Nanophotonic structures driven with ultrashort laser pulses are ideally suited to control electrons on ultrafast time scales. We will give an overview over on our quest to build a particle accelerator on a chip. Next to acceleration elements, this entails beam steering and beam confinement, all with optical nearfield forces. We will demonstrate attosecond pulse train generation as well as alternating phase focusing, representing complex electron phase space control mandatory for any particle accelerator. Furthermore, we will show free space control of an electron beam based on two-photon—electron interactions, rendering the otherwise required nearfield structures obsolete. Next to large achievable gradients, we will also show attosecond pulse train generation.

Over last decades, electron microscopy has become a very powerful and versatile technique for nano- or atomic-scale imaging and spectroscopy [1]. Major advancements and outstanding capabilities have been made possible thanks to better spatial and temporal control over the amplitude and phase of the wave function that characterizes the fast electrons used as sample probes. Control over the beam shape is commonly achieved using complex arrangements of magneto- and electrostatic electron lenses that enable sub-Ångstrom focusing and beam scanning, as well as correcting aberrations of electron optics. The phase of the electron wave function can be additionally modified by introducing static phase plates.

We envision an alternative to traditional electron-optics elements, materialized in the concept of the optically-driven electron modulator that enables dynamical shaping of electron-beam wave functions both in space and time. This approach capitalizes recent experimental demonstrations of wave function control through optical fields [2] combined with ultrafast control over the electron-light interaction [3-5]. Specifically, we propose two types of schemes to realize optical control over the electron beam shape: a photonic aberration corrector (PAC) that exploits the interaction of the electron with light scattered from a thin film; and an optical free-space electron modulator (OFEM) operating in free space. Based on realistic designs combined with detailed simulations, we demonstrate an application with high potential for improving the resolution of electron microscopes, whereby the electron-light interaction is used to correct for common aberrations introduced by electro- or magneto-static lenses in current setups [6]. In addition, we demonstrate the possibility of generating exotic electron beam shapes [6,7], with the extra advantage that fast control over such shapes is inherent to the optical elements used in our designs.

Our theoretical work suggests that the proposed PAC and OFEM elements could offer better versatility and compactness with respect to traditional static electron phase plates and corrector designs, and we foresee that they could open a new era of electron microscopy, both in aberration correction and in the generation of on-demand electron beams.

The optical shaping of free-electron beams enables a broad range of applications, from free-space acceleration [1] and attosecond bunching of electrons [2] to the implementation of laser-driven phase plates [3,4] and beam splitters [5]. Despite recent progress towards phase-matched and high-efficiency coupling [8,9], inelastic electron light scattering (IELS) and electron energy gain spectroscopy (EEGS) [10,11] typically require femtosecond high-intensity laser pulses, precluding broad usage in state-of-the-art continuous-beam electron microscopes.

Here, we show IELS on a CW-pumped Si₃N₄ microresonator with a Q-factor of >10⁵, demonstrating μeV-EEGS spectroscopy and achieving an unprecedented high coupling to a continuous electron beam.

In a custom modified Schottky-field-emission TEM [12], a continuous electron beam interacts with the optical whispering gallery mode confined in a fiber-coupled Si₃N₄ microresonator chip (fabricated in the photonic Damascene process [13], linewidth of ~390 MHz and free spectral range of ~1 THz for the quasi-TM fundamental mode). When the CW laser is tuned to a resonance of the cavity, the initially narrow energy distribution is significantly broadened. At an electron energy of 115 kV, a spectral width of ~160 eV is observed for only 4 mW of optical power coupled to the microresonator. Detuning the laser frequency enables spectral characterization of the resonance, yielding a 3.1-μeV effective linewidth in EEGS. Finally, the interaction strength between the electrons and the evanescent cavity field is mapped by energy-filtered imaging, revealing a rich spatial interaction pattern, resulting from the interplay of phase-matched electron-light interaction and the three-dimensional mode profile.

Combining electron microscopy with integrated photonics opens up new experimental pathways, ranging from versatile light-driven electron phase plates to free-electron cavity quantum optics.

Recent theoretical works on the interaction of free electrons with light[1],[2] predicted how the interaction is altered by the quantum properties of light, leading to new exciting application such as electron-electron entanglement[2], new types of quantum light source[3], novel ideas for electron shaping[4], and quantum state tomography of light[5]. Yet, to this day – in all experiments involving the interaction between free electrons and light – the light acted as a classical wave, disregarding its quantum nature.

