arXiv subjects

F. Javier García de Abajo

Publications and source records attributed to F. Javier García de Abajo.

At least 19 recordsLinked to original sources

arXiv2026

Thermal vacuum friction of objects with different dimensionality

Radiative forces acting on neutral bodies moving through a thermal bath represent a unique manifestation of the interplay between relativistic kinematics and thermal fluctuations. Vacuum friction is commonly formulated using the fluctuation--dissipation theorem or related statistical approaches, but such treatments can obscure the elementary momentum-transfer processes, especially in relativistic regimes. Here, we develop a kinetic momentum-transfer framework based on relativistic kinematics in which the radiative force and pressure are obtained by summing individual scattering and absorption events. This approach offers a transparent physical picture while ensuring a self-consistent treatment of Doppler shifts and relativistic transformations. We apply the method to three representative geometries: an isotropic dipolar particle, a thin resonant plate moving normal to its surface, and a thin resonant plate moving parallel to its surface. In the nonrelativistic limit, we derive explicit radiative drag coefficients, providing compact expressions for predicting vacuum friction in moving structures.

physics.optics↗

arXiv2026

Revealing time characteristics of optical excitations in dielectric and plasmonic structures through cathodoluminescence interferometry

Cathodoluminescence (CL) spectroscopy provides access to optical excitations with nanometer spatial resolution, but direct time-resolved measurements of optical resonances remain challenging. Here, we demonstrate that CL interferometry provides access to the temporal response, phase behavior, and modal spectral structure of resonant nanoscale scatterers without requiring ultrafast pump-probe schemes. We develop an analytical framework in which Fourier transformation angle- and frequency-resolved CL interferograms yields the decay time of optical resonances governed by the linear optical response. Multimode resonators exhibit characteristic temporal CL beating signatures associated with spectral mode splitting. By exploiting transition radiation emitted from a nearby metallic surface as a broadband reference, we further demonstrate phase retrieval and cross-correlation measurements between instantaneous and resonant emission processes. Experimental measurements on Au nanoparticles, broadband plasmonic emitters, Au nanostars, and Si nanospheres supporting multipolar Mie resonances confirm the theoretical predictions, and decay times in the range 1-10 fs are derived for each system. Our results establish CL interferometry as a powerful approach for accessing spectral, spatial, and phase information within a single nanoscale measurement with fs resolution.

cond-mat.mes-hall↗

arXiv2026

Studying electron beam coherence using plasmon interference

Energetic electrons create distinct cathodoluminescence (CL) angular distributions upon interaction with dielectric and plasmonic nanostructures, providing valuable information on coherences in the excitation pathways. A counterintuitive prediction is that the CL signals arising from the excitation associated with different lateral regions of an extended electron wave are mutually incoherent and do not interfere, while the signals originating from different structures within the electromagnetic field of a narrow electron beam are mutually coherent. We present conclusive experimental evidence of these effects by examining the angular CL emission profile from defocused electron-beam excitation of a thin silicon nitride film, which is shown to follow an incoherent sum of CL excitations within the electron beam spot. In contrast, CL interferences are observed for separated plasmonic scatterers excited within the evanescent field of a single electron. Coherence may be recovered through correlations between emitted light and post-selected electron states, for which we propose a measurement geometry that erases which-path information of the electron trajectory.

cond-mat.mes-hall↗

arXiv2026

Determining Electron Beam Lateral Coherence in a Scanning Electron Microscope Using Electron Diffraction

We develop and characterize scanning transmission electron microscopy (STEM) capabilities within a scanning electron microscope (SEM) to investigate the effective lateral coherence of the electron beam (e-beam) in the specimen plane. Using single-crystalline Au flakes and a sample composed of a monolayer of graphene, we obtain high-quality selected-area electron diffraction (SAED) maps and convergent-beam electron diffraction (CBED) patterns, validating the systems ability to probe crystallographic information at an acceleration voltage of 30 keV. Building on these capabilities, we implement a method, which is adapted from techniques traditionally used in transmission electron microscopy, to measure the degree of lateral coherence of the e-beam in the specimen plane of the SEM. By analyzing interference between electrons with two different wave vectors separated by 0.031 per angstrom, we extract a lower limit for the degree of lateral coherence over 5% of the e-beam diameter of approximately 60%. These coherence values are sufficient to enable quantum-coherent electron-light-matter interaction experiments in the SEM.

