Ultrafast transmission electron microscopy: Principles and applications

Abstract

In the realm of condensed matter physics, the properties of material are largely determined by the degrees of freedom associated with lattice, charge, orbital, and spin, as well as their intricate interplay. Unveiling the coupling mechanisms between these freedoms and effectively manipulating them necessitate a profound exploration into ultrafast dynamics, owing to the nanoseconds, femtoseconds, and even attoseconds timescales governing these fundamental interactions. Ultrafast transmission electron microscopy (UTEM) emerges as a powerful technique enabling the research of ultrafast dynamics with exceptional spatial-temporal resolution, showcasing diverse applications within nanoscale systems. Over the past decades, UTEM has witnessed significant advancements in instrument development, fostering its widespread utilization across various domains. This review firstly introduces the fundamental principles of UTEM and traces its historical evolution, intricately involving the integration of the pump-probe principle with transmission electron microscopy. Subsequently, the key performance characteristics of UTEM are succinctly summarized. Moreover, a detailed exposition is provided on the manifold applications of UTEM, encompassing structural dynamics, magnetic phenomena, and near-field optics, each delineated according to their respective time-resolved methodologies. Concluding the review, a forward-looking perspective on the UTEM technique is presented, envisioning its significance in the realm of ultrafast dynamics research.

Graphical abstract

Keywords

transimission electron microscopy (TEM) / ultrafast transmission electron microscopy (UTEM) / scanning transimission electron microscopy (STEM)

1 Introduction

Transmission electron microscopy (TEM) has found wide-ranging applications in material science, chemistry, biology, and numerous other disciplines, offering exceptional spatial resolution down to the atomic scale [1]. In the past decades, in-situ TEM has emerged as a promising avenue, enabling the observation of fundamental processes under a perturbation to the sample such as gas exposure, heating/cooling, mechanical force, electric and so on. Continuous images and/or spectroscopy are acquired to track its evolution with temporal resolutions reaching into the millisecond and even microsecond timescales, limited by the frame rate of detectors [2]. To further enhance the temporal resolution capabilities of TEM beyond the realm of nanoseconds, the pump-probe technique has been integrated into TEM setups, enabling temporal resolutions down to femtosecond or even attosecond timescales [3−5]. This innovation has led to the development of ultrafast transmission electron microscopy (UTEM), a powerful technique that enables high-spatial-resolution investigation of ultrafast phenomena. With its exceptional capabilities, UTEM provides unprecedented insights into chemical reactions, phase transitions, and electron dynamics, offering new avenues to explore fundamental interactions in materials at unprecedented temporal and spatial scales.

In recent years, the growing interest in probing ultrafast dynamics at the nanoscale has been a driving force behind the evolution of UTEM technology. Significant progresses have been made in instrument development, enhancing the performance of UTEM in terms of time resolution, coherence, and energy resolution. In this review, we aim to provide a brief overview of the historical development of UTEM and highlight the progress in instrument design and capabilities.

The versatility of UTEM renders it well-suited for investigating a diverse array of ultrafast processes, leading to significant breakthroughs across various research domains. Researchers have achieved notable advancements in studying structural dynamics, such as photo-induced phase transitions and coherent phonon excitation, as well as delving into the interactions between light and spin, exemplified by optically-induced demagnetization. Additionally, UTEM has facilitated the exploration in near-field optics and the interplay between light and free electrons. Following the instrument section, we will delve into the wide-ranging applications of UTEM and discuss the evolution of time-resolved techniques harnessed through UTEM capabilities. By elucidating the instrumental advancements and methodological developments in this field, we aim to provide an overview of the expansive utility of UTEM in pushing the boundaries of ultrafast dynamics research.

In the concluding segment, we will summarize the key insights presented and offer a brief discussion on the future prospects of UTEM, highlighting its potential trajectory and impact on advancing the frontiers of ultrafast phenomena exploration. A full-fledged review of the rapidly growing field of UTEM is beyond the scope of this mini-review. To account for the whole works in this field, we refer to other reviews, including those by Campbell et al. [6], Zewail [7], Browning [8], Plemmons [9], Carbone [10], Montgomery [11], Shimojima [12], Alcorn [13], and so on.