We observe the effect of the quantum statistics of photons on free-electron interactions with light. Our study shows interactions for different statistics passing continuously from Poissonian up to thermal statistics, unveiling a surprising manifestation of Bohr's Correspondence Principle: the continuous transition from quantum walk[6],[7] to classical random walk of a free electron on the energy ladder. The electron walker serves as the probe in non-destructive quantum detection experiments, measuring the photon statistics as well as degrees of coherence g2(0) and higher-orders gn(0). Unlike conventional quantum-optical detectors, the electron can perform both quantum weak measurements[8] and projective measurements of light by evolving into an entangled joint-state with the photons.

We achieve this free-electron–quantum-light interaction using the inverse design of silicon-photonic nanostructures recently used for miniaturizing particle accelerators[10]. We use these nanostructures in a transmission electron microscope (TEM) to facilitate a strong free-electron–light interaction, i.e., each electron exchanges multiple photons with the light field. Such strong interactions were previously only realized with intense laser pulses in photon-induced nearfield electron microscopy (PINEM). In contrast with such intense pulses that can be considered as coherent states, all other states of light are usually not so intense – necessitating an optimized interaction in specialized nanostructures.

The interaction efficiency is high enough to enable the use of continuous-wave (CW) light, while still maintaining the strong interaction. Recent experimental works have shown that even weak CW electron–light interactions (up to one photon absorbed/emitted by the electron) have intriguing applications[10],[11] Our experiment offers an avenue for taking these ideas forward to regimes of stronger interactions with CW light. Our findings suggest hitherto inaccessible concepts in quantum optics: free-electron-based non-destructive quantum tomography of light, even with ultrafast modulation of the photon statistics or the field quadratures[5]. The study of high-efficiency electron–light coupling constitutes an important step towards combined attosecond-temporal and sub-Å-spatial resolution microscopy.

Quantum carpets [1] - the spatio-temporal de Broglie density profiles- woven by an atom or an electron in the near-field region of a diffraction grating bring to light [2], in real time, the decoherence of each individual component of the interference term of the Wigner function characteristic of Schrödinger cats. The proposed experiments [2] are feasible with present-day technology.
 

Fig. captionQuantum carpet in the absence of decoherence

References:

[1] M. Berry, I. Marzoli, and W. Schleich, Quantum Carpets, Carpets of Light, Physics World

    14, 39-44 (2001)

[2] P. Kazemi, S. Chaturvedi, I. Marzoli, R.F. O’Connell, and W.P. Schleich, Quantum

    carpets - a tool to observe decoherence, New J. Phys. 15, 013052 (2013)

Photonics with superior properties can be implemented in a variety of old (silicon, silicon nitride) and new (silicon carbide, diamond) photonic materials by combining state of the art optimization and machine learning techniques (photonics inverse design) with new fabrication approaches. In addition to making  photonics more robust to errors in fabrication and temperature, more compact, and more efficient, this approach is also crucial for enabling new photonics applications, such as on chip laser driven particle accelerators, and semiconductor quantum simulators.   

We present an experimental demonstration of an integrated particle accelerator based on a silicon chip that was designed using a photonic inverse-design approach. By comparing the measured electron energy spectra with particle-tracking simulations, we infer an acceleration gradient of 30.5 mega–electron volts per meter. On-chip acceleration provides the possibility for a completely integrated mega–electron volt-scale accelerator.

Emerging research on organic semiconductors increasingly points to control of ordering as key to extending the distances achievable for exciton transport by orders of magnitude. However, sources of crystallographic disorder are not well described, and the critical link between structural order and disorder and local or non-local changes in the optical states remains unresolved. Electron microscopy techniques combining high resolution electron energy loss spectroscopy (EELS) with diffraction contrast imaging and nanobeam scanning electron diffraction techniques present crucial routes to addressing these questions, despite the sensitivity of organic semiconductors to electron beam exposure. Here, spatially resolved aloof beam scanning with <30 meV resolution EELS provides insight into spectroscopic changes at singlet exciton energies co-located with linear crystal defects observed in a perylene diimide crystal. These observations are contextualised through comparisons between two derivatives with distinctive terminal functional groups. Detection of vibrational signatures with high-resolution EELS to evaluate beam-induced damage will also be presented alongside connections to ongoing work to unravel the precise crystal structures, disorder, and its pronounced effect on optoelectronic properties.

