arXiv subjects

N. Asger Mortensen

Publications and source records attributed to N. Asger Mortensen.

At least 19 recordsLinked to original sources

arXiv2026

Spectral Anisotropy in Transition Radiation from Biaxial Media

In anisotropic optical media, the electromagnetic response depends on the orientation of the optical field relative to the material's principal dielectric axes. While this direction dependence is well understood in conventional optics, it should also influence light-generation processes driven by free electrons. Here, we experimentally observe spectrally anisotropic transition radiation from biaxial van der Waals crystals. Using cathodoluminescence spectroscopy on germanium sulphide (GeS) and molybdenum oxydichloride (MoOCl$_2$) crystals, we show that the transition-radiation spectra differ along the principal in-plane optical axes. To describe this spectral anisotropy, we develop a thin-film transition-radiation model that reproduces the experimental observations. Our results demonstrate that transition radiation is a sensitive probe of the axis-dependent dielectric response of biaxial optical media and suggest that optical anisotropy can provide an additional degree of freedom for free electron-driven spectroscopy, radiation sources, and transition-radiation-based diagnostics.

physics.optics↗

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

Beam Routing through Excitons in Transition Metal Dichalcogenide Monolayers

Routing light at the nanoscale typically relies on nanostructured surfaces to imprint directionality on the emission. Using low-temperature, angle-resolved cathodoluminescence spectroscopy, we show that the intrinsic excitonic transitions of a semiconductor can themselves produce routed emission. We probe monolayers of WSe$_2$, MoSe$_2$, and MoTe$_2$ and resolve the excitonic species of monolayer WSe$_2$ -- the bright exciton, the trion, and the spin-forbidden dark exciton -- through their distinct angular emission profiles. While the in-plane transition dipoles of the bright exciton and trion radiate predominantly toward the surface normal, the out-of-plane dipole of the dark exciton, inaccessible under normal-incidence optical excitation, produces a directional emission channel at large angles. We further tune the balance between neutral and charged exciton emission through the local dielectric environment. Our results establish dark excitons in TMD monolayers as a platform for directional light emission in compact photonic architectures without additional nanostructuring.

cond-mat.mtrl-sci↗

arXiv2026

Nanoscopy of surface polarization with oblique dipole orientations

The boundary conditions imposed by confined dipoles with arbitrary orientation on surfaces are presented, extending the conventional in-plane (IP) and out-of-plane (OOP) treatments, here applied for planar and cylindrical sheets. Examples include van der Waals heterostructures, thin films of molecular aggregates, and metal-dielectric interfaces. For large dipole strengths, the reflectance peak associated with the dipole oscillation frequency splits into two, revealing the presence of oblique dipoles. The loss function for the dipole sheet reveals pairs of polaritonic resonances originating from the IP and OOP dipole components, accessible through near-field probes. The point-dipole model for s-SNOM shows two distinct peaks, revealing higher sensitivity to dipole obliqueness than reflectance experiments. We apply the model to monolayer WSe$_2$, showing that the oblique dipole formulation with a dipole angle of $5.7^\circ$ yields significant improvements in the qualitative and quantitative agreement with an experiment reported in the literature. This work proposes a unified language for the description of two-dimensional materials, thin films, and interfaces with anisotropic dipolar responses and shows that near-field methods, sensitive to high in-plane momenta, are suitable for measuring such oblique dipoles.

physics.optics↗

arXiv2026

Crystalline metal flakes: Platforms for advanced plasmonics and hybrid 2D material architectures