2 Ultrafast transmission electron microscopy (UTEM)

2.1 Pump-probe integration with TEM

People are interested in enhancing the temporal resolution of TEM to ultrafast time scales (< ns), inspired by what has been achieved in optical pump-probe techniques. Therefore, the integration of pump-probe technique with TEM has been a topic of interest since 1980s [14, 15]. In the developmental trajectory, the initial step involves the generation of ultrashort electron pulses, a critical component for achieving high temporal resolution in UTEM setups. Two primary methods are commonly employed for introducing pulsed electrons into TEM instruments: the laser-triggered approach and the beam blanking method. These techniques play a fundamental role in shaping the temporal characteristics and performance capabilities of UTEM systems. Figure 1 illustrates the integration approach of the pump-probe principle with TEM instrument. The pulsed electrons can either be generated by probe laser pulses or e⁻ beam deflector.

The beam blanking method, initially implemented in TEM, has demonstrated the capability to achieve nanosecond time resolution [16]. The beam is deflected by applying an electronic pulse to the e⁻ beam deflector. Recent advancements have seen the integration of a radiofrequency (RF) cavity within the TEM gun, coupled with a small aperture, enabling a remarkable enhancement in temporal resolution to 10 picoseconds at gigahertz frequencies [17]. Furthermore, a pioneering concept and design have been proposed for a beam blanker featuring an integrated photoconductive switch. This novel configuration holds the promise of attaining an impressive 100 femtosecond temporal resolution [18]. Notably, the beam blanking method excels in preserving the coherence of the electron beam, thereby upholding a high level of spatial resolution critical for detailed investigations at the nanoscale.

The evolution of laser technology has paved the way for the utilization of photo-trigger cathodes as a viable option for generating pulsed electrons, leveraging both nanosecond and femtosecond lasers. In the early 2000s, dynamic transmission electron microscopy (DTEM) emerged as a groundbreaking technique by integrating nanosecond lasers, enabling single-shot electron imaging and diffraction capabilities [19, 20]. This innovation marked a significant milestone in real-time ultrafast imaging, allowing researchers to capture dynamic processes with nanosecond temporal resolution. Subsequent advancements in DTEM technology facilitates the transition to a movie mode, enabling the sequential capture of multiple frames on the same detector [21, 22]. This enhancement empowered researchers to investigate irreversible processes such as alloy nucleation and growth in detail, shedding light on intricate dynamics at the nanoscale level [23]. However, the single-shot imaging/diffraction mode in DTEM necessitates a substantial electron count, leading to space charge effects within the electron pulse and imposing limitations on temporal resolution, typically confined to the nanosecond range. To address this limitation and enhance temporal resolution in single-shot electron imaging/diffraction, RF compression techniques have been proposed for integration into UTEM setups. These techniques aim to compress the electron pulse down to femtosecond timescales at the sample position, akin to ultrafast electron diffraction instruments [24], demonstrating the possibility to improve the temporal resolution for single-shot electron imaging/diffraction.

In 2005, a significant advancement in UTEM was achieved when femtosecond laser was employed to generate electron pulses containing single or multiple electrons within a single pulse, effectively mitigating the space charge effect. This pioneering concept, demonstrated by the Zewail group, led to the realization of sub-picosecond temporal resolution in electron microscopy [25]. However, it is important to note that the equipment utilizing electron pulses with a limited number of electrons in each pulse is primarily suited for studying reversible processes where the excited state fully recovers between two consecutive pump pulses. This operational mode, known as pump-probe stroboscopic mode, enables researchers to investigate dynamic phenomena with high spatial-temporal resolution, effectively overcoming the challenges posed by the space charge effect.

