Light: Science and Applications

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Phonon-polaritonic skyrmions: transition from bubble- to Néel-type
(2026-12-01)
Mangold, Florian
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Baù, Enrico
;
Nan, Lin
;
Schwab, Julian
;
Gölz, Thorsten
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 Néel-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.
Publicationtype: Journal Article
38
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Directional dark field for nanoscale full-field transmission X-ray microscopy
(Nature, 2026-05-08)
Wirtensohn, Sami
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Flenner, Silja
;
John, Dominik
;
Qi, Peng
;
David, Christian
Dark-field X-ray imaging visualizes structural inhomogeneities through small-angle scattering, but existing directional methods are confined to the micrometer scale. While recent advances have extended dark-field capabilities to nanoscale transmission X-ray microscopy, directional scattering retrieval - critical for characterizing anisotropic nanostructures - has remained inaccessible for imaging resolutions in the sub-micrometer scale. Here, we demonstrate the first directional dark-field setup for nanoimaging, achieving orientation mapping of scattering features below the spatial resolution limit. Our method is experimentally simple to implement with existing transmission X-ray microscopy setups. We validate its performance by successfully resolving sub-resolution test structure orientations, cross-correlating orientational changes within hierarchical nanoporous materials, and mapping the directional arrangement of hydroxyapatite nanocrystals 30 - 70 nm within human tooth enamel. By utilizing shadow regions in the optical configuration, we further extend the detectable scattering vector range, demonstrating a pathway toward size-selective dark-field imaging. This advancement enables the quantitative structural characterization of anisotropic nanomaterials, which are critical to biomineralization, advanced materials, and nanotechnology applications.
Publicationtype: Journal Article
44 44
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Interface second harmonic generation enhancement in bulk WS₂/MoS₂ hetero-bilayer van der Waals nanoantennas
(2025-09-29)
Tognazzi, Andrea
;
Franceschini, Paolo
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Biechteler, Jonas
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Baù, Enrico
;
Cino, Alfonso Carmelo
Layered van der Waals (vdW) materials have emerged as a promising platform for nanophotonics due to large refractive indexes and giant optical anisotropy. Unlike conventional dielectrics and semiconductors, the absence of covalent bonds between layers allows for novel degrees of freedom in designing optically resonant nanophotonic structures down to the atomic scale: from the precise stacking of vertical heterostructures to controlling the twist angle between crystallographic axes. Specifically, although monolayers of transition metal dichalcogenides exhibit giant second-order nonlinear responses, their bulk counterparts with 2H stacking possess zero second-order nonlinearity. In this work, we investigate second harmonic generation (SHG) arising from the interface of WS₂/MoS₂ hetero-bilayer thin films with an additional SHG enhancement in nanostructured optical antennas, mediated by both the excitonic resonances and the anapole-driven field enhancement. When both conditions are met, we observe up to 10² SHG signal enhancement, compared to unstructured bilayers, with SHG conversion efficiency reaching ≈ 10⁻⁷. Our results highlights vdW materials as a platform for designing unique multilayer optical nanostructures and metamaterial, paving the way for advanced applications in nanophotonics and nonlinear optics. (Figure presented.)
Publicationtype: Journal Article
14
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Control and enhancement of optical nonlinearities in plasmonic semiconductor nanostructures
(2025-05-13)
Rossetti, Andrea
;
Hu, Huatian
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Venanzi, Tommaso
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Bousseksou, Adel
;
De Luca, Federico
The efficiency of nanoscale nonlinear elements in photonic integrated circuits is hindered by the physical limits to the nonlinear optical response of dielectrics, which cannot be engineered as it is a fundamental material property. Here, we experimentally demonstrate that ultrafast optical nonlinearities in doped semiconductors can be engineered and can easily exceed those of conventional undoped dielectrics. The electron response of heavily doped semiconductors acquires in fact a hydrodynamic character that introduces nonlocal effects as well as additional nonlinear sources. Our experimental findings are supported by a comprehensive computational analysis based on the hydrodynamic model. In particular, by studying third-harmonic generation from plasmonic nanoantenna arrays made out of heavily n-doped InGaAs with increasing levels of free-carrier density, we discriminate between hydrodynamic and dielectric nonlinearities. Most importantly, we demonstrate that the maximum nonlinear efficiency as well as its spectral location can be engineered by tuning the doping level. Crucially, the maximum efficiency can be increased by almost two orders of magnitude with respect to the classical dielectric nonlinearity. Having employed the common material platform InGaAs/InP that supports integrated waveguides, our findings pave the way for future exploitation of plasmonic nonlinearities in all-semiconductor photonic integrated circuits.
Publicationtype: Journal Article
11
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All-optical permittivity-asymmetric quasi-bound states in the continuum
(2025-05-07)
Berté, Rodrigo
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Possmayer, Thomas
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Tittl, Andreas
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Menezes, Leonardo de S.
;
Maier, Stefan A.
