arXiv subjects

Christos Tserkezis

Publications and source records attributed to Christos Tserkezis.

At least 19 recordsLinked to original sources

arXiv2026

Theoretical investigation of two-dimensional semiconductor nanoribbons and nanoparticles for tailored light--matter interactions

We explore theoretically the optical response of two-dimensional (2D) materials patterned at the nanoscale into either arrays of ribbons along a planar surface or spherical particles. Fourier-Floquet decomposition of the electromagnetic fields is used in order to obtain the reflectance, transmittance and absorbance of the nanoribbon array. The spherical particles are treated with Mie theory, with the boundary conditions modified to accommodate a surface conductivity at the interface. We consider the excitonic response of hexagonal boron nitride in the ultraviolet, and of the transition-metal dichalcogenide WS2 in the visible. Unlike what is expected from graphene, where plasmons have proven very sensitive to geometry, the excitonic response of nanoribbon arrays is found to show weak tunability with the array parameters. To enhance tunability with the geometry of the system, or via hybridization with the substrate, nanospheres with a 2D semiconductor coating are demonstrated to provide a superior platform. Overall, we find that the localized nature of excitons in 2D semiconductors largely limits tunability with geometry, but their hybridization with other optical modes in the nanopatterning setup still holds promise for controllable light--matter interactions.

cond-mat.mes-hall↗

arXiv2026

OpenMUSTANC (MUltiple Scattering Theory At Nanoplasmonic Cavities): A MATLAB toolbox for the simulation of Plasmonic Sphere Aggregates

Mesoscopic physical models, including the Hydrodynamic Drude Model (HDM), the Generalized Nonlocal Optical Response (GNOR) Model, and the Surface Response Model (SRM), have been proposed to investigate nonlocal effects in nanometric structures. The combination of classical electromagnetism with these mesoscopic material models calls for new computational electromagnetic (CEM) algorithms, or update of conventional ones, in what is termed computational mesoscopic electromagnetics (CMEM). In this work, we present a MATLAB toolbox for the simulation of multiple spherical interfaces with arbitrary relative positions, with the incorporation of the aforementioned mesoscopic models. The method exploits vector spherical wave functions to properly express electric and magnetic fields, an S matrix formulation for the efficient treatment of incident and scattered fields at spherical interfaces, and a translation matrix to deal with propagating waves with different expansion centers. Excitation sources can be chosen among arbitrarily polarized plane waves, dipoles and electron beams. For the post-processing part, the calculation of cross sections and far/near-field mapping; fluorescence enhancement, Purcell factor and quantum yield; and cathodoluminescence and electron energy-loss probability is implemented. The toolbox is built in a modular manner, and each part (routine) has its own important functionality. This paper provides a full explanation of the proposed highly efficient and general toolbox, and a detailed guideline for researchers in the nanoplasmonics community.

physics.comp-ph↗

arXiv2026

Time- and frequency-domain study for electron beams penetrating dielectric nanospheres: fingerprints of Cherenkov and transition radiation

We present a theoretical study of Cherenkov and transition radiation for swift electron beams penetrating dielectric nanospheres using material models of different sophistication. Specifically, we perform a combined time-domain (numerically, via the discontinuous Galerkin time-domain method) and frequency-domain (numerically and analytically, via Mie-based theory) study, including the induced-field distribution, cathodoluminescence (CL) multipole/directional decomposition, as well as the time-dependent angular power flow. For low velocities below the Cherenkov threshold, we show that transition radiation is dominant in the far-field CL, and the near-fields at the transition points are primarily responsible for the main features observed in the far-field. For higher velocities far beyond the Cherenkov threshold, we identify the fingerprints of the observable Cherenkov front. Specifically, a constant-permittivity model allows us to isolate the respective contributions of CR and TR to the far-field radiation, thereby facilitating the interpretation of the results for a more realistic material model that includes material resonances. Our combined time- and frequency-domain framework provides a direct view of radiative excitation channels for swift electron beams penetrating dielectric nanoparticles, thereby revealing their interplay beyond the conventional frequency-domain analyses.

physics.optics↗

arXiv2026

Gibbs Phenomenon and Friedel Oscilations: Similarities, Differences, and the Educational Potential of their Comparison

