration length with even higher energy gain can be realized. Compared with the available on-chip dielectric laser accelerators, the proposed scheme avoids serious dielectric dispersion and enhances accelerated bunch charge. Hence, it promises an attractive particle accelerator on chip.

3D-printed THz wave- and phaseplates

Three-dimensional printing based on fused deposition modeling has been shown to provide a cost-efficient and time-saving tool for fabricating a variety of THz optics for a frequency range of <0.2 THz. By using a broadband THz source, with a useful spectral range from 0.08 THz to 1.5 THz, we show that 3D-printed waveplates operate well up to 0.6 THz and have bandwidths similar to commercial products. Specifically, we investigate quarter- and half-waveplates, q-plates, and spiral phaseplates. We demonstrate a route to achieve broadband performance, so that 3D-printed waveplates can also be used with broadband, few-cycle THz pulses, for instance, in nonlinear THz spectroscopy or other THz high field applications.

Stable and Scalable Multistage Terahertz-Driven Particle Accelerator

Particle accelerators that use electromagnetic fields to increase a charged particle's energy have greatly advanced the development of science and industry since invention. However, the enormous cost and size of conventional radio-frequency accelerators have limited their accessibility. Here, we demonstrate a miniaccelerator powered by terahertz pulses with wavelengths 100 times shorter than radio-frequency pulses. By injecting a short relativistic electron bunch to a 30-mm-long dielectric-lined waveguide and tuning the frequency of a 20-period terahertz pulse to the phase-velocity-matched value, precise and sustained acceleration for nearly 100% of the electrons is achieved with the beam energy spread essentially unchanged. Furthermore, by accurately controlling the phase of two terahertz pulses, the beam is stably accelerated successively in two dielectric waveguides with close to 100% charge coupling efficiency. Our results demonstrate stable and scalable beam acceleration in a multistage miniaccelerator and pave the way for functioning terahertz-driven high-energy accelerators.

Limitation of THz conversion efficiency in DSTMS pumped by intense femtosecond pulses

Terahertz (THz) generation via optical rectification (OR) of near-infrared femtosecond pulses in DSTMS is systematically studied using a quasi-3D theoretical model, which takes into account cascaded OR, three-photon absorption (3PA) of the near-infrared radiation, and material dispersion/absorption properties. The simulation results and the comparison with experimental data for pump pulses with the center wavelength of 1.4 µm indicate that the 3PA process is one of the main limiting factors for THz generation in DSTMS at high pump fluences. The THz conversion efficiency is reduced further by the enhanced group velocity dispersion effect caused by the spectral broadening due to the cascaded OR. We predict that for broadband pump pulses with a duration of 30 fs, the THz conversion efficiency can be enhanced by a factor of 1.5 by using a positive pre-chirping that partially suppresses the cascaded OR and the 3PA effects.

Light by design: emerging frontiers in ultrafast photon sciences and light–matter interactions

Photon sciences and technologies establish the building blocks for myriad scientific and engineering frontiers in life and energy sciences. Because of their overarching functionality, the developmental roadmap and opportunities underpinned by photonics are often semiotically mediated by the delineation of subject areas of application. In this perspective article, we map current and emerging linkages between three intersecting areas of research stewarded by advanced photonics technologies, namely light by design, outlined as (a) quantum and structured photonics, (b) light–matter interactions in accelerators and accelerator-based light sources, and (c) ultrafast sciences and quantum molecular dynamics. In each section, we will concentrate on state-of-the-art achievements and present prospective applications in life sciences, biochemistry, quantum optics and information sciences, and environmental and chemical engineering, all founded on a broad range of photon sources and methodologies. We hope that this interconnected mapping of challenges and opportunities seeds new concepts, theory, and experiments in the advancement of ultrafast photon sciences and light–matter interactions. Through this mapping, we hope to inspire a critically interdisciplinary approach to the science and applications of light by design.

Systematic investigation of terahertz wave generation from liquid water lines

Understanding the process of terahertz (THz) wave generation from liquid water is crucial for further developing liquid THz sources. We present a systematic investigation of THz wave generated from laser-irradiated water lines. We show that water line in the diameter range of 0.1-0.2 mm generates the strongest THz wave, and THz frequency red shift is observed when diameter of the water line increases. The pump pulse energy dependence is decoupled from self-focusing effect by compensating the focal point displacement. As the pump pulse energy increases, saturation effect in THz peak electric field is observed, which can be mainly attributed to the intensity clamping effect inside the plasma and have never been reported previously, using water line or water film as the THz source. The proposed mechanism for saturation is supported by an independent measurement of laser pulse spectrum broadening. This work may help to further understand the laser-liquid interaction in THz generation process.