Photon bunching in cathodoluminescence reveals fundamental properties of excitation of matter with high-energy electrons. Previous work has shown that strong bunching g⁽²⁾(0)≫1 of CL photons can occur when exciting a sample with high-energy electrons, due to the fact that a single collision cascade can generate multiple CL photons [1,2]. Until now, photon bunching in CL has been modelled using Monte-Carlo simulations, which are time-consuming and do not provide a full overview of the parameters that play a role. Moreover, CL photon bunching has been limited to the study of continuous and ns-pulsed electron beams.

In this work we develop a fully analytical model to retrieve the amplitude and shape of g⁽²⁾(τ)  upon excitation with a continuous, ns-pulsed and ultrashort (ps) pulsed electron beam [3]. The model accounts for the stochastic nature of the different processes in the sample upon excitation with high-energy electrons. We obtain a simple expression relating the amplitude of bunching g⁽²⁾(0)  with electron beam current, emitter decay lifetime, pulse duration, in the case of a pulsed electron beam, and electron excitation efficiency (γ). The latter is defined as the probability that an electron creates at least one interaction with the emitter [4].

Our model shows good agreement with experimental CL data on InGaN/GaN quantum wells (QWs) obtained with a scanning electron microscope using continuous and pulsed electron beams. We evaluate two different regimes of pulsed electron beams, either with ns pulses (6-500 ns pulse width), obtained using a beam-blanker, or ultrashort (≤ps ) pulses, performed by means of a laser-driven electron cathode. In the experiments we obtain electron excitation efficiencies of the QWs of γ=0.13  and γ=0.05  for 10 and 8 keV electrons, respectively. Moreover, the QWs exhibit a linear response despite excitation with dense ultrashort electron pulses (∼500  electrons per pulse), for which we calculate a maximum carrier efficiency of 10¹⁸cm^-3 .

Overall, this work provides an extensive analytical model to evaluate the photon statistics of incoherent cathodoluminescence using three different electron beam configurations. The simplicity of the model makes CL autocorrelation measurements more accessible, thus further enabling a quantitative analysis of electron-matter interaction inside an electron microscope.

Electron energy loss spectroscopy (EELS) has become a key player in the field of plasmonics and nanophotonics, as it allows imaging of optical nearfields with nanometer spatial and high energy resolution. With the newest generation of electron microscopes, the energy resolution can be pushed to the meV range for the direct measurement of surface and bulk phonon polariton fields in ionic nanostructures [1].

In this paper, we present the combination of high-resolution EELS measurements with a plasmon tomography scheme, in order to obtain full 3D vectorial maps of the surface phonon polariton fields of a single MgO nanocube [2]. Using a highly monochromated electron beam in a scanning transmission electron microscope, we visualize varying signatures from the phonon polariton modes as a function of the beam position, energy loss, and tilt angle. The nanocube response is described in terms of geometric eigenmodes and is used for the tomo­graphic reconstruction of the phononic nearfields of the object. In comparison to previous work, our scheme includes mixing of basis modes, which is needed to properly account for the symmetry breaking of the substrate.

Detailed 3D information of the nanophotonic environment promises insights in heat transfer at the nanoscale and is invaluable to the design and optimization of nanostructures for fascinating new uses.

Coherent manipulation of quantum systems with precisely controlled electromagnetic fields is one of the key elements of quantum optics and quantum technologies. In this talk I will give an overview of our recent work [1], which theoretically demonstrates that the non-radiative electromagnetic near-field of a temporally modulated free-space electron beam can be utilized for coherent control (even on the nanoscale e.g. in an electron microscope) of quantum systems. I show that such manipulation can be performed with only classical control over the electron beam itself and that potential challenges like shot noise and decoherence through back action on the electrons are for certain parameter ranges insignificant for our approach.  I will conclude with a possible experimental realization using laser cooled, state-selected potassium atoms and point out applications such as painted potentials, which could be realized using a spatially modulated electron beam.

[1]         D. Rätzel, D. Hartley, O. Schwartz, and P. Haslinger, A Quantum Klystron -- Controlling Quantum Systems with Modulated Electron Beams, arXiv:2004.10168 (2020).