We explore the similarities and differences between the Gibbs phenomenon in partial Fourier representations of discontinuous signals and Friedel oscillations in the electron density of a solid near an anomaly. Inspired by the apparent similarities of the two phenomena, we perform a detailed exploration of both from the viewpoint of an engineer being introduced to a concept from solid-state physics. Focusing on the density of an one-dimensional electronic gas confined by a square potential, we show that, despite the similarities, Friedel oscillations cannot be attributed to the inability of a partial Fourier series to describe a discontinuity. Nevertheless, the two phenomena do exhibit similarities, which can be exploited to develop intuition. By adopting an educational style, we hope to establish some common language between electrical engineers and condensed-matter physicists, hoping that this can further inspire the two communities to seek intuition and further comprehension within neighbouring but different disciplines.

cond-mat.other↗

arXiv2026

Screened topological plasmons in graphene plasmonic crystals

We study topological effects in an one-dimensional plasmonic crystal formed by the screened plasmons emerging in a periodically modulated graphene sheet, placed on top of a metallic substrate. To this end, we develop the theory of quantization of screened plasmons, as appropriate for lossless graphene described by a Drude conductivity. By analyzing the resulting band structure, we show that the crystal sustains nontrivial topological bands, with quantized geometric phase. We further show that in a finite, open system, edge states appear within the band gap, which undergo a topological phase transition and merge with bulk states as the modulation increases. Our work provides a robust theoretical framework for the study of band structure and topology of layered media, and extends the possibilities for engineering two-dimensional materials with external modulation.

cond-mat.mes-hall↗

arXiv2026

Probe- and Substrate-Dependent Visibility of Mie Resonances in Silicon Nanospheres

Silicon nanospheres are high-quality optical resonators and promising building blocks for Mie-tronic devices. While the Mie resonances of an isolated sphere are well understood, practical implementations require substrates that inevitably modify the measured optical response. Here, we investigate how substrates alter the observable spectrum of individual nanospheres, focusing on three fundamentally different cases: a thin silicon nitride membrane, that emulates a free-standing particle, bulk silicon, which is common in experiments, and gold, where mirror charges lead to hybrid optical modes. Cathodoluminescence and dark-field spectroscopy, combined with electrodynamic simulations, show that the measured resonances are not intrinsic to the particle but depend strongly on the environment and the excitation mechanism. We find that substrate-induced effects and probe-specific selection rules can suppress, enhance, or even invert the spectral signatures of electric and magnetic modes. These results provide practical guidelines for interpreting and designing substrate-supported dielectric resonators for Mie-tronic applications.

physics.optics↗

arXiv2026

Structural Colours with Transition Metal Dichalcogenide Nanostructures

We introduce transition metal-dichalcogenide (TMD) nanostructures as a promising platform for the realisation of structural colours. Processing of semianalytically calculated reflectance spectra of TMD nanosphere arrays shows a wide range of colours, which are obtained simply through tailoring the radius and separation of spheres in the array, with the size-dependent Mie modes of the nanoparticles being the primary contributor to the spectra. Additionally, it is demonstrated that further coverage of the colour space can be obtained by employing different materials or different lattice unit cells. Theoretical examination of the impact of the excitonic attributes of TMDs on the resulting structural colours indicates that self-hybridisation between nanoparticle modes and excitonic transitions may be employed for further tuneability. Moreover, the impact of TMD anisotropy on the structural colours is shown to be negligible for small structures at typical viewing angles, while the viewing angle itself may impact the colour. This work sets out to be a general investigation of TMD nanoarchitectures, with a focus on nanosphere arrays, for structural colours, by examining both inherent material features through the lens of colourimetry, and the ability of such structures to sustain a broad range of hues.