Regarding the aspect of pump, laser pulses serve as common pump pulses due to their ultrashort pulse duration and seamless synchronization capabilities with probe pulses. The wavelength of the laser pulses can be precisely tuned across a wide spectrum, ranging from ultraviolet (UV) to terahertz (THz), allowing researchers to tailor the experimental setup according to the specific requirements of the research topic at hand. This flexibility in wavelength selection enables the investigation of a diverse range of dynamic processes with varying timescales, enhancing the versatility and applicability of UTEM. In addition to laser pulses, alternative stimuli such as electric pulses can also be employed to trigger dynamic processes in experiments [26]. By utilizing different types of stimulation pulses, researchers can explore a broader array of dynamic phenomena, further expanding the scope of investigations in UTEM.

2.2 Key performance characteristics of UTEM

UTEM shares many operational modes with traditional TEM, offering a wide array of techniques such as imaging, selected area electron diffraction (SAED), convergent beam electron diffraction (CBED), electron energy loss spectroscopy (EELS), scanning transmission electron microscopy (STEM), and holography. The versatility of UTEM allows for the application of numerous time-resolved methods, making it a valuable tool in various research fields.

The performance characteristics of UTEM, encompassing spatial resolution, temporal resolution, and energy resolution, are predominantly influenced by the electron beam source. For traditional TEM, electron guns are commonly classified into two main types: thermionic guns and field emission guns. Field emission guns are renowned for their superior performance characteristics compared to thermionic guns with better brightness, coherence, spatial resolution and energy resolution. When compared to conventional TEM, the average brightness in UTEM mode is typically lower by an order of magnitude because of the pulsed nature of electron source, irrespective of whether a field-emission gun or a thermionic electron gun is utilized [27]. In the case of thermionic guns, there may be a slight compromise in temporal resolution and coherence. However, thermionic UTEM systems offer the advantage of a higher total electron count compared to field-emission guns, owing to their larger emission cross-section (tens/hundreds of micrometers). The relatively low brightness of UTEM systems presents a challenge for detector systems, necessitating high quantum efficiency and low noise background detectors to achieve a favorable signal-to-noise ratio. Overcoming these challenges in detector design is crucial for optimizing the performance of UTEM systems and unlocking their full potential in ultrafast dynamics research.

The spatial resolution of UTEM can achieve sub-nanometer scale when operated at high repetition rates, enabling the resolution of crystalline lattices [7]. However, imaging lattices using UTEM under a pump laser excitation is more challenging compared to traditional TEM, primarily due to stability and brightness limitations. In the STEM mode, the electron beam can be focused to approximately 1 nm, indicating a possible nanometer spatial resolution in STEM based techniques [28].

When equipped with a field-emission gun, the temporal resolution of UTEM can reach below 200 femtoseconds without the need for additional pulse compression [28]. The temporal resolution for field-emission guns is slightly superior to that of thermionic guns due to the larger extraction field around the gun, which helps minimize the energy dispersion’s impact on pulse broadening [29]. However, the temporal resolution can be degraded to hundreds of femtoseconds when performing imaging and diffraction since achieving an acceptable signal-to-noise ratio often requires a higher number of electrons within one pulse. Various techniques have been proposed to further enhance the temporal resolution of UTEM, including THz compression (from 930 fs to 75 fs) [30], RF electron guns (~100 fs) [31], and MeV electron microscopy (10 ps, single shot resolution) [32], although these methods are not yet widely implemented in UTEM instruments. Recent advancements have showcased the generation of attosecond electron pulses through the modulation of electron−photon interaction, offering exciting prospects for studying electron dynamics with attosecond resolution [5, 33].

Electron energy spectroscopy has emerged as a powerful tool for investigating electron−photon interaction and laser-matter interaction in UTEM [34]. Energy resolutions below 1 eV have been achieved in UTEM. However, energy broadening can occur due to factors such as energy dispersion from the photoemission process, space charge effects, and equipment stability limitations [28, 35]. The energy resolution of UTEM is comparable to that of TEM without a monochromator. However, the use of a monochromator in UTEM is challenging due to the limited number of electrons. To further improve energy resolution, a THz pulse has been proposed to compress the electron energy dispersion without compromising the quantity of electrons [36].

Stability is a critical concern when operating in UTEM mode, particularly due to the typically long acquisition time. Magnetic and mechanical stability, temperature control, air flow management, laser pointing stability, photoemission stability, and electron optics stability are all crucial factors that need to be carefully managed in UTEM systems to ensure reliable and high-quality experimental results.

