Quantum phase modulation and retrieval in an ultrafast transmission electron microscope

Development of analytical ultrafast transmission electron microscopy based on laser-driven Schottky field emission

We present a review on the development and applications of the ultrafast transmission electron microscopy (UTEM) at RIKEN. We introduce the UTEM system for the pump-probe TEM observation in a wide temporal range. By combining the UTEM and pixelated detector, we further develop five-dimansional scanning TEM (5D STEM) which provides the ultrafast nanoscale movie of physical quantities in nano-materials, such as crystal lattice information and electromagnetic field, by convergent-beam electron diffraction (CBED) and differential phase contrast (DPC) imaging technique. We show our recent results on the nanosecond-to-microsecond magnetic skyrmion dynamics observed by Lorentz TEM (LTEM) and photo-induced acoustic wave generation in picosecond regime by bright-field TEM and electron diffraction measurements by UTEM. We also show the demonstartion of the 5D STEM on the quantitative time (t)-dependent strain mapping by CBED with an accuracy of 4 ps and 8 nm, and the ultrafast demagnetization under zero magnetic field observed by DPC with 10 ns and 400 nm resolution.

Ultrafast transmission electron microscopy (UTEM) combines sub-picosecond time-resolution with the versatility of TEM spectroscopies. It allows us to study the ultrafast materials' response using complementary techniques. However, until now, time-resolved cathodoluminescence was unavailable in UTEM. In this paper, we report time-resolved cathodoluminescence measurements in an ultrafast transmission electron microscope. We mapped the spatial variations of the emission dynamics from nano-diamonds with a high density of NV centers with a 12 nm spatial resolution and sub-nanosecond temporal resolution. This development will allow us to study the emission dynamics from quantum emitters with a unique spatiotemporal resolution and benefit from the wealth of complementary signals provided by transmission electron microscopes. It will further expand the possibilities of ultrafast transmission electron microscopes, paving the way to the investigation of the quantum aspects of an electron/sample interaction.

Ultrafast condensed-matter science seeks to unravel how photoexcited electrons exchange energy and momentum with the lattice, setting the causal pathways for relaxation, coherent phonon generation, and light-driven changes of structure and function. Establishing such causality requires simultaneous access to electronic response and structural dynamics under truly identical excitation conditions. Here, we report the development of an ultrafast transmission electron microscopy (UTEM) with an integrated transient optical spectroscopy, enabling co-registered transient optical reflectance, and ultrafast electron diffraction/imaging/spectroscopy measurements from the same microscopic region while sharing the same pump condition. As a first application, we demonstrate its unique capability in deciphering coupled electron-lattice dynamics in layered MoS 2 and GaTe semiconductors. The MoS₂ illustrates a three-stage energy-flow scenario captured by joint optical and diffraction transients, whereas the GaTe shows coherent phonons that imprint on both dielectric and structural observables, with overlapping and mode-selective oscillatory components across the two signals. This multimodal instrument provides a powerful route to causality-resolved, quantitatively constrained studies of nonequilibrium energy flow and structural dynamics with high spatiotemporal resolution, bridging electronic and lattice degrees of freedom across diverse quantum and functional materials.

Ultrafast transmission electron microscopy (UTEM) has gained wide applications in the nanoscale dynamics with femtosecond, even attosecond, resolution. The instrument development is still in progress to satisfy the various applications. At Nankai University, we built an UTEM with a laser-driven Schottky field emitter based on a field emission TEM (Talos200i) of Thermo Fisher Scientific. This study comprehensively examines the performance of the UTEM, including the continuous mode and ultrafast photoemission mode. The investigation focuses on optimizing brightness, temporal resolution, energy dispersion, and stability in ultrafast photoemission mode, achieving a temporal resolution of ∼200fs and an energy dispersion of 0.7eV with excellent stability through careful adjustments of laser parameters and equipment settings. In scanning transmission electron microscopy mode, the beam size of the photoemission mode is ∼1.4nm at specific settings with potential for further improvement. As application examples, we illustrate the results of photoinduced structural dynamics of gold film and MoS2 thin flake by ultrafast electron diffraction. We also report the polarization dependent optical interference pattern characterized by the photoinduced near field microscopy effect in a silicon thin film sample prepared by the focused ion beam method. These findings provide valuable insights for researchers aiming to leverage the UTEM's capabilities for advanced electron microscopy applications and pave the way for future advancements in UTEM technology.