cond-mat.mes-hall↗

arXiv2026

Probing strong coupling in core--shell nanoparticles with fast electron beams

Collective optical excitations, such as localized surface plasmons in metallic nanoparticles and Mie resonances in high-index dielectrics, play a central role in nanoscale light--matter interactions. When such optical modes interact with electronic transitions in matter under suitable conditions, they can couple strongly, analogous to two coupled harmonic oscillators, forming hybrid light--matter states. In this work, we probe this coupling in core--shell nanoparticles using fast electrons in electron energy-loss (EEL) and cathodoluminescence (CL) spectroscopy. Owing to their highly localized fields, fast electrons can excite modes inaccessible with light-based spectroscopies, including higher-order nonradiative modes, which offer greater field confinement and potentially stronger coupling. Here, we develop an analytical framework to calculate the EEL and CL probabilities for spherical core--shell nanoparticles under aloof and penetrating electron trajectories. This formalism is applied to two representative systems: an excitonic core with a metallic shell, and a silicon core with an excitonic shell. Our main focus is to examine how the electron beam position and velocity affect our ability to probe this coupling. Depending on the electron beam parameters, we find that the spectral signature of strong coupling remains robust in plasmonic nanospheres. In contrast, it can be significantly suppressed or even completely obscured in dielectric nanospheres. Our developed formalism enables a deeper understanding of the coupling mechanisms in electron--light--matter interactions, thereby accelerating progress in single-nanoparticle-based polaritonic studies.

physics.optics↗

arXiv2025

Overcoming Computational Bottlenecks in Quantum Hydrodynamics: A Volume-Based Integral Formalism

Mesoscopic models of the optical response of metals have emerged as fundamental building blocks in quantum plasmonics, in principle overcoming the computational bottlenecks of ab initio techniques by implementing aspects of the atomistic description of the metal in otherwise classical calculations. Nonetheless, even these approaches are eventually hindered by demanding computations due to sophisticated material response. Here, this issue is addressed for the advanced Self-Consistent Hydrodynamic Drude Model (SC-HDM), which captures both nonlocal electron dynamics and electron spill-out, through a Volume Integral Equation (VIE) method. Adopting an IE-based method shifts perspective from the commonly employed Differential Equation (DE)-based ones, demonstrating significant computational efficiency. The VIE approach is a valuable methodological scaffold: It addresses SC-HDM and simpler models, but can also be adapted to more advanced ones. For spherical nanoparticles (NPs), using the inherent symmetries, similar performance for three increasingly complicated material models is achieved, breaking the taboo that increased sophistication in material response requires taxing simulations. Mesoscopic material-response functions can be readily extracted from the VIE implementation, thus circumventing the need for lengthy microscopic calculations. This method opens a new way of modeling quantum hydrodynamic NPs and will serve as essential benchmarking tool for recipes addressing more complicated geometries.

physics.comp-ph↗

arXiv2025

An S-matrix Formalism for the Nonclassical Optical Response of Plasmonic Sphere Aggregates

A computational method for the scattering of light by multiple nonclassical plasmonic nanospheres, each of which has multiple (non-)concentric dielectric or metallic layers, is presented. The electromagnetic (EM) response of the free electrons in the metals is described by three popular mesoscopic models: the nonlocal hydrodynamic Drude model (NLHDM) and its diffusive variant, namely the generalized nonlocal optical response (GNOR) model, as well as the surface response model (SRM). The main equation behind the method is set up by detailing the evaluation of the S-matrix for each individual spherical interface and the interactions amongst the interfaces. The algorithm is numerically validated by comparing with an in-house boundary element method (BEM) solver for a spherical NP with two smaller embedded spheres, and physically checked on a trimer configuration, where the responses from the NLHDM and SRM are contrasted with the local response model (LRM). In both cases a very good agreement is seen regarding frequency shifts and field enhancements.

physics.comp-ph↗

arXiv2025

Non-Markovian effects in long-range polariton-mediated energy transfer

Intramolecular energy transfer driven by near-field effects plays an important role in applications ranging from biophysics and chemistry to nano-optics and quantum communications. Advances in strong light-matter coupling in molecular systems have opened new possibilities to control energy transfer. In particular, long-distance energy transfer between molecules has been reported as the result of their mutual coupling to cavity photon modes, and the formation of hybrid polariton states. In addition to strong coupling to light, molecular systems also show strong interactions between electronic and vibrational modes. The latter can act as a reservoir for energy to facilitate off-resonant transitions, and thus energy relaxation between polaritonic states at different energies. However, the non-Markovian nature of those modes makes it challenging to accurately simulate these effects. Here we capture them via process tensor matrix product operator (PT-MPO) methods, to describe exactly the vibrational environment of the molecules combined with a mean-field treatment of the light-matter interaction. In particular, we study the emission dynamics of a system consisting of two spatially separated layers of different species of molecules coupled to a common photon mode, and show that the strength of coupling to the vibrational bath plays a crucial role in governing the dynamics of the energy of the emitted light; at strong vibrational coupling this dynamics shows strongly non-Markovian effects, eventually leading to polaron formation. Our results shed light on polaritonic long-range energy transfer, and provide further understanding of the role of vibrational modes of relevance to the growing field of molecular polaritonics.

