arXiv subjects

Harald Giessen

Publications and source records attributed to Harald Giessen.

At least 19 recordsLinked to original sources

arXiv2026

Analysis of the Topology of a Plasmonic Target-Skyrmion Texture

Topological concepts are frequently used to describe structured optical fields, including plasmonic near fields. Topological descriptions in terms of skyrmion numbers implicitly assume the compactness of the underlying manifold. Even when skyrmion-like textures appear locally, the compactness is usually not fulfilled in extended optical fields. Here, we use photoemission electron microscopy to investigate a plasmonic nano-focus that exhibits a sequence of radially extending alternating skyrmion and antiskyrmion textures. The full spatio-temporal reconstruction of the electric field vectors and their topology is accessible by vector polarimetry. The experiments confirm the expected oscillatory behavior of the skyrmion number and demonstrate that a global skyrmion number cannot be assigned in such non-compact fields.

physics.optics↗

arXiv2026

Intrinsic plasmon canalization in the biaxial van der Waals crystal MoOCl$_2$

Anisotropic polaritons in low-symmetry crystals allow for subwavelength confinement and directional routing of light. The most extreme form of such anisotropy arises at the topological transition between elliptical and hyperbolic dispersion, where the isofrequency contours collapse into parallel lines and polaritons propagate in a diffractionless, beam-like fashion. This canalization regime has previously been accessed through twisted heterostructures or engineered metasurfaces. Here we show that natural canalization can be achieved without any fabrication or structuring by exploiting the intrinsic elliptical-to-hyperbolic transition in the van der Waals crystal MoOCl$_2$ at room temperature. Using near-field imaging, we directly visualize plasmon-polariton canalization emerging at the low-loss Drude crossing point along the [010] crystal axis. Owing to the moderate slope of the Drude permittivity, the resulting polaritons remain highly directional across a broad spectral window. This weak dispersion also enables robust thickness-dependent tuning, and we demonstrate, both experimentally and theoretically, that the canalization wavelength can be adjusted by more than 1 {\mu}m simply by varying the flake thickness. This work brings canalized polariton propagation into the 4.5 - 6 {\mu}m range, beyond the frequency limits of phonon-polariton platforms and overlapping with important molecular vibrations, opening new opportunities for mid-IR nanophotonics and sensing.

physics.optics↗

arXiv2026

Skyrmions in scalar fields of non-Hermitian optical microcavities: spontaneous formation, nonlinear control, and optical forces

Topological textures of light offer powerful routes for structuring optical fields, controlling wave transport, and manipulating matter. Skyrmions, long studied as topological solitons in vector fields, have recently been extended to scalar wave systems, including acoustics, hydrodynamics, and plasmonics. However, their realization in two-dimensional scalar wave propagation with nonlinearities and in quantum fluids remains uncharted. Here, we establish such a Skyrmion framework for scalar fields in optical microcavities. With focus on exciton-polaritons, we show that nonresonant excitation without imposed phase can spontaneously generate isolated Skyrmions and self-organized Skyrmion lattices in a polariton condensate. We trace this mechanism to gain- and loss-induced phase curvature together with outward polariton flow. We further demonstrate that polariton nonlinearities provide all-optical control of these textures, enabling switching of the Skyrmion number and reconfiguration of Skyrmion moir\'e lattices through resonant and nonresonant excitation schemes. These results establish nonlinear non-Hermitian resonators as a versatile platform for the spontaneous generation and active control of scalar topological light fields.

physics.optics↗

arXiv2026

Size, Shape, and Material matter: All-optical Mie void sensor for complex nanoplastic mixtures

The fragmentation of plastic debris and the direct release of nanoplastics have emerged as a pressing ecological concern. Once dispersed, these enduring particles infiltrate food webs, accumulate within organisms, and bind toxic co-contaminants, posing long-term risks to ecosystems and human health. Despite growing awareness, the detection and characterization of nanoplastics remain highly challenging due to their minute size. Moreover, obtaining additional critical information, such as the particle shape or material composition, further exacerbates these detection hurdles. Conventional analytical techniques capable of providing more detailed information often demand substantial experimental and lab-bound effort, costly instrumentation, and lengthy measurement times. Here, we introduce a novel photonic sensing platform based on nanoscale voids that enables the simultaneous material- and morphology-sensitive detection of particles below 500 nm. Arrays of voids embedded in a high-refractive-index material act in parallel as both sorting elements and direct color reporters. Spherical and elongated particles are selectively trapped in circular and elliptical voids, while polymer types such as PS, PMMA, and PET are distinguished via the specific color signatures arising from their refractive index contrasts. This approach offers a cheap and scalable route toward rapid optical identification of nanoplastics in complex environmental and biological settings. Its compatibility with quick, high-throughput analysis positions it as a promising tool for real-time monitoring and comparative studies of heterogeneous nanoplastic populations.