Atomically-thin layers of semiconducting transition metal dichalcogenides (TMDCs), such as tungsten diselenide (WSe₂), have attracted considerable interest because of new properties that can be obtained when artificially fabricated into van der Waals homo- or heterostructures. Due to interlayer coupling, bulk and multi-layered TMDCs are indirect gap semiconductors, while their monolayers exhibit a crossover to a direct band gap. It has been demonstrated that the interlayer coupling strength in homostructures is also sensitive to the twist angle in the cases of bilayer MoS₂ [1] and WS₂ [2]. In this work, the combined high spatial and spectral resolution of aberration-corrected scanning transmission electron microscopy (STEM) and monochromated electron energy-loss spectroscopy (EELS) in the low-loss regime are used to investigate the excitonic response of atomically-thin WSe₂, specifically in twisted bilayer WSe₂ as a function of moiré angle.

Few-layered WSe₂ flakes were mechanically exfoliated from a synthetic bulk crystal and transferred onto a Si₃N₄ TEM grid with periodic micron-sized holes. Atomically-resolved imaging has been performed on a Nion UltraSTEM200 operated at 60 keV and monochromated EELS performed on a modified Nion HERMES-S200 (also known as ChromaTEM) operated at 60 kV with the sample cooled using liquid nitrogen (T ≈ 150 K). In addition to freestanding WSe₂ monolayers, fragments of bilayers and trilayers with variable twist angle between 0–30° are also routinely observed due to folding during the mechanical exfoliation and transfer process. Well-defined hexagonal moiré patterns with nanometer periodicity are evident in the high-angle annular dark-field (HAADF) images for the low twist angles. The excitonic absorption signatures of these nanometric twisted bilayers from low-loss EELS are compared to the WSe₂ monolayer and trilayer counterparts of zero twist angle. The spectra demonstrate a good general correspondence to the thickness dependence of the A, B, and C exciton resonances, namely a pronounced decrease in C exciton energy with number of layers [3]. Comparing different twist angles in the bilayers also show sizable blueshifts in the C exciton energy up to 200 meV, which subsequently alter drastically the spectral shape between the B–C excitonic transitions, with extremes between the zero-twist and towards the anti-aligned (28°) case suggesting underlying differences in interlayer coupling with respect to the moiré angle.

Cathodoluminescence (CL) offers intriguing coherence properties. Originating from a spontaneous process in a deep subwavelength region, it is generally temporally incoherent, albeit with an exquisite spatial coherence. Thus, CL can be interfered with itself spatially and spectrally^1,2, but not with an external laser field. Coherent mixing with a reference laser at a controllable phase delay would offer novel opportunities in CL physics.

This talk discusses the concept of coherent CL emission based on free electron beams modulated by PINEM (photon-induced nearfield electron microscopy)³. In particular, we present CL-emission effects linked to the transfer of optical phase information by an electron, rather than by light waves^4,5. On one hand, the emitted CL power is independent of the PINEM modulation, hence, observing coherences require far-field interference with a reference laser. The fundamentally different nature of the electron facilitates nontrivial emission properties. For example, even a single electron exhibits inherent nonlinearity, as it can transfer coherences at discrete harmonics of the laser frequency. Additionally, the coupling of the coherent emission to radiation varies with the electron propagation as a result from the electron’s relativistic dispersion. The suppression or enhancement of the emitted power requires a different concept. We show that illuminating the reference laser directly at the sample allows the optical scattering and the coherent CL emission to interfere controllably. In the case of CL from a PINEM-modulated electron, that can lead to the suppression of the emission at the harmonic frequencies of the PINEM-driving laser.

We believe that the presented work sets the path for coherent control of optical excitations and opens new routes toward the realization of quantum-state tomography of light states with nanoscale precision.    

A key aspect of the physics of recombination in InGaN quantum wells (QWs) grown on the polar c-plane of GaN is charge carrier separation across the QW resulting from internal electric fields arising from spontaneous and piezoelectric polarisation.  A consequence of these fields is that holes in polar QWs probe the lower quantum well interface (where InGaN was grown on GaN) and electrons probe the upper interface.  Holes tend to be localised at randomly-occurring regions of increased indium content, whereas electrons may be localised at well width fluctuations[1].  These differences in localisation spatially separate carriers within the plane of the QW, in addition to the field-related separation across the well.  Signatures of this carrier separation in photoluminescence measurements include long recombination lifetimes and variation of these lifetimes across the QW emission band. This spectral dependence of the timescale of the luminescence decays results from changes in strain and hence macroscopic built-in electric field associated with the randomly-occurring variations in local indium content responsible for hole localisation. Non-polar QWs, in contrast, exhibit much shorter recombination lifetimes, with little variation in lifetime across the QW emission band.  Here, in the absence of internal fields, the electron and hole are co-localised and strong wavefunction overlap leads to fast decays for all wavelengths[2].

