Photon-induced Near-field Electron Microscopy Research Articles

Unveiling Spatial and Temporal Dynamics of Plasmon-Enhanced Localized Fields in Metallic Nanoframes through Ultrafast Electron Microscopy.

Plasmonic nanomaterials, particularly noble metal nanoframes (NFs), are important for applications such as catalysis, biosensing, and energy harvesting due to their ability to enhance localized electric fields and atomic efficiency via localized surface plasmon resonance (LSPR). Yet the fundamental structure-function relationships and plasmonic dynamics of the NFS are difficult to study experimentally and thus far rely predominately on computational methodologies, limiting their utilization. This study leverages the capabilities of ultrafast electron microscopy (UEM), specifically photon-induced near-field electron microscopy (PINEM), to probe the light-matter interactions within plasmonic NF structures. The effects of shape, size, and plasmonic coupling of Pt@Au core-shell NFs on spatial and temporal characteristics of plasmon-enhanced localized electric fields are explored. Importantly, time-resolved PINEM analysis reveals that the plasmonic fields around hexagonal NF prisms exhibit a spatially dependent excitation and decay rate, indicating a nuanced interplay between the spatial geometry of the NF and the temporal evolution of the localized electric field. These results and observations uncover nanophotonic energy transfer dynamics in NFs and highlight their potential for applications in biosensing and photocatalysis.

Coherent electron phase-space manipulation by combined elastic and inelastic light-electron scattering

Abstract Photon-Induced Near-field Electron Microscopy (PINEM), Kapitza-Dirac (KD) gratings, and ponderomotive phase plates are examples of techniques in which the wave function of an electron in free space is manipulated using light fields: Free Electron Quantum Optics (FEQO). These effects are usually treated in separate theoretical frameworks. In this paper we present a unified, two-pronged framework that can be used to describe and numerically evaluate the performance of a number of FEQO-based electron-optical elements. The first part is an extension of an existing analytical treatment of PINEM, based on solving a relativistically corrected Schrödinger equation. The extension covers both second-order contributions to PINEM and the Kapitza-Dirac effect. The second, novel element of the approach is based on electron wavefront reconstruction by evaluating the quantum mechanical phase along a bundle of classical electron trajectories. The quasi-classical (but fully relativistic) approach lends itself to simulating a wide variety of FEQO devices, including the examples mentioned. We apply both approaches to a few specific experimental configurations: mirror-based first-order PINEM, second-order PINEM in very high laser intensity, and Kapitza-Dirac diffraction. The results show excellent agreement between the analytical results and the quasi-classical simulations. Finally, we propose a setup that combines KD and PINEM to allow for simultaneous coherent energy and transverse momentum shaping of an electron beam, and present simulation results thereof.

Photon-Induced Near-Field Electron Microscopy of Nanostructured Metallic Films and Membranes

Photon-Induced Near-Field Electron Microscopy of Nanostructured Metallic Films and Membranes

Topologically Protected Strong-Interaction of Photonics with Free Electrons.

We propose a robust scheme of studying the strong interactions between free electrons and photons using topological photonics. Our study reveals that the topological corner state can be used to enhance the interaction between light and a free electron significantly. The quality factor of the topological cavity can exceed 20 000 and the corner state has a very long lifetime even after the pump pulse is off. And thus, the platform enables us to achieve a strong interaction without the need for zero delay and phase matching as in traditional photon-induced near-field electron microscopy (PINEM). This work provides the new perspective that the topological photonic structures can be utilized as a platform to shape free electron wave packets, which facilitates the control of quantum electrodynamical (QED) processes and quantum optics with free electrons in the future.

Enhanced Free-Electron Wavefunction Modulation via Photon-Induced Near-Field Electron Microscopy (PINEM) with Shaped Light Fields

Enhanced Free-Electron Wavefunction Modulation via Photon-Induced Near-Field Electron Microscopy (PINEM) with Shaped Light Fields

Free-electron Ramsey-type interferometry for enhanced amplitude and phase imaging of nearfields.