physics.optics↗

arXiv2026

Compact system development of efficient quantum-entangled photon sources towards deployable and industrial devices

Entangled photon pair sources are a key enabling technology for quantum communication and networking, yet their deployment beyond laboratory environments is hindered by system-level complexity, limited operational stability, and insufficient industry compatibility. Here, we demonstrate a rack-based, mobile quantum light source architecture based on a semiconductor quantum dot emitter that directly addresses these challenges through modular system integration and automated operation. The source generates polarization-entangled photon pairs with an entanglement negativity 2n of up to $0.98(1)$, confirming near-maximal entanglement quality. In continuous, hands-off operation over a six-hour time window, the system achieves an average single-photon emission rate of $697(8)$ kHz and a maximum rate of $740(7)$ kHz, while maintaining 2n-value of more than $95$ $\%$. These results are enabled by the integration of optical excitation, collection, cryogenic operation, and control electronics within a standardized rack footprint, together with automated monitoring. By demonstrating simultaneously high entanglement quality, sustained brightness, and long-term operational stability in an industry-aligned system architecture, this work advances semiconductor quantum dot sources toward deployable entangled photon sources for applied quantum photonics.

quant-ph↗

arXiv2026

Phonon-polaritonic skyrmions: Transition from bubble- to N\'eel-type

Optical skyrmions are members of the emerging topological branch of solid-state physics and photonics, allowing for control over topological light textures through light-matter interactions. However, in nanophotonics their practical application has been severely limited by high inherent losses in plasmonic materials, resulting in the lack of tunability between different topological properties. Here, we utilize the strong dispersion of silicon carbide thin films to realize highly confined surface phonon-polariton skyrmion lattices, which we image via near-field microscopy. We experimentally demonstrate topological tuning between bubble- and N\'eel-type skyrmions, a unique advantage that polar dielectrics offer over most existing approaches. Changing the excitation wavelength by only 10% switches the skyrmion type, revealed by examination of the skyrmion number density contrast. Analysis of domain wall size and steepness in analogy to magnetic materials also confirms this transition. Our results are a starting point to investigate other topological features in phononic systems such as merons, skyrmion bags, and other complex structured light fields. Furthermore, strong light-matter hybridization and nonlinear effects owing to anharmonicity of the phonons may be observed in the future, possibly leading towards the discovery of polaritonic skyrmion-skyrmion interactions and hence applications in topology-based information processing.

physics.optics↗

arXiv2026

Twisted multilayer moir\'e water waves topologically robust to disorder

Moir\'e patterns, stacking and twisting multilayer periodic lattices into superlattices, have become cornerstones of many physical systems from condensed matter to wave phenomena, but have never been properly studied in water waves. Here, we demonstrate twisted multilayer moir\'e water surface waves carrying robust skyrmionic topologies. Using a custom water tank of circular multi-channel phased array, we precisely generate water-wave skyrmion lattices and superimpose them into moir\'e superlattices with higher-order topological textures, e.g., skyrmion bags and clusters, programmed via the twist angle. We also quantitatively compare the topological robustness of bilayer and trilayer configurations under spatiotemporal perturbations. The trilayer moir\'e superlattices exhibit more enhanced stability, stronger field localization and energy concentration than the bilayer. Our work establishes water waves as a macroscopic, tunable, and visually accessible platform for moir\'e physics, towards robust particle manipulation and classical analogues of topological quantum phenomena.

physics.optics↗

arXiv2026

Mie Voids as broadband directional light sources

The Kerker effect arises from the interference between electric and magnetic multipoles, enabling directional light scattering in nanophotonics. However, conventional dielectric and plasmonic nanoparticles can only act as Kerker sources in narrow spectral regions, limiting their applicability. Here, we show that the recently discovered Mie voids overcome this limitation by supporting a broadband generalized Kerker effect spanning the whole visible range. We investigate the optical response of Mie voids under both plane-wave and dipolar excitation. For plane waves, the voids preferentially scatter light in the forward direction. Under dipolar excitation, the resulting radiation emission towards the void and beyond is suppressed due to destructive interference between the dipole field with the directional scattered field of the void. These findings identify Mie voids as versatile broadband directional sources, opening pathways for antenna design and energy harvesting at the nanoscale.

