arXiv subjects

Mathieu Kociak

Publications and source records attributed to Mathieu Kociak.

At least 19 recordsLinked to original sources

arXiv2026

Absolute measurement of the intrinsic helicity in nanophotonics

Helicity is a universal quantity describing the internal rotation of an object or a field. Whether it characterizes fundamental properties such as the spin of elementary particles or leads to practical consequences such as the toxicity or harmlessness of chiral molecules, defining and measuring it is essential. However, this remains ambiguous in the case of chiral photonic systems such as plasmonic nanoparticles or photonic metasurfaces, where contrary to common knowledge, the observation of circularly polarized dependent optical properties is not a relevant measure for the helicity. We demonstrate experimentally and theoretically that the helicity can be rigorously defined and measured in a nanophotonic system emitting circularly polarized light after excitation in the near-field by a focused electron beam. In the case of a model system composed of two plasmonic dipoles (Born-Kuhn systems), we show that the helicity of photonic modes takes on a very intuitive form and can be simply measured by symmetrizing the geometry of excitation and detection. The method could be extended to a variety of chiral photonic systems, whose locally enhanced properties make them promising for the engineering of local chirality.

physics.optics↗

arXiv2025

Cathodoluminescence, light injection and EELS in STEM: From comparative to coincidence experiments

Electron spectroscopy implemented in electron microscopes provides high spatial resolution, down to the atomic scale, of the chemical, electronic, vibrational and optical properties of materials. In this review, we will describe how temporal coincidence experiments in the nanosecond to femtosecond range between different electron spectroscopies involving photons, inelastic electrons and secondary electrons can provide information bits not accessible to independent spectroscopies. In particular, we will focus on nano-optics applications. The instrumental modifications necessary for these experiments are discussed, as well as the perspectives for these coincidence techniques.

cond-mat.mtrl-sci↗

arXiv2025

Single photon emitters in hBN: Limitations of atomic resolution imaging and potential sources of error

There is a growing interest in identifying the origin of single-photon emission in hexagonal boron nitride (hBN), with proposed candidates including boron and nitrogen vacancies as well as carbon substitutional dopants. Because photon emission intensity often increases with sample thickness, hBN flakes used in these studies commonly exceed 30 atomic layers. To identify potential emitters at the atomic scale, annular dark-field scanning transmission electron microscopy (ADF-STEM) is frequently employed. However, due to the intrinsic AA' stacking of hBN with vertically alternating boron and nitrogen atoms, this approach is complicated even in few-layer systems. Here, we demonstrate using STEM image simulations and experiments that, even under idealized conditions, the intensity differences between boron- and nitrogen-dominated columns and carbon substitutions become indistinguishable at thicknesses beyond 17 atomic layers (ca. 6 nm). While vacancy-type defects can remain detectable at somewhat larger thicknesses, also their detection becomes unreliable at thicknesses typically used in photonic studies. We further show that common residual aberrations, particularly threefold astigmatism, can lead to artificial contrast differences between columns, which may result in misidentification of atomic defects. We systematically study the effects of non-radially symmetric aberrations on multilayer hBN and demonstrate that even small residual threefold astigmatism can significantly distort the STEM contrast, leading to misleading interpretations.

cond-mat.mtrl-sci↗

arXiv2025

Advancing Time-Resolved Spectroscopies with Custom Scanning Units and Event-Based Electron Detection

Direct electron detection is revolutionizing electron microscopy by offering lower noise, reduced point-spread function, and increased quantum efficiency. Among these advancements, the Timepix3 hybrid-pixel direct electron detector stands out for its unique ability to output temporal information about individual hits within its pixel array. Its event-based architecture enables data-driven detection, where individual events are immediately read out from the chip. While recent studies have demonstrated the potential of event-based detectors in various applications in the context of continuous-gun scanning transmission electron microscopes (STEM), the use of such detectors with standard scanning units remains underdeveloped. In this work, we present a custom-designed, Timepix3-compatible scanning unit specifically developed to leverage the advantages of event-based detection. We explore its performance in enabling spatially and temporally resolved experiments, as well as its seamless interfacing with other time-resolved instruments, such as pulsed lasers and electron beam blankers. Additionally, we examine the prospects for achieving enhanced temporal resolution in time-resolved experiments using continuous-gun electron microscopes, identifying key challenges and proposing solutions to improve performance. This combination of custom hardware with advanced detection technology promises to expand the capabilities of electron microscopy in both fundamental research and practical applications.

