arXiv subjects

Mario Hentschel

Publications and source records attributed to Mario Hentschel.

16 recordsLinked to original sources

arXiv2026

Control of helix orientation in chiral magnets via lateral confinement

Helimagnetic materials offer a versatile platform for spin-based device concepts owing to their long-range, tunable spiral order. Here, we demonstrate controlled manipulation of the helimagnetic propagation vector q by geometrical confinement, using FeGe as a model Dzyaloshinskii-Moriya interaction (DMI)-driven chiral magnet. Micromagnetic simulations based on the nonlinear sigma model reveal that open boundaries give rise to a chiral surface twist acting as an effective surface anisotropy, which dictates the preferred helix orientation in the absence of magnetostatic shape effects. This geometry-induced anisotropy is quantitatively captured by an analytical model derived from the DMI boundary condition. Magnetic force microscopy measurements on focused-ion-beam structured FeGe confirm the predicted orientation behavior and establish geometry-controlled helimagnetic order as a robust, tunable mechanism for steering DMI-stabilized spin-spiral states. The concept provides a general route toward device-level control of chiral magnetic order in non-centrosymmetric systems.

cond-mat.mtrl-sci↗

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

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

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

Attoliter Mie Void Sensing

Traditional nanophotonic sensing schemes utilize evanescent fields in dielectric or metallic nanoparticles, which confine far-field radiation in dispersive and lossy media. Apart from the lack of a well-defined sensing volume that can be accompanied by moderate sensitivities, these structures suffer from the generally limited access to the modal field, which is key for sensing performance. Recently, a novel strategy for dielectric nanophotonics has been demonstrated, namely, the resonant confinement of light in air. So-called Mie voids created in high-index dielectric host materials support localized resonant modes with exceptional properties. In particular, due to the confinement in air, these structures benefit from the full access to the modal field inside the void. We utilize these Mie voids for refractive index sensing in single voids with volumes down to 100 attoliters and sensitivities on the order of 400 nm per refractive index unit. Taking the signal-to-noise ratio of our measurements into account, we demonstrate detection of refractive index changes as small as 6.9 x 10-4 in a defined volume of just 850 attoliters. The combination of our Mie void sensor platform with appropriate surface functionalization will even enable specificity to biological or other analytes of interest, as the sensing volumes are on the order of cellular signaling chemicals of single vesicles in cellular synapses.

physics.optics↗

arXiv2024

Ultrafast phonon-mediated dephasing of color centers in hexagonal boron nitride probed by electron beams

Defect centers in hexagonal boron nitride have been extensively studied as room temperature single photon sources. The electronic structure of these defects exhibits strong coupling to phonons, as evidenced by the observation of phonon sidebands in both photoluminescence and cathodoluminescence spectra. However, the dynamics of the electron phonon coupling as well as phonon mediated dephasing of the color centers in hexagonal boron nitride remain unexplored. Here, we apply a novel time resolved CL spectroscopy technique to explore the population decay to phonon states and the dephasing time T2 with sub femtosecond time resolution. We demonstrate an ultrafast dephasing time of only 200 fs and a radiative decay of about 585 fs at room temperature, in contrast with all optical time resolved photoluminescence techniques that report a decay of a few nanoseconds. This behavior is attributed to efficient electron-beam excitation of coherent phonon polaritons in hexagonal boron nitride, resulting in faster dephasing of electronic transitions. Our results demonstrate the capability of our sequential cathodoluminescence spectroscopy technique to probe the ultrafast dephasing time of single emitters in quantum materials with sub femtosecond time resolution, heralding access to quantum path interferences in single emitters coupled to their complex environment.

quant-ph↗

arXiv2023

Quasi-Babinet principle in dielectric resonators and Mie voids

Advancing resonant nanophotonics requires novel building blocks. Recently, cavities in high-index dielectrics have been shown to resonantly confine light inside a lower-index region. These so-called Mie voids represent a counterpart to solid high-index dielectric Mie resonators, offering novel functionality such as resonant behavior in the ultraviolet spectral region. However, the well-known and highly useful Babinet's principle, which relates the scattering of solid and inverse structures, is not strictly applicable for this dielectric case as it is only valid for infinitesimally thin perfect electric conductors. Here, we show that Babinet's principle can be generalized to dielectric systems within certain boundaries, which we refer to as the quasi-Babinet principle and demonstrate for spherical and more generically shaped Mie resonators. Limitations arise due to geometry-dependent terms as well as material frequency dispersion and losses. Thus, our work not only offers deeper physical insight into the working mechanism of these systems but also establishes simple design rules for constructing dielectric resonators with complex functionalities from their complementary counterparts.