cond-mat.mes-hall↗

arXiv2025

Roadmap on Nonlocality in Photonic Materials and Metamaterials

Photonic technologies continue to drive the quest for new optical materials with unprecedented responses. A major frontier in this field is the exploration of nonlocal (spatially dispersive) materials, going beyond the local, wavevector-independent assumption traditionally made in optical material modeling. On one end, the growing interest in plasmonic, polaritonic and quantum materials has revealed naturally occurring nonlocalities, emphasizing the need for more accurate models to predict and design their optical responses. This has major implications also for topological, nonreciprocal, and time-varying systems based on these material platforms. Beyond natural materials, artificially structured materials--metamaterials and metasurfaces--can provide even stronger and engineered nonlocal effects, emerging from long-range interactions or multipolar effects. This is a rapidly expanding area in the field of photonic metamaterials, with open frontiers yet to be explored. In the case of metasurfaces, in particular, nonlocality engineering has become a powerful tool for designing strongly wavevector-dependent responses, enabling enhanced wavefront control, spatial compression, multifunctional devices, and wave-based computing. Furthermore, nonlocality and related concepts play a critical role in defining the ultimate limits of what is possible in optics, photonics, and wave physics. This Roadmap aims to survey the most exciting developments in nonlocal photonic materials, highlight new opportunities and open challenges, and chart new pathways that will drive this emerging field forward--toward new scientific discoveries and technological advancements.

cond-mat.mes-hall↗

arXiv2025

Self-similar plasmonic nanolenses: mesoscopic ensemble averaging and chiral light-matter interactions

We investigate how the near-field enhancement of self-similar nanolenses, made of three plasmonic nanospheres with decreasing sizes and separations, is affected by quantum corrections in the optical response of the metal, and by fabrication imperfections related to the positioning of the spheres in the nanolens. In particular, we integrate mesoscopic phenomena, such as electron spill-in and -out and surface-enabled Landau damping, via the surface-response formalism, focusing particularly on the role of spill-out in alkali metals. In addition, we take realistic imperfections in the nanofabrication process into account, through numerical averaging of both far- and near-field spectra for large collections of nanolenses. Statistical analysis of hundreds of trimers shows that inevitable deviations from the perfectly aligned chain only slightly, if at all, impair the field enhancement, as long as the average misplacement is kept within 1 nm from the ideal situation. Wishing to explore whether such imperfections can be harvested for practical applications, we probe the potential for triggering chiral response. Our results verify that imperfect nanolenses can display considerable light-induced optical activity and circular dichroism, while providing a means to manipulate the optical chirality density. This highlights how promising the nanolensing effect is for chiral light-matter interactions. Nonetheless, we emphasize that quantification of chiral light-matter interactions can be largely affected by mesoscopic phenomena, which cannot be ignored when near-field quantities like optical chirality density are investigated.

cond-mat.mes-hall↗

arXiv2024

Quantum-hydrodynamic modal perspective on plasmonic gap structures

Plasmonic gap structures are among the few configurations capable of generating extreme light confinement, finding applications in surface-enhanced spectroscopy, ultrasensitive detection, photocatalysis and more. Their plasmonic response undergoes a dramatic, quantum effect-driven transition as the gap size approaches zero. Modal analysis can reveal insights into the mechanisms governing this process, which are otherwise obscured by nonlocal damping effects. Here, we offer a fresh modal perspective on the transition of the plasmonic response using quantum hydrodynamic theory (QHT)-based quasinormal mode (QNM) analysis. Focusing on the bonding dipolar and charge-transfer plasmons of a nanosphere dimer, we examine the detailed mode transition through the touching regime as well as the asymptotic behavior compared with the classical results as the constituent nanoparticles either separate or overlap. The complex eigenfrequency particularly provides accurate information on the linewidth and quality factor of the plasmon modes. We introduce an index to characterize charge-transfer efficiency, especially for the charge-transfer plasmon. The significant role of nonlocal damping in the mode evolution is elucidated by our mode-resolved QHT-QNM analysis. The insights from our theoretical study provide an integrated understanding of mode evolution in plasmonic gap structures, which can further advance gap structure-based applications.