Tip-based photoemission electron sources offer unique properties for ultrafast imaging, diffraction, and spectroscopy experiments with highly coherent few-electron pulses. Extending this approach to increased bunch-charges requires a comprehensive experimental study on Coulomb interactions in nanoscale electron pulses and their impact on beam quality. For a laser-driven Schottky field emitter, we assess the transverse and longitudinal electron pulse properties in an ultrafast transmission electron microscope at a high photoemission current density. A quantitative characterization of electron beam emittance, pulse duration, spectral bandwidth, and chirp is performed. Due to the cathode geometry, Coulomb interactions in the pulse predominantly occur in the direct vicinity to the tip apex, resulting in a well-defined pulse chirp and limited emittance growth. Strategies for optimizing electron source parameters are identified, enabling advanced ultrafast transmission electron microscopy approaches, such as phase-resolved imaging and holography.

As charged particles surpass the speed of light in an optical medium they produce radiation - analogously to the way jet planes surpass the speed of sound and produce a sonic boom. This radiation emission, known as the Cherenkov effect, is among the most fundamental processes in electrodynamics. As such, it is used in numerous applications of particle detectors, particle accelerators, light sources, and medical imaging. Surprisingly, all Cherenkov-based applications and experiments thus far were fully described by classical electrodynamics even though theoretical work predicts new Cherenkov phenomena coming from quantum electrodynamics. The quantum description could provide new possibilities for the design of highly controllable light sources and more efficient accelerators and detectors. Here, we provide a direct evidence of the quantum nature of the Cherenkov effect and reveal its intrinsic quantum features. By satisfying the Cherenkov condition for relativistic electron wavefunctions and maintaining it over hundreds of microns, each electron simultaneously accelerates and decelerates by absorbing and emitting hundreds of photons in a coherent manner. We observe this strong interaction in an ultrafast transmission electron microscope, achieving for the first time a phase-matching between a relativistic electron wavefunction and a propagating light wave. Consequently, the quantum wavefunction of each electron evolves into a coherent plateau, analogous to a frequency comb in ultrashort laser pulses, containing hundreds of quantized energy peaks. Our findings prove that the delocalized wave nature of electrons can become dominant in stimulated interactions. In addition to prospects for known applications of the Cherenkov effect, our work provides a platform for utilizing quantum electrodynamics for applications in electron microscopy and in free-electron pump-probe spectroscopy.

Photon-induced near-field electron microscopy (PINEM), developed from ultrafast transmission electron microscopy, enables near-field imaging with nanometer spatial resolution and femtosecond temporal resolution. We report the plasmonic near-field distribution and lifetime analysis for an asymmetric copper bowtie nanostructure having unequal opening angles. The PINEM images show surface plasmon polaritons and local surface-plasmon resonance excitations with various polarizations. Combined with finite-element simulations, the polarization- and structure-dependent distribution and intensity variations of the near-field were analyzed. The lifetime difference of a plasmonic near-field excited by different polarizations is also discussed. The temporal and spatial characterization of the plasmonic near-field is important for the further studies of plasmonic near-field manipulation and designs of plasmonic devices having specific functions.

ABSTRACTWe report on the development of an ultrafast Transmission Electron Microscope based on a laser-driven cold-field emission source. We first describe the instrument before reporting on numerical simulations of the laser-driven electron emission. These simulations predict the temporal and spectral properties of the femtosecond electron pulses generated in our ultrafast electron source. We then discuss the effects that contribute to the spatial, temporal and spectral broadening of these electron pulses during their propagation from the electron source to the sample and finally to the detectors of the electron microscope. The spectro-temporal properties are then characterized in an electron/photon cross-correlation experiment based on the detection of electron energy gains. We finally illustrate the potential of this instrument for ultrafast electron holography and ultrafast electron diffraction.

We probe near-field THz pulses with nm-fs spatio-temporal resolution, through their interaction with free-electron pulses in an ultrafast transmission electron microscope. Our experiment can help understand THz generation and nanoscale THz phenomena.