Resonances are usually associated with finite systems—the vibrations of clamped strings in a guitar or the optical modes in a cavity defined by mirrors. In optics, resonances may be induced in infinite continuous media via periodic modulations of their optical properties. Here we demonstrate that periodic modulations of the permittivity of a featureless thin film can also act as a symmetry-breaking mechanism, allowing the excitation of photonic quasi-bound states in the continuum (qBICs). By interfering two ultrashort laser pulses in the unbounded film, transient resonances can be tailored through different parameters of the pump beams. We show that the system offers resonances tunable in wavelength and quality-factor, and spectrally selective enhancement of third-harmonic generation. Due to a fast decay of the permittivity asymmetry, we observe ultrafast dynamics, enabling time-selective near-field enhancement with picosecond precision. Optically induced permittivity asymmetries may be exploited in on-demand weak to ultrastrong light-matter interaction regimes and light manipulation at dynamically chosen wavelengths in lithography-free metasurfaces.
Publicationtype: Journal Article
8
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Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality
(Nature Publishing Group, 2023-10-12)
Kühner, Lucca
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Wendisch, Fedja J.
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Antonov, Alexander A.
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Bürger, Johannes
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Hüttenhofer, Ludwig
The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures, constraining experimental feasibility and hampering practical implementations. Even though the three-dimensional assembly of metallic nanostructures has been demonstrated previously, the resulting plasmonic resonances suffer from high intrinsic and radiative losses. The concept of photonic bound states in the continuum (BICs) is instrumental for tailoring radiative losses in diverse geometries, especially when implemented using lossless dielectrics, but applications have so far been limited to planar structures. Here, we introduce a novel nanofabrication approach to unlock the height of individual resonators within all-dielectric metasurfaces as an accessible parameter for the efficient control of resonance features and nanophotonic functionalities. In particular, we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness. Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics, but also promise advances in the realization of efficient generation of optical angular momentum, holographic metasurfaces, and parity-time symmetry-broken optical systems.
Publicationtype: Journal Article
7
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Tunable structural colors on display
(Nature Publishing Group, 2022-05-25)
Tittl, Andreas
Structural coloration takes inspiration from the bright hues found in nature to control the reflection and transmission of light from artificially structured materials. Combining them with active electrical tuning heralds breakthrough applications in optical displays.
Publicationtype: Journal Article
6
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Double-layer graphene for enhanced tunable infrared plasmonics
(Nature Publishing Group, 2017-01-09)
Rodrigo, Daniel
;
Tittl, Andreas
;
Limaj, Odeta
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De Abajo, F. Javier García
;
Pruneri, Valerio
Graphene is emerging as a promising material for photonic applications owing to its unique optoelectronic properties. Graphene supports tunable, long-lived and extremely confined plasmons that have great potential for applications such as biosensing and optical communications. However, in order to excite plasmonic resonances in graphene, this material requires a high doping level, which is challenging to achieve without degrading carrier mobility and stability. Here, we demonstrate that the infrared plasmonic response of a graphene multilayer stack is analogous to that of a highly doped single layer of graphene, preserving mobility and supporting plasmonic resonances with higher oscillator strength than previously explored single-layer devices. Particularly, we find that the optically equivalent carrier density in multilayer graphene is larger than the sum of those in the individual layers. Furthermore, electrostatic biasing in multilayer graphene is enhanced with respect to single layer due to the redistribution of carriers over different layers, thus extending the spectral tuning range of the plasmonic structure. The superior effective doping and improved tunability of multilayer graphene stacks should enable a plethora of future infrared plasmonic devices with high optical performance and wide tunability.
Publicationtype: Journal Article
5
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DNA-assembled bimetallic plasmonic nanosensors
(2014-12-19)
Li, Na
;
Tittl, Andreas
;
Yue, Song
;
Giessen, Harald
;
Song, Chen
Plasmonic hybrid nanomaterials are highly desirable in advanced sensing applications. Different components in these materials undertake distinct roles and work collectively. One material component may act as an efficient light concentrator and optical probe, whereas another provides specific chemical or biological functionality. In this work, we present DNA-assembled bimetallic plasmonic nanostructures and demonstrate their application for the all-optical detection of hydrogen. Gold (Au) nanorods are functionalized with DNA strands, which serve both as linkers and seeding sites for the growth of palladium (Pd) nanocrystals and facilitate reliable positioning of Pd satellites around an Au nanorod at an ultrashort spacing in the nanometer range. Dark-field scattering spectra of single Au-DNA-Pd nanorods were recorded during controlled cycles of hydrogen gas exposure, and an unambiguous concentration-dependent optical response was observed. Our method enables, for the first time, the all-optical detection of hydrogen-induced phase-change processes in sub-5-nm Pd nanocrystals at the single-antenna level. By substituting the Pd satellites with other functional materials, our sensor platform can be extended to plasmonic sensing of a multitude of chemical and biological reagents, both in liquid and gaseous phases.
Publicationtype: Journal Article
7

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