2.3 Applications of UTEM

To a significant extent, the capabilities of TEM can be expanded to enable time-resolved studies of dynamic phenomena. In this section, we will present the diverse applications of UTEM, categorized by the techniques employed. These include ultrafast imaging, ultrafast diffraction, and ultrafast Lorentz transmission electron microscopy (ULTEM), as well as 5D-STEM and photo-induced near-field microscopy (PINEM).

2.4 Ultrafast electron imaging and diffraction

Upon photoexcitation, the atomic structure undergoes energy transfer from the electronic system to the phonon system, resulting in the generation of coherent phonons, phase transitions, and other structural responses. The study of structural dynamics enables the investigation of electron−phonon coupling and phonon−phonon scattering processes in various intriguing materials. Ultrafast electron diffraction (UED) has been extensively employed to study structural dynamics in solid state materials, gases, and liquid samples in reciprocal space since the 1980s [37, 38]. In TEM, SAED is a typical technique used to analyze the crystalline structure of materials. UTEM in SAED mode is analogous to UED and has achieved sub-picosecond temporal resolution [39].

In comparison to UED, UTEM offers a versatile and relatively cost-effective approach to simultaneously combine imaging, diffraction, and spectroscopy. While UED can provide brighter images at lower repetition rates due to the high-performance of RF photocathode gun, the UTEM platform allows for kinetic studies at small-sized samples, including individual nanoparticles. This allows for the investigation of size-dependent, and defect-dependent behaviors that are often obscured in conventional UED studies. Furthermore, the high spatial resolution of UTEM enables the possibility of studying nanoscale dynamics in imaging mode. Figure 2(a) demonstrates a graphitized carbon lattice with a high spatial resolution of 3.4 angstroms, acquired by UTEM [7]. The high spatiotemporal resolution provides a unique means to capture phonon dynamics at nanoscale materials, enabling the tracking of coherent phonon dynamics under pulsed laser excitation and the investigation of how nanostructures influence phonon nucleation and propagation. For instance, Flannigan et al. [40] imaged the photoexcited strain wave of multilayer 2H-MoS₂. They observed that step edges and terraces in crystals with fewer layers allow increased freedom of motion compared to regions away from defects, leading to preferential excitation of phonon modes. Ultrafast dark-field (UDF) imaging, in comparison to ultrafast bright-field (UBF) imaging, tends to capture crystal structure information with higher contrast. Zewail et al. [41] selectively imaged specific crystal planes to track phase transitions and structural dynamics within single-spin crossover (SCO) nanoparticles of metal-organic frameworks. Ji et al. [42] demonstrated the influence of local strain on the photo-induced structural phase transition in Td-WTe₂ by combining ultrafast electron diffraction and UDF imaging. By employing a customized dark-field aperture array, Danz et al. [43] tracked the evolution of charge-density wave domains with simultaneous femtosecond temporal and 5-nanometer spatial resolution, elucidating relaxation pathways and domain wall dynamics in 1T-TaS₂. Real-space and real-time imaging of the formation, stabilization, and relaxation processes in CDW domains provided direct insights into the phase transition mechanisms following optical excitation [Fig. 2(c)]. Wang et al. [44] utilized UDF imaging to investigate the selective excitation of bending/torsional modes in Au nanoprisms, and found that internal stresses at the corners were responsible for this behavior [Fig. 2(d)]. Additionally, they studied the vibrational dynamics in Au heterodimers composed of nanospheres and nanoprisms on an Si₃N₄ substrate and discovered a frequency coupling of acoustic vibrations between the two nanoparticles [45]. Lobastov et al. [46] demonstrated the application of UTEM in photo-induced phase transition, studying the temporal evolution of the structure order parameter and real-space images of vanadium dioxide [Fig. 2(e)]. Most recently, Alfred et al. [47] characterized the structural dynamics of a free-standing antiferromagnetic film, FePS₃, in both reciprocal space and real space using ultrafast electron diffraction and microscopy in transmission geometry as shown in Fig. 2(f). Ultrafast demagnetization releases the built-in micro strain and reduces the monoclinic angle, leading to large-amplitude, coherent shearing oscillations throughout the photoexcited volume.