We probe plasma dynamics with nm-fs resolution through electron spectroscopy performed in an ultrafast transmission electron microscope. This approach would allow to disentangle the dynamic evolution of plasma parameters and observe previously inaccessible energy inhomogeneities.

Coherent and incoherent effects on the imaging and scattering process in transmission electron microscopy and off-axis electron holography

We describe a new methodology that sheds light on the fundamental electronic processes that occur at the subsurface regions of inorganic solid photocatalysts. Three distinct kinds of microscopic imaging are used that yield spatial, temporal, and energy-resolved information. We also carefully consider the effect of photon-induced near-field electron microscopy (PINEM), first reported by Zewail et al. in 2009. The value of this methodology is illustrated by studying afresh a popular and viable photocatalyst, hematite, α-Fe2O3 that exhibits most of the properties required in a practical application. By employing high-energy electron-loss signals (of several hundred eV), coupled to femtosecond temporal resolution as well as ultrafast energy-filtered transmission electron microscopy in 4D, we have, inter alia, identified Fe4+ ions that have a lifetime of a few picoseconds, as well as associated photoinduced electronic transitions and charge transfer processes.

Abstract

The electromagnetic ‘ponderomotive force’ was discovered in 1957 in a radio-frequency experiment by Boot and Harvie 1. The force arises due to gradients in an inhomogeneous oscillating electric field, and can be used to control and accelerate electrons, with applications in ion traps, plasma accelerators and high-harmonic generation. However, being a second-order nonlinear effect in the applied field, the ponderomotive force is relatively weak. The availability of high-power, short laser pulses has enabled new practical realizations of this effect 2. In a Letter in this issue, Batelaan and co-workers predict that metal nanoantennas can produce sufficient ponderomotive force to deflect electrons 4. The phenomenon relies on the surface plasmon resonance properties of the nanoantenna, which produces a high local field enhancement combined with large field gradients in a small volume around the nanoantenna. To achieve the required field strengths, they propose to use high energy laser pulses of femtosecond time duration, tuned to the surface plasmon resonance wavelength. These short laser pulses offer yet another advantage, as they allow producing an ultrafast modulation of an incident electron beam. Such an electron switch would be of interest for a range of applications, for example ultrafast electron microscopy 4. Ultrafast electron microscopy is a powerful tool for characterizing chemical and physical processes at nanometer length scales and ultrafast time scales. Currently, the generation of ultrafast electron pulses is done by directly exciting the electron emission source with a high-power UV laser pulse. Plasmonic electron switches may be a feasibly alternative route toward the development of ultrafast electron beams at higher repetition frequencies and using lower-power femtosecond oscillators. As a first step, the authors measured the coherence time of surface plasmon modes in an array of nanoparticles, so-called optical antennas 3. A short, resonant laser pulse can drive a coherent plasmon oscillation, however this oscillation rapidly dephases via collisions of the electrons (intraband) or through excitation of electron-hole pairs (interband). Surface plasmons in isolated metal nanospheres have a dephasing time of several femtoseconds, depending on the type of metal antenna under study and details of the damping mechanism. Detailed measurements of plasmon dephasing were pioneered in the 1990s by Franz Aussenegg and co-workers, who used high harmonic generation as a nonlinear process to make an autocorrelation of the plasmon pulse 5. High-harmonic generation in nanoparticles is complicated, since second-harmonic requires noncentrosymmetric structures, while third harmonic signals are comparably weak. Becker et al. use a different, more direct method of measuring both the amplitude and phase of the plasmon field using cross-correlation of the plasmon field with a reference pulse with known width taken directly from the laser. The prospect of electron beam modulation using ultrafast plasmon fields is promising as a fundamental phenomenon and may further open up the field of ultrafast electron microscopy. Both the coherence time and local field enhancement can be tailored by the geometry of the nanoantenna, allowing for a rational design of nanostructures optimized for specific applications 6. Of particular interest will also be the application of techniques from coherent control to achieve local plasmonic hot-spots in both space and time 7, potentially resulting in even stronger field gradients at shorter time scales. Several hurdles will have to be overcome, related to the high field strengths required and the breakdown of metal nanoantenna arrays under such high power excitation. Similar problems are encountered in higher-harmonic generation experiments, and require careful optimization of material properties of the antennas and substrate. However, if successful, the plasmonic antenna approach may result in a new generation of electron modulation devices.