physics.optics↗

arXiv2025

Efficient fiber coupling of telecom single-photons from circular Bragg gratings

Deterministic sources of quantum light are becoming increasingly relevant in the development of quantum communication, particularly in deployed fiber networks. Therefore, efficient fiber-coupled sources at telecom wavelength are highly sought after. With this goal in mind, we systematically investigate the fiber coupling performance of quantum dots in optical resonators under three experimental configurations. We quantify coupling efficiency and sensitivity to spatial displacement for single-mode fibers with 3D printed optics on their tip, and benchmark their behavior over a commercial cleaved-cut fiber and a standard optical setup. The reduction of the required optical elements when operating with a lensed or a bare fiber allows for an increased end-to-end efficiency by a factor of up to 3.0 +/- 0.2 over a standard setup. For the perspective of realizing a mechanically stable fiber-coupled source, we precisely quantify the spatial tolerance to fiber-cavity misalignment, observing less than 50 % count rate drop for several micrometers displacement. These results will play a key role in the future development of fiber-coupled sources of quantum light.

quant-ph↗

arXiv2025

Compact vacuum levitation and control platform with a single 3D-printed fiber lens

Levitated dielectric particles in a vacuum have emerged as a new platform in quantum science, with applications ranging from precision acceleration and force sensing to testing quantum physics beyond the microscopic domain. Traditionally, particle levitation relies on optical tweezers formed by tightly focused laser beams, which typically require multiple bulk optical elements aligned in free space, limiting robustness and scalability of the system. To address these challenges, we employ a single optical fiber equipped with a high numerical aperture (NA) lens directly printed onto the fiber facet. This enables a compact yet robust optical levitation and detection system composed entirely of fiber-based components, eliminating the need for complex alignment. The high NA of the printed lens allows stable single-beam trapping of a dielectric nanoparticle in a vacuum, even while the fiber is in controlled motion. The high NA also allows for collecting scattered light from the particle with excellent collection efficiency, thus enabling efficient detection and feedback stabilization of the particle's motion. Our platform paves the way for practical and portable sensors based on levitated particles and provides simple yet elegant solutions to complex experiments requiring the integration of levitated particles.

physics.optics↗

arXiv2025

Exciton Energy Routing via Defect Networks in hBN/2D-Perovskite Hybrids

Excitons in two dimensional Ruddlesden Popper perovskites (RPPs) exhibit large and tunable binding energies, making them promising candidates for optoelectronic applications. In particular, long-range exciton energy transfer in these materi-als holds potential for light-harvesting technologies and nanoscale interconnects. Here, using cathodoluminescence spectros-copy, we demonstrate that exciton energy can be transferred over ultralong distances, up to 150 micrometers, in heterostructures composed of hexagonal boron nitride (hBN) and RPPs. This transfer is enabled by efficient exciton coupling to defect centers in hBN and subsequent defect defect interactions. This mechanism not only facilitates long-range energy transfer, but also leads to enhanced luminescence intensity, narrower emission linewidths, extended exciton lifetimes, and reduced electron-beam-induced degradation. Owing to the high density of emitters within the hBN layers, the investigated van der Waals heterostructure emerges as a robust and stable hybrid platform. Our findings open promising pathways for room-temperature excitonic devices with enhanced performance, including quantum transducers, light-harvesting systems, and optoelectronic interconnects.

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

A Multi-Dimensional Cathodoluminescence Detector with 3D Printed Micro-Optics on a Fiber

Cathodoluminescence, i.e. the radiation caused by the interaction of high-energy electron beams with matter, has gained a major interest in the analysis of minerals, semiconductors, and plasmonic resonances in nanoparticles. This radiation can either be coherent or incoherent, depending on the underlying interaction mechanism of electrons with nanostructured matter. Thanks to their high spatial resolution and large spectral excitation bandwidth, the evanescent near-field of a moving electron in a scanning electron microscope is used to probe locally photonic modes at the nanoscale, e.g., exciton or plasmon polaritons. The properties of these excitations can be analyzed through both spectral and temporal statistics of the emitted light. Here, we report on the technical design and implementation of a novel fiber-based cathodoluminescence detector for a scanning electron microscope. Moreover, we present first characterization measurements to prove the ability for raster scanning the cathodoluminescence emission using optical fibers with 3D printed micro-optics. The functionality and flexibility of this fiber-based detector is highlighted by resolving the spatial far-field distribution of the excited light, as well as cathodoluminescence spectroscopy and time-correlated single photon counting. Our findings pave the way for a better understanding of the characteristic of the light emitted from electron beams interacting with nanostructures and two-dimensional materials.

physics.optics↗

arXiv2024

Inorganic electrochromic metasurface in the visible

Colour printing based on metallic or dielectric nanostructures has revolutionized colour science due to its unprecedented subwavelength resolution. Evidently, the evolution towards the active control of such structural colours with smart materials is in progress for real applications. Here we experimentally demonstrate a large colour gamut with high intensity and purity, as well as its switching on and off based solely on tungsten trioxide (WO3) cylindrical resonators. The strong resonances in the visible spectral range in these WO3 metasurfaces can be reversibly switched on and off due to its electrochromism by applying alternating voltages of +2.0 V and -0.3 V. Our approach opens up possibilities for the functional diversification of commercial smart windows, as well as the development of new display technologies in the future.