Electron tomography techniques are extended to time domain. Using 4D electron tomography, which combine volume imaging with ultrafast time resolution, Kwon et al. [48] revealed the structural and morphological dynamics of multiwalled carbon nanotubes and observed nanotube’s oscillation frequency was about tens of megahertz [Fig. 2(b)]. Aided by 4D reconstruction, Kwon et al. [50] visualized anisotropic spatiotemporal behavior in black phosphorus such as bulging, buckling, and flattening which is driven by impulsive thermal stress upon photoexcitation in real time.

As for irreversible processes, the analytical pulse must contain enough electrons to capture transient events. Using a photoemitted electron pulse to probe dynamic events with “snapshot”, diffraction and imaging can be detected at nanosecond resolution inside of a dynamic TEM (DTEM) [51]. Kim et al. [23] observed metastable morphologies and reaction fronts during rapid phase transitions in reactive multilayer foils (RMLFs) composed of five Al/Ni_0.91V_0.09 bilayers. As shown in Fig. 2(g), the dynamic single-shot diffraction patterns and images capture the structural evolution of RMLFs before, during, and after the passage of the exothermic mixing reaction front.

The combination of in-situ holder with UTEM greatly extended the dynamic research topic. One example is the application of liquid environment [49, 52, 53]. As shown in Fig. 2(h), Fu et al. [49, 52] demonstrated the first liquid-phase UTEM technique and studied the gold nanoparticles dynamics in liquid, revealing ballistic and superfast diffusive rotations and translations of the nanoparticles at different timescales in water induced by a strong driving force arising from the photoinduced steam nanobubbles near the nanoparticle surface.

2.5 Ultrafast Lorentz TEM

Topological spin structures such as magnetic skyrmions and vortices are considered as one of the ideal information carriers in non-volatile data memory with high speed, high density, and low power consumption, while the key for developing their applications is the understanding and control of their ultrafast dynamic behaviors including the fast generation, in-plane motion, annihilation and spin oscillation etc. in real time and space [52, 53]. Intriguingly, combing the Fresnel Lorentz phase imaging methodology with the UTEM can enable imaging of magnetic structures with nanometer and fs spatiotemporal resolutions, namely, ultrafast Lorentz transmission electron microscopy (ULTEM). The high spatiotemporal resolution makes ULTEM an ideal platform for the investigation of magnetization dynamics of topological spin structures.

The first ULTEM experiment was demonstrated by Zewail group and has been used to observe the magnetization inversion and domain wall motion dynamics in ferromagnetic thin films under the out-of-focus Fresnel imaging mode [54]. The nanoscale-fs Lorentz phase imaging of photo-induced magnetization dynamics in topological spin structures was achieved by Schäfer group [Fig. 3(a)], in which they obtained fs demagnetization and remagnetization dynamics of a single magnetic vortex in a permalloy (Ni₈₁Fe₁₉) disk under femtosecond laser excitation [55]. Using the similar ultrafast Lorentz phase imaging mode, Carbone group has realized nanoscale-fs imaging of breathing, rotation and magnetization dynamics of the hexagonal skyrmion lattice in FeGe thin films under fs laser stimulus [Fig. 3(b)] [58]. Most recently, Shimojima et al. [57] have observed the processes of skyrmion proliferation, contraction, drift and coalescence in defect-introduced Co₉Zn₉Mn₂ sample at nanosecond time scale with nanosecond photothermal excitation, providing the physical picture of dynamical life cycle of magnetic skyrmions. Moreover, by combing two coherent fs laser pulses for transient grating pumping within the ULTEM, Cao et al. [60] have realized controlling and detecting the magnetization dynamics in Ni₈₀Fe₂₀ thin films with combined nanometer and picosecond resolutions, revealing local magnetization, precession frequency and relevant decay factors of the sample. On the other hand, by introducing radio frequency (RF) stimulus to the sample in the ULTEM, Möller et al. [61] have realized ultrafast Lorentz phase imaging on topological spin structures and directly captured the RF-induced gyration dynamics of a magnetic vortex core in a permalloy square disk [Fig. 3(c)]. The above dynamical studies demonstrate the powerful and unique capability of the ULTEM in revealing the dynamic response of topological spin structures under ultrafast multi-field modulations at high spatiotemporal resolution. Because of the unprecedented spatiotemporal resolution, ULTEM has become a unique and powerful tool for the investigation of microscopic dynamics of magnetic structures, especially the nanoscale topological spin structures with great potential in the applications of spintronic devices.