This understanding of the impact of internal electric fields on the recombination dynamics of InGaN QWs can be used to interpret time-resolved cathodoluminescence data from a range of InGaN nano-objects, thus providing new nanoscale insights not achieveable by photoluminescence.  For InGaN/GaN core-shell nanorods which have various polar, non-polar and semi-polar facets, the polar facets exhibit longer recombination lifetimes and a marked variation in lifetime across the lineshape, which is reduced for the non- and semi-polar planes. For trench defects in InGaN/GaN polar QWs, strain relaxation within a region surrounded by a trench reduces the internal fields, which is again observed as a reduced wavelength dependence of the carrier lifetime.  Overall, the measurement of the spectral variations in recombination lifetimes at the nanoscale provides a valuable probe of the impact of nanoscale structure on carrier recombination mechanisms in nitride-based materials.  

[1] Dawson et al. The nature of carrier localisation in polar and nonpolar InGaN/GaN quantum wells J. Applied Physics 119, 181505(2016)

[2] Schulz et al. Structural, electronic, and optical properties of m-plane InGaN/GaN quantum wells: Insights from experiment and atomistic theory Phys. Rev. B 92, 235419(2015)

In cathodoluminescence (CL) performed in a scanning transmission electron microscope (STEM), angle- and energy-resolved light detection enables the extraction of the coherently generated light phase due to interference. We demonstrate nanoscale field mapping, taking advantage of the light interference. The interference of multiple modes inside a single structure generates asymmetric mapping contrasts depending on the detection angle.[1,2] By controlling the electron beam and light detection, circularly polarized light with a chosen parity can also be generated from a spherical particle, based on the interference of the two orthogonal modes with a relative phase shift.[3] With reference light, for which we use transition radiation from a flat metal surface, one can also map the interference pattern of the scattering through surface plasmon polaritons. We extracted the phase of plasmonic particles placed on a metal surface and visualized the propagating surface plasmon polaritons around the particle.[4] Such measurement is most efficiently performed by 4D-STEM-CL which enables simultaneous acquisition of energy-, angle-, and 2D space-information.[3] If time allows, our recent development of TEM with a linear accelerator and decelerator will also be presented. In this instrument, electrons gain and lose energy of a few hundreds of keV from RF electromagnetic waves.[5]

The development of time-resolved Cathodoluminescence (TR-CL) in a scanning electron microscope (SEM) enabled the measurement of the lifetime of excited states in semiconductors with a sub-wavelength spatial resolution. It was used for example to measure the influence of stacking faults on the GaN exciton lifetime ¹, to probe the role of a silver layer on the dynamics of a YAG crystal ^2, or to show the influence of stress on the optical properties of ZnO nanowires ³. These results demonstrate that TR-CL is essential to study the correlation between semiconductor optical and structural properties (composition, defects, strain…). Despite giving invaluable information, TR-CL in a scanning electron microscope is still limited in spatial resolution. CL in the transmission electron microscope (TEM) proved to dramatically improve the spatial resolution with respect to SEM-CL, in addition, to give access to multiple complementary analysis tools (from atomic-scale imaging to electron energy loss spectroscopy). The advent of the yet undemonstrated TR-CL in a STEM is therefore expected to be a major next step for the investigation of the optical properties of nanomaterials.

In this presentation, we will discuss our first experimental demonstration of time-resolved cathodoluminescence within a transmission electron microscope. They were performed in a unique femtosecond pulsed transmission electron microscope, with a cold-FEG electron gun ⁴. This technology allows among other things sub-ps temporal resolution while preserving a spatial resolution of a few nanometers, essential for the study of nanophotonic materials. We will present the first lifetime maps acquired in a TEM both on nano-diamonds and InGaN quantum wells and discuss the unique features and opportunities of this technique.