Jitter-free terahertz pulses from LiNbO3

Intense terahertz pulses are indispensable for modern science and technology, but time-critical applications require ultimate stability of the field cycles with respect to a reference clock. Here we report the nonlinear optical generation of terahertz single-cycle fields by femtosecond laser pulses under passive compensation of timing jitter. The converter is based on optical rectification in a LiNbO₃ slab with two silicon prisms for extracting and combining the emitted Cherenkov radiation from both sides into a single beam. In this way, we achieve suppression of timing jitter to <200 as/µm of beam displacement, a factor of >70 better than in conventional non-collinear geometries.

Selection rules for the orbital angular momentum of optically produced THz radiation

In this work, we theoretically study the transduction of orbital angular momentum (OAM) l for infrared pump lasers into the THz domain. In the case of optical rectification, the transduction of OAM occurs only through a spin-orbit interaction, with the selection rule on the OAM l=0 valid for any kind of polarization of the pump, which means that there is no transfer of OAM along the propagation axis. In difference frequency generation, the selection rule for the difference Δl between the OAM of the pump fields with linear or circular polarization is l=Δl, whereas l ranges from Δl-2 to Δl+2 in cases of both radial and azimuthal polarization. Moreover, for THz generation in the latter case, the high diffraction obtained with tightly focused pumps yields l tending to Δl±2, while l tends to zero in the opposite case of large pump beams.

Control of quantum electrodynamical processes by shaping electron wavepackets

Fundamental quantum electrodynamical (QED) processes, such as spontaneous emission and electron-photon scattering, encompass phenomena that underlie much of modern science and technology. Conventionally, calculations in QED and other field theories treat incoming particles as single-momentum states, omitting the possibility that coherent superposition states, i.e., shaped wavepackets, can alter fundamental scattering processes. Here, we show that free electron waveshaping can be used to design interferences between two or more pathways in a QED process, enabling precise control over the rate of that process. As an example, we show that free electron waveshaping modifies both spatial and spectral characteristics of bremsstrahlung emission, leading for instance to enhancements in directionality and monochromaticity. The ability to tailor general QED processes opens up additional avenues of control in phenomena ranging from optical excitation (e.g., plasmon and phonon emission) in electron microscopy to free electron lasing in the quantum regime.

Efficient generation of a high-field terahertz pulse train in bulk lithium niobate crystals by optical rectification

We demonstrate a highly efficient method for the generation of a high-field terahertz (THz) pulse train via optical rectification (OR) in congruent lithium niobate (LN) crystals driven by temporally shaped laser pulses. A narrowband THz pulse has been successfully achieved with sub-percent level conversion efficiency and multi MV/cm peak field at 0.26 THz. For the single-cycle THz generation, we achieved a THz pulse with 373-μJ energy in a LN crystal excited by a 100-mJ laser pulse at room temperature. The conversion efficiency is further improved to 0.77 % pumped by a 20-mJ laser pulse with a smaller pump beam size (6 mm in horizontal and 15 mm in vertical). This method holds great potential for generating mJ-level narrow-band THz pulse trains, which may have a major impact in mJ-scale applications like terahertz-based accelerators and light sources.

Terahertz aqueous photonics

Developing efficient and robust terahertz (THz) sources is of incessant interest in the THz community for their wide applications. With successive effort in past decades, numerous groups have achieved THz wave generation from solids, gases, and plasmas. However, liquid, especially liquid water has never been demonstrated as a THz source. One main reason leading the impediment is that water has strong absorption characteristics in the THz frequency regime.A thin water film under intense laser excitation was introduced as the THz source to mitigate the considerable loss of THz waves from the absorption. Laser-induced plasma formation associated with a ponderomotive force-induced dipole model was proposed to explain the generation process. For the one-color excitation scheme, the water film generates a higher THz electric field than the air does under the identical experimental condition. Unlike the case of air, THz wave generation from liquid water prefers a sub-picosecond (200-800 fs) laser pulse rather than a femtosecond pulse (~50 fs). This observation results from the plasma generation process in water.For the two-color excitation scheme, the THz electric field is enhanced by one-order of magnitude in comparison with the one-color case. Meanwhile, coherent control of the THz field is achieved by adjusting the relative phase between the fundamental pulse and the second-harmonic pulse.To eliminate the total internal reflection of THz waves at the water-air interface of a water film, a water line produced by a syringe needle was used to emit THz waves. As expected, more THz radiation can be coupled out and detected. THz wave generation from other liquids were also tested.