2.6 5D-STEM

4D-STEM (Four-dimensional STEM) is an advanced electron microscopy technique that integrates real-space scanning with momentum-space diffraction pattern acquisition, enabling comprehensive materials characterization at the nanoscale. This approach has garnered significant attention in recent years due to its versatility in structural analysis, electric/magnetic field mapping, and low-dose imaging of beam-sensitive materials. To extend the temporal resolution of conventional 4D-STEM, the emerging 5D-STEM methodology synergizes 4D-STEM with ultrafast optical pump-probe techniques. This innovative integration facilitates dynamic high-resolution mapping of lattice dynamics, magnetic domain evolution, and electric field distributions in nanomaterials, as illustrated in Fig. 4(a). The principle involves utilizing ultrashort electron pulses for two-dimensional scanning across a sample while simultaneously recording electron diffraction or deflection signals at each scanning point, thereby acquiring five-dimensional information: spatial coordinates (x, y), time (t), and momentum (k_x, k_y) [appearing in Fig. 4(b)].

The development of 5D-STEM can be traced back to 2009, when Zewail et al. [63] first proposed the concept and used CBED to observe the propagation of acoustic waves in silicon. Instead of using parallel electron-beam illumination with a single-electron wave vector, they applied a convergent electron beam on the specimen to achieve high-precision determination of the 3D structures of local areas. Ropers et al. [62] further developed the ultrafast CBED (U-CBED) technique to probe the strain dynamics in nanostructures induced by optical excitation. They demonstrated its capabilities by investigating the ultrafast acoustic deformations in a single-crystalline graphite membrane and quantitatively mapped the strain changes and the dispersion relation of acoustic waves along arbitrary directions [62]. The U-CBED technique was also applied to a patterned bilayer system consisting of a semiconductor membrane with a platinum stripe [63]. This revealed a multi-modal distortion wave propagating through the membrane, primarily driven by local rotations with minor strain and shear contributions. The results demonstrate the significant effects of nanoscale geometries and interfacial coupling mechanisms in heterogeneous material systems.

In 2022, Nakamura et al. [66] extended U-CBED to 5D-STEM-CBED, enabling quantitative time-domain strain mapping at the nanoscale [Figs. 4(c)−(g)]. Their work demonstrated the generation and propagation of optically induced acoustic wavesin nanofabricated silicon thin plates with a tungsten disk acting as a source of acoustic phonon. Through CBED analysis, they quantitatively determined the polarization and amplitude of these waves as they propagated along the silicon plate. Additionally, by applying Fourier transform analysis, they resolved the strain distribution in momentum-frequency space, thereby deriving the dispersion relations along arbitrary crystallographic directionsin the plate.

5D-STEM is an innovative technique that allows for the observation of ultrafast dynamics of nanomaterials with high spatial and temporal resolution. Unlike conventional STEM methods, it can capture the transient changes in lattice structure such as strain, phase transitions, and domain formation in nanomaterials induced by optical excitation. This approach is particularly valuable for studying complex nanofabricated systems. Moreover, 5D-STEM can probe diverse nonequilibrium phenomena such as dielectric responses and magnetic dynamics [67]. With ongoing improvements such as the electron source, detectors, and synchronization systems, 5D-STEM holds the potential to further enhance its spatiotemporal resolution, thereby offering new perspectives and opportunities for ultrafast science.