physics.optics↗

arXiv2022

Phase-locked photon-electron interaction without a laser

Ultrafast electron-photon spectroscopy in electron microscopes commonly requires ultrafast laser setups. Photoemission from an engineered electron source is used to generate pulsed electrons, interacting with a sample that is excited by the ultrafast laser pulse at a specified time delay. Thus, developing an ultrafast electron microscope demands the exploitation of extrinsic laser excitations and complex synchronization schemes. Here, we present an inverse approach based on cathodoluminescence spectroscopy to introduce internal radiation sources in an electron microscope. Our method is based on a sequential interaction of the electron beam with an electron-driven photon source (EDPHS) and the investigated sample. An electron-driven photon source in an electron microscope generates phase-locked photons that are mutually coherent with the near-field distribution of the swift electron. Due to their different velocities, one can readily change the delay between the photons and electrons arriving at the sample by changing the distance between the EDPHS and the sample. We demonstrate the mutual coherence between the radiations from the EDPHS and the sample by performing interferometry with a combined system of an EDPHS and a WSe2 flake. We assert the mutual frequency and momentum-dependent correlation of the EDPHS and sample radiation, and determine experimentally the degree of mutual coherence of up to 27%. This level of mutual coherence allows us to perform spectral interferometry with an electron microscope. Our method has the advantage of being simple, compact and operating with continuous electron beams. It will open the door to local electron-photon correlation spectroscopy of quantum materials, single photon systems, and coherent exciton-polaritonic samples with nanometric resolution.

cond-mat.mes-hall↗

arXiv2022

Dielectric Mie Voids: Confining Light in Air

Manipulating light on the nanoscale has become a central challenge in metadevices, resonant surfaces, nanoscale optical sensors, and many more, and it is largely based on resonant light confinement in dispersive and lossy metals and dielectrics. Here, we experimentally implement a novel strategy for dielectric nanophotonics: Resonant subwavelength confinement of light in air. We demonstrate that voids created in high-index dielectric host materials support localized resonant modes with exceptional optical properties. Due to the confinement in air, the modes do not suffer from the loss and dispersion of the dielectric host medium. We experimentally realize these resonant Mie voids by focused ion beam milling into bulk silicon wafers and experimentally demonstrate resonant light confinement down to the UV spectral range at 265 nm (4.68 eV). Furthermore, we utilize the bright, intense, and naturalistic colours for nanoscale colour printing. The combination of resonant dielectric Mie voids with dielectric nanoparticles will more than double the parameter space for the future design of metasurfaces and other micro- and nanoscale optical elements and push their operation into the blue and UV spectral range. In particular, this extension will enable novel antenna and structure designs which benefit from the full access to the modal field inside the void as well as the nearly free choice of the high-index material.

physics.optics↗

arXiv2021

Nanofabricated and integrated colour centres in silicon carbide with high-coherence spin-optical properties

Optically addressable spin defects in silicon carbide (SiC) are an emerging platform for quantum information processing. Lending themselves to modern semiconductor nanofabrication, they promise scalable high-efficiency spin-photon interfaces. We demonstrate here nanoscale fabrication of silicon vacancy centres (VSi) in 4H-SiC without deterioration of their intrinsic spin-optical properties. In particular, we show nearly transform limited photon emission and record spin coherence times for single defects generated via ion implantation and in triangular cross section waveguides. For the latter, we show further controlled operations on nearby nuclear spin qubits, which is crucial for fault-tolerant quantum information distribution based on cavity quantum electrodynamics.

quant-ph↗

arXiv2021

Reconfigurable Plasmonic Chirality: Fundamentals and Applications

Molecular chirality is a geometric property that is of great importance in chemistry, biology, and medicine. Recently, plasmonic nanostructures that exhibit distinct chiroptical responses have attracted tremendous interest, given their ability to emulate the properties of chiral molecules with tailored and pronounced optical characteristics. However, the optical chirality of such human-made structures is in general static and cannot be manipulated postfabrication. Herein, different concepts to reconfigure the chiroptical responses of plasmonic nano- and micro-objects are outlined. Depending on the utilized strategies and stimuli, the chiroptical signature, the 3D structural conformation, or both can be reconfigured. Optical devices based on plasmonic nanostructures with reconfigurable chirality possess great potential in practical applications, ranging from polarization conversion elements to enantioselective analysis, chiral sensing, and catalysis.

physics.optics↗

arXiv2021

Chiral plasmonics

We present a comprehensive overview of chirality and its optical manifestation in plasmonic nanosystems and nanostructures. We discuss top-down fabricated structures that range from solid metallic nanostructures to groupings of metallic nanoparticles arranged in three dimensions. We also present the large variety of bottom-up synthesized structures. Using DNA, peptides, or other scaffolds, complex nanoparticle arrangements of up to hundreds of individual nanoparticles have been realized. Beyond this static picture, we also give an overview of recent demonstrations of active chiral plasmonic systems, where the chiral optical response can be controlled by an external stimulus. We discuss the prospect of using the unique properties of complex chiral plasmonic systems for enantiomeric sensing schemes.