cond-mat.mes-hall↗

arXiv2024

Tunable exciton polaritons in band-gap engineered hexagonal boron nitride

We show that hexagonal boron nitride (hBN), a two-dimensional insulator, when subjected to an external superlattice potential forms a new paradigm for electrostatically tunable excitons in the near- and mid-ultraviolet (UV). The imposed potential has three consequences: (i) it renormalizes the effective mass tensor, leading to anisotropic effective masses; (ii) it renormalizes the band gap, eventually reducing it; (iii) it reduces the exciton binding energies. All these consequences depend on a single dimensionless parameter, which includes the product of strength of the external potential with its period. In addition to the excitonic energy levels, we compute the optical conductivity along two orthogonal directions, and from it the absorption spectrum. The results for the latter show that our system is able to mimic a grid polarizer. These characteristics make one-dimensional hBN superlattices a viable and unexplored platform for fine-tuned polaritonics in the UV to visible spectral range.

cond-mat.mes-hall↗

arXiv2024

Dirac plasmon polaritons and magnetic modes in topological-insulator nanoparticles

We demonstrate the existence of previously unreported magnetic modes with record-high magnetic Purcell factors in topological-insulator nanospheres. Focusing on bismuth selenide (Bi$_{2}$Se$_{3}$), and based on full electromagnetic Mie theory, we find magnetic modes arising from both displacement current loops in the bulk, and surface currents due to delocalized surface states, induced by electronic transitions between topologically protected states within the Dirac cone and discretized due to the sphere finite size. Furthermore, we discuss how Dirac plasmon polaritons, resulting from the interaction between THz photons and Dirac electrons, dramatically influence both the magnetic and the electric transitions of quantum emitters placed near Bi$_2$Se$_3$ nanospheres, significantly enhancing the corresponding Purcell factors. These findings position Bi$_{2}$Se$_{3}$ nanospheres, whose optical response is related to a richness of physical mechanisms, among the most promising candidates for enhancing light--matter interactions in nanophotonics and THz technologies.

cond-mat.mes-hall↗

arXiv2024

Electron beams traversing spherical nanoparticles: analytic and numerical treatment

We present an analytic, Mie theory-based solution for the energy-loss and the photon-emission probabilities in the interaction of spherical nanoparticles with electrons passing nearby and through them, in both cathodoluminescence and electron energy-loss spectroscopies. In particular, we focus on the case of penetrating electron trajectories, for which the complete fully electrodynamic and relativistic formalism has not been reported as yet. We exhibit the efficiency of this method in describing collective excitations in matter through calculations for a dispersive and lossy system, namely a sphere described by a Drude permittivity. Subsequently, we use the analytic solution to corroborate the implementation of electron-beam sources in a state-of-the-art numerical method for problems in electrodynamics, the discontinuous Galerkin time-domain (DGTD) method. We show that the two approaches produce spectra in good mutual agreement, and demonstrate the versatility of DGTD via simulations of spherical nanoparticles characterized by surface roughness. The possibility of simultaneously employing both kinds of calculations (analytic and numerical) facilitates a better understanding of the rich optical response of nanophotonic architectures excited by fast electron beams.

cond-mat.mes-hall↗

arXiv2023

Quantum-informed plasmonics for strong coupling: the role of electron spill-out

The effect of nonlocality on the optical response of metals lies at the forefront of research in nanoscale physics and, in particular, quantum plasmonics. In alkali metals, nonlocality manifests predominantly as electron density spill-out at the metal boundary, and as surface-enabled Landau damping. For an accurate description of plasmonic modes, these effects need be taken into account in the theoretical modelling of the material. The resulting modal frequency shifts and broadening become particularly relevant when dealing with the strong interaction between plasmons and excitons, where hybrid modes emerge and the way they are affected can reflect modifications of the coupling strength. Both nonlocal phenomena can be incorporated in the classical local theory by applying a surface-response formalism embodied by the Feibelman parameters. Here, we implement surface-response corrections in Mie theory to study the optical response of spherical plasmonic--excitonic composites in core--shell configurations. We investigate sodium, a jellium metal dominated by spill-out, for which it has been anticipated that nonlocal corrections should lead to an observable change in the coupling strength, appearing as a modification of the width of the mode splitting. We show that, contrary to expectations, the influence of nonlocality on the anticrossing is minimal, thus validating the accuracy of the local response approximation in strong-coupling photonics.

cond-mat.mes-hall↗