physics.ins-det↗

arXiv2025

Optical absorption in hexagonal-diamond Si and Ge nanowires: insights from STEM-EELS experiments and ab initio theory

Hexagonal-diamond (2H) group IV nanowires are key for advancing group IV-based lasers, quantum electronics, and photonics. Understanding their dielectric response is crucial for performance optimization, but their optical absorption properties remain unexplored. We present the first comprehensive study of optical absorption in 2H-Si and 2H-Ge nanowires, combining high-resolution STEM, monochromated EELS, and ab initio simulations. The nanowires, grown in situ in a TEM as nanobranches on GaAs stems, show excellent structural quality: single crystalline, strain-free, minimal defects, no substrate contamination, enabling access to intrinsic dielectric response. 2H-Si exhibits enhanced absorption in the visible range compared to cubic Si, with a marked onset above 2.5 eV. 2H-Ge shows absorption near 1 eV but no clear features at the direct bandgap, as predicted by ab initio simulations. A peak around 2 eV in aloof-beam spectra is attributed to a thin 3C-Ge shell. These findings clarify structure-optical response relationships in 2H materials.

cond-mat.mes-hall↗

arXiv2025

Plasmon-interband hybridization and anomalous production of hot electrons in aluminum nanoantennas

Strong coupling typically occurs between two separate objects or between an object and its environment (such as an atom and a cavity). However, it can also occur between two different excitations within the same object, a situation that has been much less studied. In this study, we observe strong coupling between localized surface plasmon resonances and the interband transition in aluminum nanorods, as evidenced by optical spectroscopy and electron energy loss spectroscopy, and corroborated with numerical simulations. Strong coupling is observed between the interband transition and multiple orders of the surface plasmon mode, including dark ones. We also obtain experimental maps of the hybrid modes at the nanoscale. In each case, the associated Rabi energy, which corresponds to the energy splitting between the two polaritonic branches, is obtained. Finally, a dedicated numerical model was employed to calculate the hot electron generation rate in the nanorods. The calculations demonstrate that efficient generation of hot electrons can be achieved in the near-infrared region, when the interband transition is strongly coupled with a plasmon resonance. This high generation rate stems from the hybrid nature of the mode, as its plasmonic component provides a high absorption cross-section, while the IT part ensures efficient conversion to hot electrons. Consequently, aluminum nanorods represent an efficient source of hot electrons in the visible and near-infrared regions, with potential applications in local photochemistry, photodetection, and solar energy harvesting.

physics.optics↗

arXiv2025

Roadmap for Quantum Nanophotonics with Free Electrons

Over the past century, continuous advancements in electron microscopy have enabled the synthesis, control, and characterization of high-quality free-electron beams. These probes carry an evanescent electromagnetic field that can drive localized excitations and provide high-resolution information on material structures and their optical responses, currently reaching the sub-{\aa}ngstr\"om and few-meV regime. Moreover, combining free electrons with pulsed light sources in ultrafast electron microscopy adds temporal resolution in the sub-femtosecond range while offering enhanced control of the electron wave function. Beyond their exceptional capabilities for time-resolved spectromicroscopy, free electrons are emerging as powerful tools in quantum nanophotonics, on par with photons in their ability to carry and transfer quantum information, create entanglement within and with a specimen, and reveal previously inaccessible details on nanoscale quantum phenomena. This Roadmap outlines the current state of this rapidly evolving field, highlights key challenges and opportunities, and discusses future directions through a collection of topical sections prepared by leading experts.