3 Photon-induced near-field electron microscopy

Barwick et al. [34] demonstrated that free electrons can interact with optical near-field mediated by nanostructures due to the satisfied energy-momentum conservation condition in UTEM and developed photon-induced near-field electron microscopy (PINEM). In PINEM, electrons pass through the near-field and absorb/lose integer multiples of photons, resulting in the symmetrical sideband peaks on both sides of the zero-loss peak in electron energy spectrum [Fig. 5(a)]. PINEM is also a near field imaging technique. When only the electrons with an increased energy are selected by an energy filtering slit, the spatial distribution of the optical near-field can be imaged at the nanoscale.

As an ultrafast imaging technique, PINEM has been widely used to study the ultrafast evolution of optical near-field in physics, biology, materials and other fields, giving an enhancing image contrast with nanometer-femtosecond resolution [68−86]. Compared to bright field TEM image, PINEM image can give an enhancing contrast of weak contrast structures such as protein vesicles [71], cells [78, 81], graphene-layered steps [87] and grain boundaries on sliver film [88] due to the near field on these structures. On the other hand, the high temporal and spatial resolution of PINEM is powerful for studying the generation, propagation and decay of polaritons, such as surface plasmon polaritons skyrmions in gold film [89] [Fig. 5(b)] and phonon-polariton in hBN [83, 90] [Fig. 5(c)], allowing us to directly capture polariton dynamics, obtain their group velocity and dispersion relationship and discover exotic nonlinear optical phenomena in polaritons. By changing the wavelength of the pump laser, polaritons in the spectral range from visible to far-infrared can be studied with meV resolution [76, 80, 88]. Compared with time-resolved photoemission electron microscopy (TR-PEEM) and time-resolved scanning near-field optical microscopy (TR-SNOM), the analysis of transmitted electrons allows PINEM to study the polaritons at the buried interfaces and avoid the influence of the SNOM tips on the intrinsic polaritons signal, which is a non-destructive detection method [92−94].

Free-electron−photon interaction is a quantum coherent phase modulation of electrons, providing a way to control the electron wave function on the period of attosecond scale [5, 76, 80, 95−98]. In 2015, Feist et al. [99] focused the electron pulse to 15 nm, extended the duration of the pump laser pulse to 3.4 ps, and allowed the electrons to interact with the uniform light field in time and space to achieve free electron rabbinic oscillation [Fig. 5(d)], which experimentally proved that light coherently reshapes electron density due to quantum walk. By changing the photon statistics, the electron evolves into an entangled joint state with the photons and can change from quantum walk to classical walks, meaning that the quantum statistics of photons can be imprinted on free electrons [100] [Fig. 5(e)]. Realizing the coherent transfer between electrons and light and creating unique optical quantum states through free electrons are of great significance for quantum optics [101−103]. As a method for modulating the electron wave function, researchers have achieved strong longitudinal and transverse phase modulation of the electron, showing light-driven Ramsey-type phase control [95], multi-color interactions [5], nonlinear interactions [85, 104] and electron vortex beam [76, 105] [Fig. 5(f)], Hermite-Gaussian beam [106], tunable spatial modulation of free electrons [89], even polarizing electron beams using optical near fields [107]. Coherent shaping electrons are expected to be used in atomic-resolution quantum measurement [108], phase-resolved imaging [109], electron beam aberration correction [110] and other areas.

In order to better control free electrons, it is necessary to achieve strong free electron−photon interactions. Optical cavities are widely used to enhance the interaction between light and matter. In 2020, Wang et al. and Kfir et al. [111] greatly enhanced the coupling of electrons and photons through the optical Bloch mode of photonic crystals [Fig. 5(g)] and the optical whisper gallery mode of dielectric microsphere cavity [112] [Fig. 5(h)], respectively. In addition to strong field enhancement, increasing the distance which electrons interact with photons while satisfying phase matching can also enhance the coupling of electron-photon. By interacting with an evanescent field generated by a laser which is totally internally reflected from a planar interface, Dahan et al. [113] achieved electron energy combs with a bandwidth of more than 1700 eV [Fig. 5(i)]. By increasing the light field intensity and interaction distance simultaneously, integrated photonics [114] and dielectric laser accelerators [100, 115, 116] enable the control of continuous beam electrons.