cond-mat.mes-hall↗

arXiv2026

Rotational Vacuum Friction of Nonabsorbing Particles

A nonabsorbing particle rotating in vacuum can lose angular momentum only by converting mechanical energy into electromagnetic radiation. Here, we develop a quantum theory of rotational vacuum friction for small lossless particles and show that axial symmetry qualitatively changes the leading dissipation channel. At zero temperature, the frictional torque scales as $M\proptoΩ^7$ with rotation frequency $\ Omega$ in anisotropic particles due to the emission of correlated photon pairs whose frequencies sum to $2Ω$, while a contribution to the torque linear in $\ Omega$ is found at finite temperature. In contrast, axisymmetric particles are protected against photon-assisted friction regardless of temperature.

quant-ph↗

arXiv2026

Free-electron decoherence: Theory and applications

Electron microscopy relies on the spatial coherence of electron beams to generate atomic-scale images using interference and diffraction, which can be degraded by inelastic scattering processes that induce decoherence. Here, we present a theoretical study of decoherence arising from the electromagnetic interaction of free electrons with bulk materials and planar surfaces. We show that bulk plasmons dominate decoherence in Al and Au, while electronic excitations above the band gap, supplemented by weaker coupling to phononic and guided modes, are the primary channels in ionic insulators such as LiF. A thermal population of electromagnetic modes leads to a divergence in the energy-loss probability at low frequencies, which in turn produces a pronounced temperature dependence. We show that this effect can be exploited for nanoscale thermometry, predicting that optimized energy-filtered holography enables $\sim0.1\%$ changes in fringe visibility for physically viable temperature variations in metals. Through these results, we establish a unified theoretical framework to describe free-electron decoherence in the bulk and surfaces of arbitrary materials.

cond-mat.mtrl-sci↗

arXiv2026

Radiative loss of coherence in free electrons: a long-range quantum phenomenon

Quantum physics rules the dynamics of small objects as they interact over microscopic length scales. Nevertheless, quantum correlations involving macroscopic distances can be observed between entangled photons as well as in atomic gases and matter waves at low temperatures. The long-range nature of the electromagnetic coupling between charged particles and extended objects could also trigger quantum phenomena over large distances. Here, we reveal a manifestation of quantum mechanics that involves macroscopic distances and results in a nearly complete depletion of coherence associated with which-way free-electron interference produced by electron-radiation coupling in the presence of distant extended objects. This is a ubiquitous effect that we illustrate through a rigorous theoretical analysis of a two-path electron beam interacting with a semi-infinite metallic plate and find the inter-path coherence to vanish proportionally to the path separation at zero temperature and exponentially at finite temperature. The investigated regime of large distances involves the coupling of the electron to radiative modes assisted by diffraction at material structures but without any involvement of material excitations. Besides the fundamental interest of this macroscopic quantum phenomenon, our results suggest an approach to measuring the vacuum temperature and nondestructively sensing the presence of distant objects.

quant-ph↗

arXiv2025

Active steering of cathodoluminescence through a generalized Smith-Purcell effect

Optical metasurfaces can shape the near fields of energetic electrons, enabling Smith-Purcell (SP) emission. We introduce a generalized SP effect relying on finite periodic arrays whose elements possess individually tunable polarizabilities, allowing us to explore higher-order SP radiation. By controlling the amplitude and phase of each of the elements, we show through rigorous theory the ability to create an SP steering device. In particular, we explore the active tuning capabilities of doped graphene, and thermally driven phase-change materials, which we compare with standard passive plasmonic structures made of gold and silver. Our results establish programmable electron-driven light sources and spectroscopic probes spanning the terahertz-to-visible range, advancing tunable metasurfaces for next-generation electron-photon technologies.

physics.optics↗

arXiv2025

Nonreciprocal plasmons in one-dimensional carbon nanostructures

The directional control of light in miniaturized plasmonic waveguides holds appealing possibilities for emerging nanophotonic technologies, but is hindered by the intrinsic reciprocal optical response of conventional plasmonic materials. While the ability of graphene to sustain large electrical currents shows promise for nonreciprocal plasmonics, studies have been limited to extended samples characterized by linear electrical dispersion. Here, we theoretically explore quantum finite-size and nonlocal effects in the nonreciprocal response of mesoscale plasmonic waveguides comprised of drift-biased graphene nanoribbons (GNRs) and carbon nanotubes (CNTs). Using atomistic simulation methods based on tight-binding electronic states and self-consistent mean-field optical response, we reveal that a moderate electrical bias can significantly break reciprocity for propagation of guided plasmon modes in GNRs and CNTs exhibiting electronic band gaps. The excitation by a nearby point dipole emitter and subsequent propagation of guided plasmon modes can thus be actively controlled by the applied current, which can further be leveraged to mediate nonlocal interactions of multiple emitters. Our results establish graphene nanostructures as a promising atomically thin platform for nonreciprocal nanophotonics.