The complex range of interactions between electrons and electromagnetic fields gave rise to countless scientific and technological advances. A prime example is photon-induced nearfield electron microscopy (PINEM), enabling the detection of confined electric fields in illuminated nanostructures with unprecedented spatial resolution. However, PINEM is limited by its dependence on strong fields, making it unsuitable for sensitive samples, and its inability to resolve complex phasor information. Here, we leverage the nonlinear, overconstrained nature of PINEM to present an algorithmic microscopy approach, achieving far superior nearfield imaging capabilities. Our algorithm relies on free-electron Ramsey-type interferometry to produce orders-of-magnitude improvement in sensitivity and ambiguity-immune nearfield phase reconstruction, both of which are optimal when the electron exhibits a fully quantum behavior. Our results demonstrate the potential of combining algorithmic approaches with state-of-the-art modalities in electron microscopy and may lead to various applications from imaging sensitive biological samples to performing full-field tomography of confined light.

Weak measurements and quantum-to-classical transitions in free electron–photon interactions

How does the quantum-to-classical transition of measurement occur? This question is vital for both foundations and applications of quantum mechanics. Here, we develop a new measurement-based framework for characterizing the classical and quantum free electron–photon interactions and then experimentally test it. We first analyze the transition from projective to weak measurement in generic light–matter interactions and show that any classical electron-laser-beam interaction can be represented as an outcome of weak measurement. In particular, the appearance of classical point-particle acceleration is an example of an amplified weak value resulting from weak measurement. A universal factor, exp left(-{Gamma }^{2}/2right), quantifies the measurement regimes and their transition from quantum to classical, where Gamma corresponds to the ratio between the electron wavepacket size and the optical wavelength. This measurement-based formulation is experimentally verified in both limits of photon-induced near-field electron microscopy and the classical acceleration regime using a DLA. Our results shed new light on the transition from quantum to classical electrodynamics, enabling us to employ the essence of the wave-particle duality of both light and electrons in quantum measurement for exploring and applying many quantum and classical light–matter interactions.

Lorentz microscopy of optical fields

In electron microscopy, detailed insights into nanoscale optical properties of materials are gained by spontaneous inelastic scattering leading to electron-energy loss and cathodoluminescence. Stimulated scattering in the presence of external sample excitation allows for mode- and polarization-selective photon-induced near-field electron microscopy (PINEM). This process imprints a spatial phase profile inherited from the optical fields onto the wave function of the probing electrons. Here, we introduce Lorentz-PINEM for the full-field, non-invasive imaging of complex optical near fields at high spatial resolution. We use energy-filtered defocus phase-contrast imaging and iterative phase retrieval to reconstruct the phase distribution of interfering surface-bound modes on a plasmonic nanotip. Our approach is universally applicable to retrieve the spatially varying phase of nanoscale fields and topological modes.

Capturing the non-equilibrium state in light–matter–free-electron interactions through ultrafast transmission electron microscopy

Ultrafast transmission electron microscope (UTEM) with the multimodality of time-resolved diffraction, imaging, and spectroscopy provides a unique platform to reveal the fundamental features associated with the interaction between free electrons and matter. In this review, we summarize the principles, instrumentation, and recent developments of the UTEM and its applications in capturing dynamic processes and non-equilibrium transient states. The combination of the transmission electron microscope with a femtosecond laser via the pump–probe method guarantees the high spatiotemporal resolution, allowing the investigation of the transient process in real, reciprocal and energy spaces. Ultrafast structural dynamics can be studied by diffraction and imaging methods, revealing the coherent acoustic phonon generation and photo-induced phase transition process. In the energy dimension, time-resolved electron energy-loss spectroscopy enables the examination of the intrinsic electronic dynamics of materials, while the photon-induced near-field electron microscopy extends the application of the UTEM to the imaging of optical near fields with high real-space resolution. It is noted that light–free-electron interactions have the ability to shape electron wave packets in both longitudinal and transverse directions, showing the potential application in the generation of attosecond electron pulses and vortex electron beams.

Imaging the field inside nanophotonic accelerators

Controlling optical fields on the subwavelength scale is at the core of nanophotonics. Laser-driven nanophotonic particle accelerators promise a compact alternative to conventional radiofrequency-based accelerators. Efficient electron acceleration in nanophotonic devices critically depends on achieving nanometer control of the internal optical nearfield. However, these nearfields have so far been inaccessible due to the complexity of the devices and their geometrical constraints, hampering the design of future nanophotonic accelerators. Here we image the field distribution inside a nanophotonic accelerator, for which we developed a technique for frequency-tunable deep-subwavelength resolution of nearfields based on photon-induced nearfield electron-microscopy. Our experiments, complemented by 3D simulations, unveil surprising deviations in two leading nanophotonic accelerator designs, showing complex field distributions related to intricate 3D features in the device and its fabrication tolerances. We envision an extension of our method for full 3D field tomography, which is key for the future design of highly efficient nanophotonic devices.