physics.optics↗

arXiv2024

Plasmonic Twistronics: Discovery of Plasmonic Skyrmion Bags

The study of van der Waals heterostructures with an interlayer twist, known as "twistronics", has been instrumental in advancing contemporary condensed matter research. Most importantly, it has underpinned the emergence of a multitude of strongly-correlated phases, many of which derive from the topology of the physical system. Here, we explore the application of the twistronics paradigm in plasmonic systems with nontrivial topology, by creating a moir\'e skyrmion superlattice using two superimposed plasmonic skyrmion lattices, twisted at a "magic" angle. The complex electric field distribution of the moir\'e skyrmion superlattice is measured using time-resolved vector microscopy, revealing that each super-cell possesses very large topological invariants and harbors a "skyrmion bag", the size of which is controllable by the twist angle and center of rotation. Our work shows how twistronics leads to a diversity of topological features in optical fields, providing a new route to locally manipulate electromagnetic field distributions, which is crucial for future structured light-matter interaction.

physics.optics↗

arXiv2024

Spatio-temporal topology of plasmonic spin meron pairs revealed by polarimetric photo-emission microscopy

Topology is the study of geometrical properties and spatial relations unaffected by continuous changes, and has become an important tool for understanding complex physical systems. Although recent optical experiments have inferred the existence of vector fields with the topologies of merons, the inability to extract the full three dimensional vectors misses a richer set of topologies that have not yet been fully explored. In our work, we extend the study of the topology of electromagnetic fields on surfaces to a spin quasi-particle with the topology of a meron pair, formed by interfering surface plasmon polaritons, and show that the in-plane vectors are constrained by the embedding topology of the space as dictated by the Poincare-Hopf theorem. In addition we explore the time evolution of the three dimensional topology of the spin field formed by femtosecond laser pulses. These experiments are possible using our here developed method called polarimetric photoemission electron microscopy (polarimetric PEEM) that combines an optical pump-probe technique and polarimetry with photo-emission electron microscopy. This method allows for the accurate generation of surface plasmon polariton fields and their subsequent measurement, revealing both the spatial distribution of the full three-dimensional electromagnetic fields at deep sub-wavelength resolution and their time evolution.

physics.optics↗

arXiv2024

Skyrmion Bag Robustness in Plasmonic Bilayer and Trilayer Moir\'e Superlattices

Twistronics is studied intensively in twisted 2D heterostructures and its extension to trilayer moir\'e structures has proven beneficial for the tunability of unconventional correlated states and superconductivity in twisted trilayer graphene. Just recently, the concept of twistronics has been applied to plasmonic lattices with nontrivial topology, demonstrating that bilayer moir\'e skyrmion lattices harbor multi-skyrmion textures called skyrmion bags. Here, we explore the properties of plasmonic trilayer moir\'e superlattices that are created by the interference of three twisted skyrmion lattices. More specifically, we explore the properties of periodic superlattices and their topological invariants. We also demonstrate that twisted trilayer skyrmion lattices harbor the same skyrmion bags as twisted bilayer skyrmion lattices. We quantify the robustness of these skyrmion bags by the stability of their topological numbers against certain disturbance fields that leads to experimental designs for topological textures with maximum robustness.

physics.optics↗

arXiv2024

3D-printed axicon enables extended depth-of-focus intravascular optical coherence tomography

A fundamental challenge in endoscopy is how to fabricate a small fiber-optic probe that can achieve comparable function to probes with large, complicated optics (e.g., high resolution and extended depth of focus). To achieve high resolution over an extended depth of focus (DOF), the application of needle-like beams has been proposed. However, existing methods using miniaturized needle beam designs fail to adequately correct astigmatism and other monochromatic aberrations, limiting the resolution of at least one axis. Here, we describe a novel approach to realize freeform beam-shaping endoscopic probes via two-photon direct laser writing, also known as micro 3D-printing. We present a design achieving approximately 8-micron resolution with a DOF of >0.8 mm at a central wavelength of 1310 nm. The probe has a diameter of 0.25 mm (without the catheter sheaths) and is fabricated using a single printing step directly on the optical fiber. We demonstrate our device in intravascular imaging of living atherosclerotic pigs at multiple time points, as well as human arteries with plaques ex vivo. This is the first step to enable beam-tailoring endoscopic probes which achieve diffraction-limited resolution over a large DOF.

physics.optics↗