All-Optical Scheme for Generation of Isolated Attosecond Electron Pulses

Field-Induced Rocking-Curve Effects in Attosecond Electron Diffraction

Recent advances in electron microscopy trigger the question of whether attosecond electron diffraction can resolve atomic-scale electron dynamics in crystalline materials in space and time. Here, we explore the ultrafast dynamics of the relevant electron-lattice scattering process. We drive a single-crystalline silicon membrane with the optical cycles of near-infrared laser light and use phase-locked attosecond electron pulses to produce electron diffraction patterns as a function of delay. For all Bragg spots, we observe time-dependent intensity changes and position shifts that are correlated with a time shift of 0.5-1.2  fs. For single-cycle excitation pulses with strong peak intensity, the correlations become nonlinear. The origins of these effects are local and integrated beam deflections by the optical electric and magnetic fields at the crystal membrane. Those deflections modify the diffraction intensities in addition to the atomic structure factor dynamics by time-dependent rocking-curve effects. However, the measured time delays and symmetries allow one to disentangle both effects. Future attosecond electron diffraction and microscopy experiments need to be based on these results.

Self-Trapping of Slow Electrons in the Energy Domain

The interaction of light and swift electrons has enabled phase-coherent manipulation and acceleration of electron wave packets. Here, we investigate this interaction in a new regime where low-energy electrons (∼20-200  eV) interact with a phase-matched light field. Our analytical and one-dimensional numerical study shows that slow electrons are subject to strong confinement in the energy domain due to the nonvanishing curvature of the electron dispersion. The spectral trap is tunable and an appropriate choice of light field parameters can reduce the interaction dynamics to only two energy states. The capacity to trap electrons expands the scope of electron beam physics, free-electron quantum optics and quantum simulators.

Ultrashort and high-collimation X/γ-rays generated by nonlinear inverse Thomson scattering between off-axis electrons and circularly polarized intense laser pulses

The properties of nonlinear inverse Thomson scattering (NITS) are investigated in the collision between a circularly polarized tightly focused intense laser pulse and a relativistic off-axis electron with numerical simulations. Due to the asymmetric effect of the laser field on the off-axis electrons, the electron trajectory is torqued to the off-axis direction, and the symmetry of the spatial radiation is also destroyed, which causes the concentrations of the radiation in the off-axis direction. With the increase of laser intensity, the torsion effect is more obvious, the radiation collimation improves, the direction turns to sideways. With the increase of electron's initial energy, the direction turns back to backwards and the degree of off-axis effect decreases. In both cases, the power exponentially enhances, the pulse width shortens, the spectrum broadens and super-continuity appears. With the laser intensity, the duration of sideways X-ray pulse from the low-energy (2.61MeV) electron is only 0.2 as, and the normalized intensity reaches 109. While using ultra-high-energy (100MeV) electrons, the duration of backwards γ-ray pulse reaches 1.22 zs, and the normalized intensity reaches 1017. These results help the understanding of nonlinear Thomson scattering and provide important numerical references for the research of NITS as high-quality X-ray and γ-ray sources.

Attosecond electron microscopy of sub-cycle optical dynamics

The primary step of almost any interaction between light and materials is the electrodynamic response of the electrons to the optical cycles of the impinging light wave on sub-wavelength and sub-cycle dimensions¹. Understanding and controlling the electromagnetic responses of a material^2-11 is therefore essential for modern optics and nanophotonics^12-19. Although the small de Broglie wavelength of electron beams should allow access to attosecond and ångström dimensions²⁰, the time resolution of ultrafast electron microscopy²¹ and diffraction²² has so far been limited to the femtosecond domain^16-18, which is insufficient for recording fundamental material responses on the scale of the cycles of light^1,2,10. Here we advance transmission electron microscopy to attosecond time resolution of optical responses within one cycle of excitation light²³. We apply a continuous-wave laser²⁴ to modulate the electron wave function into a rapid sequence of electron pulses, and use an energy filter to resolve electromagnetic near-fields in and around a material as a movie in space and time. Experiments on nanostructured needle tips, dielectric resonators and metamaterial antennas reveal a directional launch of chiral surface waves, a delay between dipole and quadrupole dynamics, a subluminal buried waveguide field and a symmetry-broken multi-antenna response. These results signify the value of combining electron microscopy and attosecond laser science to understand light-matter interactions in terms of their fundamental dimensions in space and time.

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.