cond-mat.mes-hall↗

arXiv2024

Nanosecond nanothermometry in an electron microscope

Thermal transport in nanostructures plays a critical role in modern technologies. As devices shrink, techniques that can measure thermal properties at nanometer and nanosecond scales are increasingly needed to capture transient, out-of-equilibrium phenomena. We present a novel pump-probe photon-electron method within a scanning transmission electron microscope (STEM) to map temperature dynamics with unprecedented spatial and temporal resolutions. By combining focused laser-induced heating and synchronized time-resolved monochromated electron energy loss spectroscopy (EELS), we track phonon, exciton and plasmon signals in various materials, including silicon nitride, aluminum thin film, and transition metal dichalcogenides. Our results demonstrate the technique's ability to follow temperature changes at the nanometer and nanosecond scales. The experimental data closely matched theoretical heat diffusion models, confirming the method's validity. This approach opens new opportunities to investigate transient thermal phenomena in nanoscale materials, offering valuable insights for applications in thermoelectric devices and nanoelectronics.

cond-mat.mtrl-sci↗

arXiv2024

Quantum Confined Luminescence in Two dimensions

Achieving localized light emission from monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) embedded in the matrix of another TMD has been theoretically proposed but not experimentally proven. In this study, we used cathodoluminescence performed in a scanning transmission electron microscope to unambiguously resolve localized light emission from 2D monolayer MoSe2 nanodots of varying sizes embedded in monolayer WSe2 matrix. We observed that the light emission strongly depends on the nanodot size wherein the emission is dominated by MoSe2 excitons in dots larger than 85 nm, and by MoSe2/WSe2 interface excitons below 50 nm. Interestingly, at extremely small dot sizes (< 10 nm), the electron energy levels in the nanodot become quantized, as demonstrated by a striking blue-shift in interface exciton emission, thus inducing quantum confined luminescence. These results establish controllable light emission from spatially confined 2D nanodots, which holds potential to be generalized to other 2D systems towards future nanophotonic applications.

cond-mat.mtrl-sci↗

arXiv2024

Selective probing of longitudinal and transverse plasmon modes with electron phase-matching

The optical properties of metallic nanoparticles are dominated by localized surface plasmons (LSPs). Their properties only depend on the constituting material, the size and shape of the nano-object as well as its surrounding medium. In anisotropic structures, such as metallic nanorods, two families of modes generally exist, transverse and longitudinal. Their spectral and spatial overlaps usually impede their separate measurements in electron energy loss spectroscopy (EELS). In this work, we propose three different strategies enabling to overcome this difficulty and selectively probe longitudinal and transverse modes. The first strategy is numeric and relies on morphing of nano-structures, rooted in the geometrical nature of LSPs. The two other strategies exploit the relativistic and wave nature of the electrons in an EELS experiment. The first one is the phase-matching between the electron and the plasmon excitation to enhance their coupling by either tilting the sample and modifying the electron kinetic energy. The second one - polarized EELS (pEELS) - exploits the wave nature of electrons to mimic selection rules analogous to the one existing in light spectroscopies. The above-mentioned strategies are exemplified - either experimentally or numerically - on a canonical plasmonic toy model: the nano-rod. The goal of the paper is to bring together the state-of-the-art concepts of EELS for plasmonics to tackle a pedestrian problem in this field.

physics.optics↗

arXiv2023

Engineering 2D material exciton lineshape with graphene/h-BN encapsulation

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Remarkable advances have been achieved through alloying, chemical and electrical doping, and applied strain. However, the integration of TMDs with other 2D materials in van der Waals heterostructures (vdWHs) to tailor novel functionalities remains largely unexplored. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton lineshape and charge state. Fano-like asymmetric spectral features are produced in WS$_{2}$, MoSe$_{2}$ and WSe$_{2}$ vdWHs combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe$_{2}$/graphene with a neutral exciton redshift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron-beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

cond-mat.mes-hall↗

arXiv2023

Time calibration studies for the Timepix3 hybrid pixel detector in electron microscopy