physics.optics↗

arXiv2021

Chiral Plasmonic Nanostructures Enabled by Bottom-Up Approaches

We present a comprehensive review of recent developments in the field of chiral plasmonics. Significant advances have been made recently in understanding the working principles of chiral plasmonic structures. With advances in micro- and nanofabrication techniques, a variety of chiral plasmonic nanostructures have been experimentally realized; these tailored chiroptical properties vastly outperform those of their molecular counterparts. We focus on chiral plasmonic nanostructures created using bottom-up approaches, which not only allow for rational design and fabrication but most intriguingly in many cases also enable dynamic manipulation and tuning of chiroptical responses. We first discuss plasmon-induced chirality, resulting from the interaction of chiral molecules with plasmonic excitations. Subsequently, we discuss intrinsically chiral colloids, which give rise to optical chirality owing to their chiral shapes. Finally, we discuss plasmonic chirality, achieved by arranging achiral plasmonic particles into handed configurations on static or active templates. Chiral plasmonic nanostructures are very promising candidates for real-life applications owing to their significantly larger optical chirality than natural molecules. In addition, chiral plasmonic nanostructures offer engineerable and dynamic chiroptical responses, which are formidable to achieve in molecular systems. We thus anticipate that the field of chiral plasmonics will attract further widespread attention in applications ranging from enantioselective analysis to chiral sensing, structural determination, and in situ ultrasensitive detection of multiple disease biomarkers, as well as optical monitoring of transmembrane transport and intracellular metabolism.

physics.optics↗

arXiv2021

Electrically switchable metasurface for beam steering using PEDOT

Switchable and active metasurfaces allow for the realization of beam steering, zoomable metalenses, or dynamic holography. To achieve this goal, one has to combine high-performance metasurfaces with switchable materials that exhibit high refractive index contrast and high switching speeds. In this work, we present an electrochemically switchable metasurface for beam steering where we use the conducting polymer poly(3,4-ethylene-dioxythiophene) (PEDOT) as an active material. We show beam diffraction with angles up to 10° and change of the intensities of the diffracted and primary beams employing an externally applied cyclic voltage between -1 V and +0.5 V. With this unique combination, we realize switching speeds in the range of 1 Hz while the extension to typical display frequencies in the tens of Hz region is possible. Our findings have immediate implications on the design and fabrication of future electronically switchable and display nanotechnologies, such as dynamic holograms.

physics.optics↗

arXiv2020

Interaction of edge exciton polaritons with engineered defects in the van der Waals material Bi2Se3

Hyperbolic materials exhibit unique properties that enable a variety of intriguing applications in nanophotonics. The topological insulator Bi2Se3 represents a natural hyperbolic optical medium, both in the THz and visible range. Here, using cathodoluminescence spectroscopy and electron energy-loss spectroscopy, we demonstrate that Bi2Se3, in addition to being a hyperbolic material, supports room-temperature exciton polaritons. Moreover, we explore the behavior of hyperbolic edge exciton polaritons in Bi2Se3. Edge polaritons are hybrid modes that result from the coupling of the polaritons bound to the upper and lower edges of Bi2Se3 nanoplatelets. In particular, we use electron energy-loss spectroscopy to compare Fabry-Pérot-like resonances emerging in edge polariton propagation along pristine and artificially structured edges of the nanoplatelets. The experimentally observed scattering of edge polaritons by defect structures was found to be in good agreement with finite-difference time-domain simulations. Moreover, we experimentally proved coupling of localized polaritons in identical open and closed circular nanocavities to the propagating edge polaritons. Our findings are testimony to the extraordinary capability of the hyperbolic polariton propagation to cope with the presence of defects. This provides an excellent basis for applications such as nanooptical circuitry, cloaking at the nanometer scale, as well as nanoscopic quantum technology on the nanoscale.

physics.app-ph↗

arXiv2011

Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle

Optical nanoantennas are a novel tool to investigate previously unattainable dimensions in the nanocosmos. Just like their radio-frequency equivalents, nanoantennas enhance the light-matter interaction in their feed gap. Antenna enhancement of small signals promises to open a new regime in linear and nonlinear spectroscopy on the nanoscale. Without antennas especially the nonlinear spectroscopy of single nanoobjects is very demanding. Here, we present for the first time antenna-enhanced ultrafast nonlinear optical spectroscopy. In particular, we utilize the antenna to determine the nonlinear transient absorption signal of a single gold nanoparticle caused by mechanical breathing oscillations. We increase the signal amplitude by an order of magnitude which is in good agreement with our analytical and numerical models. Our method will find applications in linear and nonlinear spectroscopy of nanoobjects, ranging from single protein binding events via nonlinear tensor elements to the limits of continuum mechanics.

cond-mat.mes-hall↗