Crystalline noble metal flakes are emerging as versatile platforms in nanophotonics, enabling a broad range of optical phenomena and applications. Their atomically flat surfaces, high crystallinity, and superior optical quality open new avenues in advanced plasmonics, quantum light generation, and hybrid photonic systems. In contrast to conventional polycrystalline metal films, which typically suffer from higher optical losses due to grain boundaries, surface roughness, and structural disorder, these monocrystalline flakes provide minimal scattering and enhanced performance. They serve as templates for precise nanostructuring through techniques like focused-ion beam (FIB) milling and are crucial for advanced applications in sensing and optoelectronics. Additionally, they facilitate frontier research in quantum plasmonics, enabling fundamental studies of nonlocal optical effects and the generation of nonclassical light. Furthermore, the well-defined $\{111\}$ facets of these flakes host Tamm--Shockley surface states that support 2D plasmons coexisting with bulk modes. At near-infrared wavelengths and beyond, crystalline flakes act as nearly ideal metallic mirrors, featuring surface roughness limited only to atomic terrace steps, making them highly suitable for integration with 2D materials in hybrid photonic architectures. This review surveys the key roles these flakes play, highlighting recent developments and discussing future prospects while emphasizing their unique benefits in addressing fundamental and applied challenges in modern nanophotonics.

physics.optics↗

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

Velocity-tunable exciton-photon hybridization in cathodoluminescence

Exciton-photon hybridization is typically realised in geometrically defined optical cavities, where tunability is achieved by modifying either the cavity or the excitonic medium. Here we investigate transition-radiation interferences in suspended subwavelength films resembling a free-electron-defined resonance and explore their interaction with excitons in transition metal dichalcogenides. We demonstrate that these resonances hybridize with excitonic transitions and can be tuned continuously by varying the electron energy. The resulting detuning depends on both film thickness and electron velocity, establishing the latter as an external and continuous knob for exciton-photon coupling. This approach enables tunable hybridization without structural modification and provides a free-electron-driven nanoscale platform for studying exciton-light interactions.

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

Substrate-Assisted Cathodoluminescence

Electron-beam-induced luminescence typically relies on direct excitation by high energy primary electrons. Here, we explore properties of an alternative excitation approach where cathodoluminescence (CL) is driven by substrate-generated electrons rather than by the primary electron beam. Using color centers in diamond as sensitive and durable local probes, we investigate the spatial profiles of such indirect CL in different geometries and substrates. Photon-correlation experiments demonstrate increased synchronization of emitters at reduced currents, which we propose as a method for extracting the effective indirect excitation currents experienced by the emitters. This approach enables the estimation of remarkably low currents, down to 0.1 pA, highlighting the potential of substrate-assisted excitation for minimally invasive probing of sensitive emitters in CL microscopy.

physics.optics↗

arXiv2025

Nonlocal electrodynamics of two-dimensional anisotropic magneto-plasmons

We present a hydrodynamic model, grounded in Madelung's formalism, to describe collective electronic motion in anisotropic materials. This model incorporates nonlocal contributions from the Thomas-Fermi quantum pressure and quantum effects arising from the Bohm potential. We derive analytical expressions for the magnetoplasmon dispersion and nonlocal optical conductivity. To demonstrate the applicability of the model, we examine electrons in the conduction band of monolayer phosphorene, an exemplary anisotropic two-dimensional electron gas. The dispersion of plasmons derived from our hydrodynamic approach is closely aligned with that predicted by ab~initio calculations. Then, we use our model to analyze few-layer black phosphorus, whose measured infrared optical response is hyperbolic. Our results reveal that the incorporation of nonlocal and quantum effects in the optical conductivity prevents black phosphorus from supporting hyperbolic surface plasmon polaritons. We further demonstrate that the predicted wavefront generated by an electric dipole exhibits a significant difference between the local and nonlocal descriptions for the optical conductivity. This study underscores the necessity of moving beyond local approximations when investigating anisotropic systems capable of hosting strongly confined plasmon-polaritons.

cond-mat.mes-hall↗

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↗

arXiv2025

Application of Madelung Hydrodynamics to Plasmonics and Nonlinear Optics in Two-Dimensional Materials