Semiconducting transition metal dichalcogenides (TMDC) combined with photonic cavities, exhibit strong light-matter interactions, leading to the emergence of new quasi particles [1], namely exciton-polaritons (EP). EPs have been extensively studied using near-field techniques such as SNOM and EELS due to their fascinating fundamental properties [2-3]. In this work, we used cathodoluminescence (CL) spectroscopy for the first time to probe EPs and stemming spatial correlations in atomically-flat TMDCs such as Wse₂ [4]. Being based on spontaneous interactions, the resolved spatial interference maps are the direct proof of the spontaneous coherences associated with propagating exciton polaritons.

Here, we demonstrate that the transversal one-dimensional optical confinement within the thin film and the propagation of the optical waves along the longitudinal orientation, allow for strong exciton-photon couplings leading to an energy split and spatially-resolved interference patterns, associated with propagating optical modes. In particular the combination of both aspects confirm the excitation of exciton polaritons. Experimental results followed by numerical simulations, provide deep inside into the electron-photon interaction mechanisms inside the specimen.

CL radiation collected from thin WSe₂ flakes demonstrate a  wavelength splitting on the order of 100 nm to 250 nm, depending on the thickness of the film, comparable to the predicted spectral and spatial maps. A deep at the A exciton wavelength of 751 nm and broad peaks associated with lower and upper polariton branches were observed, respectively; below and above the A exciton. Numerous interference fringes in the wavelength-distance CL map are clear signature of the spontaneous coherence caused by the excitation of EPs. Spatial interference fringes up to several orders confined along the edges revealed the excitation of edge exciton polaritons. Our results demonstrate that CL spectroscopy can be used to probe coherent optical modes of semiconducting van der Waals materials.

In this seminar, we review recent experimental developments in reshaping the wavefunction of free particles. In particular, the possibility to maniupulate free electrons to generate attosecond or even zeptosecond pulses as well as attosecond-controlled vortex electron beams will be discussed. These advanced electron beams can enable  new microscopy techniques having ultra-high time resolution in diffraction or spectroscopy. Recently, it has been shown that it is possible to map the structural dynamics of solids, or to characterize and control plasmonic fields in nanostructures at the attosecond/nanometer scale or to probe the local magnetization dynamics. More recently, a discussion emerged on using  prepared electron packets to unveil new phenomena in nuclear physics. We will depict the state of the art in this field and show that the coherent control of both the transverse and longitudinal wave function of individual electrons can have applications in controlling nuclear excitations of long lived isomers.

EELS is a very powerful technique as it allows for the measurement of energy state of the electrons after inelastic scattering. For this reason, it had incredible success in measuring atomic and chemical properties at the lowest scale. Often the energy information alone is sufficient to measure chemical species concentration, plasmon or even phonon excitations, especially with the recent improvements in energy resolution using high resolution spectrometers and source monochromators. However, it is sometimes desirable to add a second dispersion variable, to produce, for example, a momentum and energy double dispersion EELS or acquire a spectrum for a given momentum transfer.

But we can think of associating different quantities to the energy loss and most notably the Orbital Angular Momentum (OAM), the component of the angular momentum along the main electron propagation direction. Thanks to the new idea of miniaturized electron optics we developed the OAM sorter [1,2]: probably one of the most innovative electron optics elements now being developed now controlled by deep learining autoalignment procedure.

The advantage of a doubly dispersed OAM_EELS experiments is that some measurements becomes feasible or much easier if the OAM is chosen as projection state. Among these Electron Magnetic Circular Dichroism (EMCD) [3], multipolar transition, excitation of Plasmon with specific OAM, plasmonic dichroism.

The example of EMCD is particularly  relevant. In fact, while such a technique is quite cumbersome in the case of linear momentum (q, E) dispersed EELS and requires the measurement of the EELS signal interference, it results completely natural in the OAM basis [4] (ℓ, E) where the magnetism introduces an asymmetry between the ℓ = -1 and ℓ = +1 channels.

More in general Laurenco-Martins [5] demonstrated in different context that an dichroic +1/-1 OAM measurement is equivalent to a circularly polarized optical absorption measurement. Probing OAM is therefore also relevant in the case of plasmon modes excitation [6].

Another potential use of OAM sorter is to more easily single out specific character of multipolar transitions. For example we studied the transitions to σ, π states in 2D materials.