One-Dimensional “Ghost Imaging” in Electron Microscopy of Inelastically Scattered Electrons

Entanglement and correlation are at the basis of quantum mechanics and have been used in optics to create a framework for “ghost imaging”. We propose that a similar scheme can be used in an electron microscope to exploit the correlation of electrons with the coincident detection of collective mode excitations in a sample. In this way, an image of the sample can be formed on an electron camera even if electrons never illuminated the region of interest directly. This concept, which can be regarded as the inverse of photon-induced near-field electron microscopy, can be used to probe delicate molecules with a resolution that is beyond the wavelength of the collective mode.

Plasmonic near-field spatiotemporal characterizations of an asymmetric copper bowtie nanostructure

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.

Loss effects on quantum surface plasmon polaritons excited by a traveling electron beam

We present the modeling of a quantum regime for surface plasmon polaritons (SPPs) excited by an electron beam skimming parallel to the surface of a metallic structure. The theoretical approach resembles that used to describe the quantum Cherenkov radiation in which the quantized free electrons interact with a classical radiation field. In this paper, we rephrase the model in a more rigorous way, considering the detrimental effects of losses on coherent light. In the quantum regime of SPPs, each electron emits a single photon due to the transition between two successive momentum states. It is shown that the quantum nature of SPPs is realized in the low beam current limit where the radiation (gain in the field strength) operates in discrete frequency bands with a remarkably narrow linewidth. When losses are negligible, the photon emission occurs in periodic bursts along the interaction length. We show also that the Ohmic loss effects in the SPP process set an intrinsic limit on the coherent production of photons and also have severe detrimental effects on the radiation intensity. The findings of this study can describe recent experimental observations of the surface plasmonic near-field based on the photon-induced near-field electron microscopy.

Tunable photon-induced spatial modulation of free electrons

Spatial modulation of electron beams is an essential tool for various applications such as nanolithography and imaging, yet its conventional implementations are severely limited and inherently non-tunable. Conversely, proposals of light-driven electron spatial modulation promise tunable electron wavefront shaping, for example, using the mechanism of photon-induced near-field electron microscopy. Here we present tunable photon-induced spatial modulation of electrons through their interaction with externally controlled surface plasmon polaritons (SPPs). Using recently developed methods of shaping SPP patterns, we demonstrate a dynamic control of the electron beam with a variety of electron distributions and verify their coherence through electron diffraction. Finally, the nonlinearity stemming from energy post-selection provides us with another avenue for controlling the electron shape, generating electron features far below the SPP wavelength. Our work paves the way to on-demand electron wavefront shaping at ultrafast timescales, with prospects for aberration correction, nanofabrication and material characterization.

Miniature light-driven nanophotonic electron acceleration and control

Dielectric laser accelerators (DLAs) are fundamentally based on the interaction of photons with free electrons, where energy and momentum conservation are satisfied by mediation of a nanostructure. In this scheme, the photonic nanostructure induces near-fields which transfer energy from the photon to the electron, similar to the inverse-Smith–Purcell effect described in metallic gratings. This, in turn, may provide ground-breaking applications, as it is a technology promising to miniaturize particle accelerators down to the chip scale. This fundamental interaction can also be used to study and demonstrate quantum photon-electron phenomena. The spontaneous and stimulated Smith–Purcell effect and the photon-induced near-field electron-microscopy (PINEM) effect have evolved to be a fruitful ground for observing quantum effects. In particular, the energy spectrum of the free electron has been shown to have discrete energy peaks, spaced with the interacting photon energy. This energy spectrum is correlated to the photon statistics and number of photon exchanges that took place during the interaction. We give an overview of DLA and PINEM physics with a focus on electron phase-space manipulation.

Chiral assemblies of pinwheel superlattices on substrates.