This paper explores the application of Madelung hydrodynamic models to study two-dimensional electron gases, with a focus on nonlocal plasmonics and nonlinear optics. We begin by reviewing the derivation of the Madelung equations. Using the Madelung equations in conjunction with Poisson's equation, we calculate the spectrum of magnetoplasmons and the magneto-optical conductivity in the electrostatic regime, incorporating nonlocal corrections due to the Fermi pressure. In the absence of a magnetic field, we analyze nonlinear and nonlocal second-harmonic generation, demonstrating how plasmon excitation enhances this process. We further discuss the emergence of self-modulation phenomena driven by nonlinearity, leading to the renormalization of the plasmon dispersion. Notably, we show that nonlinearity amplifies nonlocal effects and, leveraging the hydrodynamic formalism, derive a simple analytic expression for the renormalized spectra.

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

Prospects to bypass nonlocal phenomena in metals using phonon-polaritons

Electromagnetic design relies on an accurate understanding of light-matter interactions, yet often overlooks electronic length scales. Under extreme confinement, this omission can lead to nonclassical effects, such as nonlocal response. Here, we use mid-infrared phonon-polaritons in hexagonal boron nitride (hBN) screened by monocrystalline gold flakes to push the limits of nanolight confinement unobstructed by nonlocal phenomena, even when the polariton phase velocity approaches the Fermi velocities of electrons in gold. We employ near-field imaging to probe polaritons in nanometre-thin crystals of hBN on gold and extract their complex propagation constant, observing effective indices exceeding 90. We further show the importance of sample characterisation by revealing a thin low-index interfacial layer naturally forming on monocrystalline gold. Our experiments address a fundamental limitation posed by nonlocal effects in van der Waals heterostructures and outline a pathway to bypass their impact in high-confinement regimes.

physics.optics↗

arXiv2025

An atlas of photonic and plasmonic materials for cathodoluminescence microscopy

Cathodoluminescence (CL) microscopy has emerged as a powerful tool for investigating the optical properties of materials at the nanoscale, offering unique insights into the behavior of photonic and plasmonic materials under electron excitation. We introduce an atlas of bulk CL spectra and intensity for a broad range of materials used in photonics and plasmonics. Through a combination of experimental CL microscopy and Monte Carlo simulations, we characterize spectra and intensity of coherent and incoherent CL, electron penetration depth and energy deposition, offering a foundational reference for interpreting CL signals and understanding material behavior under electron excitation. Our atlas captures CL signals across a wide range of materials, offering valuable insight into intrinsic emission properties for informed material selection and device design in photonics and plasmonics.

physics.optics↗

arXiv2025

Near-field refractometry of van der Waals crystals

Common techniques for measuring refractive indices, such as ellipsometry and goniometry, are ineffective for van der Waals crystal flakes because of their high anisotropy and small, micron-scale, lateral size. To address this, we employ near-field optical microscopy to analyze the guided optical modes within these crystals. By probing these modes in MoS$_2$ flakes with subwavelength spatial resolution at a wavelength of $1570\,\mathrm{nm}$, we determine both the in-plane and out-of-plane permittivity components of MoS$_2$ as $16.11$ and $6.25$, respectively, with a relative uncertainty below $1\%$, while overcoming the limitations of traditional methods.

cond-mat.mes-hall↗

arXiv2025

Tunable exciton polaritons in biased bilayer graphene

By harnessing the unique properties of bilayer graphene, we present a flexible platform for achieving electrically tunable exciton polaritons within a microcavity. Using a semiclassical approach, we solve Maxwell's equations within the cavity, approximating the optical conductivity of bilayer graphene through its excitonic response as described by the Elliott formula. Transitioning to a quantum mechanical framework, we diagonalize the Hamiltonian governing excitons and cavity photons, revealing the resulting polariton dispersions, Hopfield coefficients and Rabi splittings. Our analysis predicts that, under realistic exciton lifetimes, the exciton-photon interaction reaches the strong coupling regime. Furthermore, we explore the integration of an epsilon-near-zero material within the cavity, demonstrating that such a configuration can further enhance the light-matter interaction.

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↗