Direct electron detection is currently revolutionizing many fields of electron microscopy due to its lower noise, its reduced point-spread function, and its increased quantum efficiency. More specifically to this work, Timepix3 is a hybrid-pixel direct electron detector capable of outputting temporal information of individual hits in its pixel array. Its architecture results in a data-driven detector, also called event-based, in which individual hits trigger the data off the chip for readout as fast as possible. The presence of a pixel threshold value results in an almost readout-noise-free detector while also defining the hit time of arrival and the time the signal stays over the pixel threshold. In this work, we have performed various experiments to calibrate and correct the Timepix3 temporal information, specifically in the context of electron microscopy. These include the energy calibration, and the time-walk and pixel delay corrections, reaching an average temporal resolution throughout the entire pixel matrix of $1.37 \pm 0.04$ ns. Additionally, we have also studied cosmic rays tracks to characterize the charge dynamics along the volume of the sensor layer, allowing us to estimate the limits of the detector's temporal response depending on different bias voltages, sensor thickness, and the electron beam ionization volume. We have estimated the uncertainty due to the ionization volume ranging from about 0.8 ns for 60 keV electrons to 8.8 ns for 300 keV electrons.

physics.ins-det↗

arXiv2023

High efficiency coupling of free electrons to sub-$\lambda^3$ modal volume, high-Q photonic cavities

We report on the design, realization and experimental investigation by spatially resolved monochromated electron energy loss spectroscopy (EELS) of high quality factor cavities with modal volumes smaller than $\lambda^3$, with $\lambda$ the free-space wavelength of light. The cavities are based on a slot defect in a 2D photonic crystal slab made up of silicon. They are optimized for high coupling of electrons accelerated to 100 kV, to quasi-Transverse Electrical modes polarized along the slot direction. We studied the cavities in two geometries. The first geometry, for which the cavities have been designed, corresponds to an electron beam travelling along the slot direction. The second consists in the electron beam travelling perpendicular to the slab. In both cases, a large series of modes is identified. The dielectric slot modes energies are measured to be in the 0.8- 0.85 eV range, as per design, and surrounded by two bands of dielectric and air modes of the photonic structure. The dielectric even slot modes, to which the cavity mode belongs, are highly coupled to the electrons with up to 3.2$\%$ probability of creating a slot photon per incident electron. Although the experimental spectral resolution (around 30 meV) alone does not allow to disentangle cavity photons from other slot photons, the remarkable agreement between the experiments and FDTD simulations permits us to deduce that amongst the photons created in the slot, around 30$\%$ are stored in the cavity mode. A systematic study of the energy and coupling strength as a function of the photonic band gap parameters permits to foresee increase of coupling strength by fine-tuning phase matching. Our work demonstrates free electron coupling to high quality factor cavities with low mode density, sub-$\lambda^3$ modal volume, making it an excellent candidate for applications such as quantum nano-optics with free electrons.

physics.optics↗

arXiv2023

Excitation's lifetime extracted from electron-photon (EELS-CL) nanosecond-scale temporal coincidences

Electron-photon temporal correlations in electron energy loss (EELS) and cathodoluminescence (CL) spectroscopies have recently been used to measure the relative quantum efficiency of materials. This combined spectroscopy, named Cathodoluminescence excitation spectroscopy (CLE), allows the identification of excitation and decay channels which are hidden in average measurements. Here, we demonstrate that CLE can also be used to measure excitation's decay time. In addition, the decay time as a function of the excitation energy is accessed, as the energy for each electron-photon pair is probed. We used two well-known insulating materials to characterize this technique, nanodiamonds with \textit{NV$^0$} defect emission and h-BN with a \textit{4.1 eV} defect emission. Both also exhibit marked transition radiations, whose extremely short decay times can be used to characterize the instrumental response function. It is found to be typically 2 ns, in agreement with the expected limit of the EELS detector temporal resolution. The measured lifetimes of \textit{NV$^0$} centers in diamond nanoparticles (20 to 40 ns) and \textit{4.1 eV} defect in h-BN flakes ($<$ 2 ns) matches those reported for those materials previously.