The unique topology and physics of chiral superlattices make their self-assembly from nanoparticles highly sought after yet challenging in regard to (meta)materials1-3. Here we show that tetrahedral gold nanoparticles can transform from a perovskite-like, low-density phase with corner-to-corner connections into pinwheel assemblies with corner-to-edge connections and denser packing. Whereas corner-sharing assemblies are achiral, pinwheel superlattices become strongly mirror asymmetric on solid substrates as demonstrated by chirality measures. Liquid-phase transmission electron microscopy and computational models show that van der Waals and electrostatic interactions between nanoparticles control thermodynamic equilibrium. Variable corner-to-edge connections among tetrahedra enable fine-tuning of chirality. The domains of the bilayer superlattices show strong chiroptical activity as identified by photon-induced near-field electron microscopy and finite-difference time-domain simulations. The simplicity and versatility of substrate-supported chiral superlattices facilitate the manufacture of metastructured coatings with unusual optical, mechanical and electronic characteristics.

Defocus Phase Contrast in Photon-Induced Near-field Electron Microscopy

Defocus Phase Contrast in Photon-Induced Near-field Electron Microscopy

Energy-filtered dark-field imaging of nanoparticles by PINEM in 4D electron microscopy

Time-resolved dark-field imaging of alpha Fe2O3 nanoparticle and Ag nanowires using scattered electrons by selected crystal planes are realized by photon-induced near-field electron microscopy (PINEM) selecting probing electrons which absorb energy from a transient laser field during their passage through the target particles in a four-dimensional transmission electron microscope (4D-TEM). The high laser fluence illuminated on the particles causes significant part of probing swift electrons exchange energy with the laser light, creating enough PINEM electrons required for the dark-field imaging of particles with high spatiotemporal resolution at nanometer and femtosecond scale. Different from the bright-field PINEM imaging of particles where the outerspace with a close distance to the particle are illuminated by transmitted PINEM electrons, illumination is confined on the particles by selected scattered PINEM electrons, leading to a much more defined and sharp imaging of particles compared with a bright-field PINEM image. In combination with PINEM temporal gating and dark-field selective imaging, the PINEM dark-field imaging technique in the 4D-TEM enables the studies of structural dynamics of selective crystal planes or elements with high spatiotemporal resolution.

Accurate time zero determination in an ultrafast transmission electron microscope without energy filter

In ultrafast transmission electron microscopy, time zero can be accurately determined by making use of the photon-induced near-field electron microscopy (PINEM) effect, which causes electrons interacting with the near fields of a nanoparticle to coherently gain or lose energy in multiples of the photon energy when the laser pump and electron probe pulse overlap in time. If the instrument is not equipped with an energy filter, which is required to observe the PINEM effect, the response of a sample is frequently monitored instead. However, the gradual or delayed onset of this response can render an accurate measurement as challenging. Here, we demonstrate a simple and accurate method for determining time zero without an energy filter that is based on the observation that the outline of a nanoparticle blurs when the electron and laser pulse overlap in time. We show that this phenomenon arises from the PINEM effect, which causes some electrons to gain a large energy spread, thus blurring the image due to the chromatic aberration of the imaging system. This effect can also be used to characterize the instrument response and determine the laser polarization in situ. Furthermore, it may find applications for mapping out the near fields of a nanoparticle without the help of an energy filter.

Nanoscale-Femtosecond Imaging of Evanescent Surface Plasmons on Silver Film by Photon-Induced Near-Field Electron Microscopy.

Surface plasmons on silver nanostructures have a broad range of tunable resonance properties in visible and near-infrared regimes, which possess wide applications in nanophotonics and optoelectronics. Here we use a femtosecond laser to excite surface plasmons on a silver film and trace the subsequent transient dynamics via photon-induced near-field electron microscopy (PINEM). A polarization experiment of PINEM demonstrates a conspicuous polarization dependence of the transient surface plasmon field on the silver film; however, unlike silver nanowires and nanorods, there is no polarization dependence for the PINEM intensity. This compelling finding suggests a thin film platform can be more easily used to identify the temporal and spatial overlaps between the pump laser and probe electron pulses in 4D ultrafast electron microscopy (UEM). Our work illustrates the femtosecond excitation and transient behavior of the surface plasmons on silver film and paves a universal, simple way for identifying the time zero in 4D UEM.