cond-mat.mes-hall↗

arXiv2025

Atomically thin silver films for enhanced nanoscale nonlinear optics

The inherently weak nonlinear optical response of bulk materials remains a fundamental limitation in advancing photonic technologies. Nanophotonics addresses this challenge by tailoring the size and morphology of nanostructures to manipulate the optical near field, thus modulating the nonlinear response. Here, we explore a complementary strategy based on engineering the electronic band structure in the mesoscopic regime to enhance optical nonlinearities. Specifically, we demonstrate an increase in second-harmonic generation (SHG) from crystalline silver films as their thickness is reduced down to just a few atomic monolayers. Operating at the boundary between bulk and two-dimensional systems, these ultra-thin films exhibit a pronounced enhancement of SHG with decreasing thickness. This enhancement stems from quantum confinement effects that modify the interaction between electronic states and incident light, which we explain based on quantum-mechanical calculation. Our atomically-thin crystalline silver films provide a new means to overcome the small interaction volumes inherent to nanophotonic platforms, enabling efficient nanoscale nonlinear optics with potential applications in photonics, sensing, and quantum technologies.

physics.optics↗

arXiv2025

Wave-mixing cathodoluminescence microscopy of low-frequency excitations

Nonlinear optical phenomena such as parametric amplification and frequency conversion are typically driven by external optical fields. Free electrons can also act as electromagnetic sources, offering unmatched spatial precision. Combining optical and electron-induced fields via the nonlinear response of material structures therefore holds potential for revealing new physical phenomena and enabling disruptive applications. Here, we theoretically investigate wave mixing between external light and the evanescent fields of free electrons, giving rise to inelastic photon scattering mediated by the second-order nonlinear response of a specimen. Specifically, an incident photon may be blue- or red-shifted, while the passing electron correspondingly loses or gains energy. These processes are strongly enhanced when the frequency shift matches an optical resonance of the specimen. We present a general theoretical framework to quantify the photon conversion probability and demonstrate its application by revealing far-infrared vibrational fingerprints of retinal using only visible light. Beyond its fundamental interest, this phenomenon offers a practical approach for spatially mapping low-frequency excitations with nanometer resolution using visible photon energies and existing electron microscopes.

cond-mat.mes-hall↗

arXiv2025

Photonics in Flatland: Challenges and Opportunities for Nanophotonics with 2D Semiconductors

Two-dimensional (2D) semiconductors are emerging as a versatile platform for nanophotonics, offering unprecedented tunability in optical properties through exciton resonance engineering, van der Waals heterostructuring, and external field control. These materials enable active optical modulation, single-photon emission, quantum photonics, and valleytronic functionalities, paving the way for next-generation optoelectronic and quantum photonic devices. However, key challenges remain in achieving large-area integration, maintaining excitonic coherence, and optimizing amplitude-phase modulation for efficient light manipulation. Advances in fabrication, strain engineering, and computational modelling will be crucial to overcoming these limitations. This perspective highlights recent progress in 2D semiconductor-based nanophotonics, emphasizing opportunities for scalable integration into photonics.

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-ångström 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↗

arXiv2025

Realization of a Pre-Sample Photonic-based Free-Electron Modulator in Ultrafast Transmission Electron Microscopes

Spatial and temporal light modulation is a well-established technology that enables dynamic shaping of the phase and amplitude of optical fields, significantly enhancing the resolution and sensitivity of imaging methods. Translating this capability to electron beams is highly desirable within the framework of a transmission electron microscope (TEM) to benefit from the nanometer spatial resolution of these instruments. In this work, we report on the experimental realization of a photonic-based free-electron modulator integrated into the column of two ultrafast TEMs for pre-sample electron-beam shaping. Electron-photon interaction is employed to coherently modulate both the transverse and longitudinal components of the electron wave function, while leveraging dynamically controlled optical fields and tailored design of electron-laser-sample interaction geometry. Using energy- and momentum-resolved electron detection, we successfully reconstruct the shaped electron wave function at the TEM sample plane. These results demonstrate the ability to manipulate the electron wave function before probing the sample, paving the way for the future development of innovative imaging methods in ultrafast electron microscopy.