cond-mat.mtrl-sci↗

arXiv2022

$\mu$eV electron spectromicroscopy using free-space light

The synergy between free electrons and light has recently been leveraged to reach an impressive degree of simultaneous spatial and spectral resolution, enabling applications in microscopy and quantum optics. However, the required combination of electron optics and light injection into the spectrally narrow modes of arbitrary specimens remains a challenge. Here, we demonstrate microelectronvolt spectral resolution in the nanoscale mapping of photonic modes with quality factors as high as 10$^4$. We rely on mode-matching of a tightly focused laser beam to whispering gallery modes to achieve a 10$^8$-fold increase in light-electron coupling efficiency. By adapting the shape and size of free-space optical beams to address specific physical questions, our approach allows us to interrogate any type of photonic structure with unprecedented spectral and spatial detail

cond-mat.mes-hall↗

arXiv2022

Tomographic reconstruction of quasistatic surface polariton fields

We theoretically investigate the tomographic reconstruction of the three-dimensional photonic environment of nanoparticles. As input for our reconstruction we use electron energy loss spectroscopy (EELS) maps for different rotation angles. We perform the tomographic reconstruction of surface polariton fields for smooth and rough nanorods, and compare the reconstructed and simulated photonic local density of states, which are shown to be in very good agreement. Using these results, we critically examine the potential of our tomography scheme, and discuss limitations and directions for future developments.

physics.optics↗

arXiv2022

Design and implementation of a device based on an off-axis parabolic mirror to perform luminescence experiments in a scanning tunneling microscope

We present the design, implementation, and illustrative results of a light collection/injection strategy based on an off-axis parabolic mirror collector for a low-temperature Scanning Tunneling Microscope (STM). This device allows us to perform STM induced Light Emission (STM-LE) and Cathodoluminescence (STM-CL) experiments and in situ Photoluminescence (PL) and Raman spectroscopy as complementary techniques. Considering the \'Etendue conservation and using an off-axis parabolic mirror, it is possible to design a light collection and injection system that displays 72% of collection efficiency (considering the hemisphere above the sample surface) while maintaining high spectral resolution and minimizing signal loss. The performance of the STM is tested by atomically resolved images and scanning tunneling spectroscopy results on standard sample surfaces. The capabilities of our system are demonstrated by performing STM-LE on metallic surfaces and two-dimensional semiconducting samples, observing both plasmonic and excitonic emissions. In addition, we carried out in situ PL measurements on semiconducting monolayers and quantum dots and in situ Raman on graphite and hexagonal boron nitride (h-BN) samples. Additionally, STM-CL and PL were obtained on monolayer h-BN gathering luminescence spectra that are typically associated with intragap states related to carbon defects. The results show that the flexible and efficient light injection and collection device based on an off-axis parabolic mirror is a powerful tool to study several types of nanostructures with multiple spectroscopic techniques in correlation with their morphology at the atomic scale and electronic structure.

physics.ins-det↗

arXiv2022

Cathodoluminescence excitation spectroscopy: nanoscale imaging of excitation pathways

Following the lifespan of optical excitations from their creation to decay into photons is crucial in understanding materials optical properties. Macroscopically, techniques such as the photoluminescence excitation spectroscopy provide unique information on the photophysics of materials with applications as diverse as quantum optics or photovoltaics. Materials excitation and emission pathways are affected by nanometer scale variations directly impacting devices performances. However, they cannot be directly accessed, despite techniques, such as optical spectroscopies with free electrons, having the relevant spatial, spectral or time resolution. Here, we explore optical excitation creation and decay in two representative optical devices: plasmonic nanoparticles and luminescent 2D layers. The analysis of the energy lost by an exciting electron that is coincident in time with a visible-UV photon unveils the decay pathways from excitation towards light emission. This is demonstrated for phase-locked interactions, such as in localized surface plasmons, and non-phase-locked ones, such as the light emission by individual point defects. This newly developed cathodoluminescence excitation spectroscopy images energy transfer pathways at the nanometer scale. It widens the toolset available to explore nanoscale materials.

cond-mat.mes-hall↗