Recently, three different methods have been proposed to implement attosecond electron microscopy. Several theoretical and experimental works have demonstrated that probe electron pulses can be modulated into attosecond electron pulse trains after interacting with photons and propagating over a well-defined distance in UTEM [5, 96, 99, 117]. Nabben et al. [118] modulated the continuous electron beam into electron pulses with attosecond duration by a continuous-wave laser and used a second continuous-wave laser to pump the sample so that the optical responses within one cycle of pump laser can be recorded by an imaging energy filter [Fig. 5(j)]. However, this method can only provide dynamics information in the optical cycle, since the pump light is a continuous laser. Photon gating is another way to improve the time revolution of UTEM [65, 79, 116, 117]. In this method, the temporal duration of probe pluses is actually determined by the gating laser pulse due to the electron–light coupling. Fu et al. [82] increased the temporal resolution of UTEM to 50 fs and captured the dielectric response of Mott insulators. Hui et al. [121] used a polarization gating (PG) process to generate a 625 attosecond optical gating pulse so that they can image the field-induced electron dynamics in neutral multilayer graphene with attosecond time resolution [Fig. 5(k)]. The time delay between the gating pulses and pump pulses can be adjusted independently, so that the dynamics of longer time scales can be probed. But only 0.1% of the total electrons can be gated. Free-electron homodyne detection is the third way to achieve attosecond electron microscopy [119]. Gaida et al. [122] used a phase-controlled reference interaction and free-electron quantum-state reconstruction to readout of coherent amplitude or phase modulations of the free-electron wave function, imaging the plasmonic fields with attosecond temporal resolution without a need for electron density bunching [Fig. 5(l)].

PINEM provides a method for controlling free electrons through the field strength, polarization, cycle, phase, orbital angular momentum, and spatial distribution of the laser, enabling imaging of optical near-field such as polaritons at the femtosecond- nanometer scale. PINEM is no longer limited to UTEM and has achieved the interaction of slow electrons [123] and photon in SEM [124] and point-projection microscope [125]. Free electrons can even interact with photons without near-field mediation [126]. Further improvement of the temporal and spatial resolution of UTEM through PINEM, study of exotic optical phenomena of other polaritons, study of new phenomena of interaction between slow electrons and photons, and realizing more degrees of freedom interaction between electrons and photons still need to be explored.

4 Outlooks

This review highlights the versatility of UTEM and its growing importance across multiple disciplines in various fields such as physics, chemistry, material science, and biology due to its ability to capture ultrafast dynamics with high spatial and temporal resolution. However, despite its strengths, UTEM also faces certain limitations that researchers are actively working to address.

One key challenge in UTEM is the low brightness of the electron beam in UTEM mode, which can impact the quality of images and diffraction patterns obtained. To ensure reversibility and avoid sample damage in stroboscopic mode, the repetition rate of laser pulses is typically kept below the MHz range. And the probe laser power is kept relatively low to avoid significant space charge effect induced temporal elongation and energy dispersion broadening. The conflicts between the resolution and brightness make it quite challenging to realize the spatial resolution as the conventional TEM mode in practice. To enhance the capabilities of UTEM, researchers are exploring new detector technologies, such as direct electron detectors, which offer the potential for high-resolution imaging and spectroscopy at lower electron doses. Additionally, techniques like RF/THz compressors [31, 32] are being investigated to further improve the temporal resolution of UTEM experiments.

Exciting developments in UTEM include the modulation of electrons by lasers to generate attosecond electron pulses, opening up new opportunities to study ultrafast dynamics at the attosecond scale. Moreover, the use of specialized in-situ holders that provide electric or magnetic stimuli, control environmental conditions like liquid or atmosphere, apply strain fields, or facilitate precise heating and cooling can offer researchers a way to explore complex ultrafast dynamics under specific experimental conditions [124]. By addressing the challenges and leveraging emerging technologies and techniques, researchers are pushing the boundaries of what is possible with UTEM, paving the way for new discoveries and insights into ultrafast processes across a wide range of scientific disciplines.

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