Asynchronous Inelastic Scattering of Electrons at the Ponderomotive Potential of Optical Waves

We study free electron dynamics during inelastic interaction with the ponderomotive potential of a traveling optical wave using classical and quantum-mechanical models. We show that in the strong interaction regime, the electrons trapped in the periodic potential oscillate leading to periodic revolutions of sharp peaks of the density distributions in the real and momentum spaces. In this regime, the synchronicity between the velocity of the optical wave and the electron propagation velocity is not required. Asynchronous interaction enables acceleration or deceleration of a significant fraction of the electrons to a final spectrum with a relative spectral width of 0.5%-2.5%. This technique allows one to accelerate electrons from rest to keV energies while reaching a narrow spectrum of kinetic energies and femtosecond pulsed operation.

Real-time ultrafast oscilloscope with a relativistic electron bunch train

The deflection of charged particles is an intuitive way to visualize an electromagnetic oscillation of coherent light. Here, we present a real-time ultrafast oscilloscope for time-frozen visualization of a terahertz (THz) optical wave by probing light-driven motion of relativistic electrons. We found the unique condition of subwavelength metal slit waveguide for preserving the distortion-free optical waveform during its propagation. Momentary stamping of the wave, transversely travelling inside a metal slit, on an ultrashort wide electron bunch enables the single-shot recording of an ultrafast optical waveform. As a proof-of-concept experiment, we successfully demonstrated to capture the entire field oscillation of a THz pulse with a sampling rate of 75.7 TS/s. Owing to the use of transversely-wide and longitudinally-short electron bunch and transversely travelling wave, the proposed “single-shot oscilloscope” will open up new avenue for developing the real-time petahertz (PHz) metrology.

A travelling wave inside a metal slit can reveal its own waveform by probing deflecting motions of charged particles. Here, a real-time THz oscilloscope was demonstrated by utilizing the relativistic electrons and the subwavelength slit waveguide.

Space-Time Wave Packets from Smith-Purcell Radiation

Space-time wave packets are electromagnetic waves with strong correlations between their spatial and temporal degrees of freedom. These wave packets have gained much attention for fundamental properties like propagation invariance and user-designed group velocities, and for potential applications like optical microscopy, micromanipulation, and laser micromachining. Here, free-electron radiation is presented as a natural and versatile source of space-time wave packets that are ultra-broadband and highly tunable in frequency. For instance, ab initio theory and numerical simulations show that the intensity profile of space-time wave packets from Smith-Purcell radiation can be directly tailored through the grating properties, as well as the velocity and shape of the electron bunches. The result of this work indicates a viable way of generating space-time wave packets at exotic frequencies such as the terahertz and X-ray regimes, potentially paving the way toward new methods of shaping electromagnetic wave packets through free-electron radiation.

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.

Optical Modulation of Electron Beams in Free Space

We exploit free-space interactions between electron beams and tailored light fields to imprint on-demand phase profiles on the electron wave functions. Through rigorous semiclassical theory involving a quantum description of the electrons, we show that monochromatic optical fields focused in vacuum can be used to correct electron beam aberrations and produce selected focal shapes. Stimulated elastic Compton scattering is exploited to imprint the required electron phase, which is proportional to the integral of the optical field intensity along the electron path and depends on the transverse beam position. The required light intensities are attainable in currently available ultrafast electron microscope setups, thus opening the field of free-space optical manipulation of electron beams.

Single-Cycle Optical Control of Beam Electrons

We report the single-cycle optical control of a freely propagating electron beam with an isolated cycle of midinfrared light. In particular, we produce and characterize a modulated electron current with peak-cycle-specific subfemtosecond structure in time. The direct effects of the carrier-envelope phase, amplitude, and dispersion of the optical waveform on the temporal composition, pulse durations, and chirp of the free-space electron wave function demonstrate the subcycle nature of our control. These results and concept may create novel opportunities in free-electron lasers, laser-driven particle accelerators, ultrafast electron microscopy, and wherever else high-energy electrons are needed with the temporal structure of single-cycle light.

The Development of Ultrafast Electron Microscopy

Time-resolved electron microscopy is based on the excitation of a sample by pulsed laser radiation and its probing by synchronized photoelectron bunches in the electron microscope column. With femtosecond lasers, if probing pulses with a small number of electrons—in the limit, single-electron wave packets—are used, the stroboscopic regime enables ultrahigh spatiotemporal resolution to be obtained, which is not restricted by the Coulomb repulsion of electrons. This review article presents the current state of the ultrafast electron microscopy (UEM) method for detecting the structural dynamics of matter in the time range from picoseconds to attoseconds. Moreover, in the imaging mode, the spatial resolution lies, at best, in the subnanometer range, which limits the range of observation of structural changes in the sample. The ultrafast electron diffraction (UED), which created the methodological basis for the development of UEM, has opened the possibility of creating molecular movies that show the behavior of the investigated quantum system in the space-time continuum with details of sub-Å spatial resolution. Therefore, this review on the development of UEM begins with a description of the main achievements of UED, which formed the basis for the creation and further development of the UEM method. A number of recent experiments are presented to illustrate the potential of the UEM method.