physics.optics↗

arXiv2025

Quantum sensing and metrology with free electrons

The quantum properties of matter and radiation can be leveraged to surpass classical limits of sensing and detection. Quantum optics does so by creating and measuring nonclassical light. However, better performance requires higher photon-number states, which are challenging to generate and detect. Here, we combine photons and free electrons to solve the problem of generating and detecting high-number states well beyond those reachable with light alone and further show that an unprecedented level of sensitivity and resolution is gained based on the measurement of free-electron currents after suitably designed electron-light interaction events. Our enabling ingredient is the strong electron-light coupling produced by aloof electron reflection on an optical waveguide, leading to the emission or absorption of a high number of guided photons by every single electron. We theoretically demonstrate that, by combining electron-beam splitters with two electron-waveguide interactions, the sensitivity to detect optical-phase changes can be enhanced tenfold using currently attainable technology. We further show that waveguided NOON states comprising tens of photons can be generated at megahertz rates based on electron post-selection after electron-waveguide interaction. These results inaugurate a disruptive quantum technology relying on free electrons and their strong interaction with waveguided light.

quant-ph↗

arXiv2025

Zeptosecond free-electron compression through temporal lensing

The pursuit of ever-shorter time scales is a frontier in modern physics, exemplified by the synthesis of attosecond light pulses -- an achievement made possible by coherently superimposing a broad range of photon energies, as required by the uncertainty principle. However, extending this progress into the zeptosecond regime poses significant challenges, as it demands phase-correlated optical spectra spanning hundreds of electronvolts. In this context, electrons offer a compelling alternative to light because they can be coherently manipulated to form broad energy superpositions, as demonstrated by the generation of attosecond pulses in ultrafast electron microscopes. Here, we propose a practical scheme for compressing free electrons into the zeptosecond domain by modulating their wave functions using suitably tailored broadband light fields. Building on recent advances in {free-electron--light--matter} interactions, our method introduces the concept of temporal lensing -- an extension of conventional optical lensing to the time domain -- to produce electron pulses with arbitrarily short durations.

cond-mat.mes-hall↗

arXiv2025

Spectrometer-Free Electron Spectromicroscopy

We introduce an approach for performing spectrally resolved electron microscopy without the need for an electron spectrometer. The method involves an electron beam prepared as a coherent superposition of multiple paths, one of which passes near a laser-irradiated specimen. These paths are subsequently recombined, and their interference is measured as a function of laser frequency and beam position. Electron--light scattering introduces inelastic components into the interacting path, thereby disturbing the interference pattern. We implement this concept using two masks placed at conjugate image planes. The masks are complementary and act in tandem to fully suppress electron transmission in the absence of a specimen. However, electron interaction with an illuminated specimen perturbs the imaging condition, enabling electron transmission through the system. For a fixed external light intensity, the transmitted electron current is proportional to the strength of the local optical response in the material. The proposed technique does not require monochromatic electron beams, dramatically simplifying the design of spectrally resolved electron microscopes.

cond-mat.mtrl-sci↗

arXiv2025

Roadmap for Photonics with 2D Materials

Triggered by the development of exfoliation and the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals currently constitute a wide research field protruding in multiple directions in combination with layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary collection of those directions, where 2D materials contribute with polaritons of unique characteristics such as strong spatial confinement, large optical-field enhancement, long lifetimes, high sensitivity to external stimuli (e.g., electric and magnetic fields, heating, and strain), a broad spectral range from the far infrared to the ultraviolet, and hybridization with spin and momentum textures of electronic band structures. The explosion of photonics with 2D materials as a vibrant research area is producing breakthroughs, including the discovery and design of new materials and metasurfaces with unprecedented properties as well as applications in integrated photonics, light emission, optical sensing, and exciting prospects for applications in quantum information, and nanoscale thermal transport. This Roadmap summarizes the state of the art in the field, identifies challenges and opportunities, and discusses future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners.

cond-mat.mtrl-sci↗