The talk will explore the theoretical possibilities, the properties of the new OAM sorter and the early results in doubly dispersive OAM-EELS experiments. We will conclude with a prospective also on different experiments made available by miniaturized optics.

We report on the experimental realisation of an electrostatic programmable phase plate for electrons making use of a series of microscopic electrostatic elements that can be individually controlled to shift the phase of a coherent electron beam passing through them [1]. The device offers a total of 48 pixels and provides rapid and hysteresis free control over the phase. We show experimental proof of the ability to implement basic electron optical functions as focus, astigmatism and spherical aberration correction and discuss multiple design patterns to optimise for a given function. We discuss the potential applications of this device as an aberration corrector, contrast enhancement for weakly scattering objects, seeing through thick objects, solving inverse problems and increased selectivity in inelastic scattering. Its compact design, lack of magnetic hysteresis and microsecond response time make this an attractive tool to significantly improve the capabilities of future electron microscopes. We discuss the possibility to adaptively correct for aberrations keeping contrast and resolution in the most optimal imaging conditions without lengthy manual tuning procedures allowing for unattended measurement procedures that could increase the attractiveness of electron microscopy in e.g. industrial and clinical settings.

Development of a pulsed laser phase plate inside a cavity-based transmission electron microscope

A conventional Transmission Electron Microscope (TEM) makes use of amplitude contrast to visualize samples. This enables the visualization of structures on the sub-nanosecond scale. Interestingly, even more information on the sample is stored in phase changes induced in the electrons. This phase contrast can be made visible via a phase plate. However, present-day electron phase plates suffer from contamination, limited lifetimes and charging.

Therefore, we propose a pulsed laser phase plate, which does not require any structures to be placed in the electron beam’s path, is spectroscopically invisible and has a tunable strength. Compared to a continuous wave laser, a relatively simple laser system is sufficient for the pulsed laser approach and no custom-made laser cavities for world record intensity levels are needed. The microwave cavities utilized in our TEM allow for the creation and modification of high-brightness electron pulses, which can be synchronised with our laser pulses. A theoretical framework for the laser-electron interaction has been developed and is supplemented by numerical simulations. Calculations show that conventional pulsed lasers can induce the desired phase shift in the electrons. Construction and initial alignment of the laser system has started. The laser phase plate in combination with our cavity-based TEM will be the ideal system to visualize phase contrast.

Coherent optical excitations in van der Waals materials (2D polaritons) span a large range of optical phenomena which arise from their extraordinary dispersion relations [1]. Among the different polaritons, it is particularly interesting to examine polaritons in hyperbolic materials (hyperbolic phonon-polaritons, hPhP) which show rich physical behavior, ranging from negative refraction and subdiffraction imaging. The phononic resonance creates a dispersion relation that contains multiple branches, which were shown to be tunable by the 2D material geometry, thickness, and surrounding environment [2], reaching relatively low losses in room temperature [3].

Among the various experimental techniques that are used in the field of 2D polaritons, scanning nearfield optical microscopy (SNOM) and its variants dominates the direct nearfield imaging of 2D polaritons. Specifically, recent advances in time-resolved SNOM allowed adressing the polariton’s dynamical properties as group velocity from the changes of interference of scattered polaritons with different time delays [4]. However, this interferometric technique cannot image the wavepacket dynamics, as it excludes the phase difference between different wavelengths. Different imaging techniques such as electron energy loss spectroscopy (EELS), where the 2D polariton dispersion was recently extracted [5], obtains only time-independent information on the polaritonic modes.

Here we present how an ultrafast transmission electron microscope (UTEM), driven by femtosecond mid-infrared pulses, can probe the spatiotemporal dynamics of 2D wavepackets.

We record how the mid-infrared pulses excites hPhP wavepacket and measure the wavepacket's trajectory inside the sample, revealing unforseen physical behaviors: multi-branch wavepacket splitting and wavepacket acceleration and deceleration. These phenomena are especially surprising since conventional wavepackets propagate in a fixed group velocity. We show that it is the unique dispersion relations of the hyperbolic PhPs that facilitates these phenomena, while recording group velocities between c/45 to c/850. These results examplify the wide physical spatiotemporal dynamical phenomena that can be probed through a non-destructive measurement using the unique combination of femtosecond temporal and nanometer spatial resolution of the UTEM.