Stefan Partel

Polariton interferometry through optical near-field microscopy has become a powerful tool in nanophotonics, enabling direct spatial access to the propagation characteristics of strongly confined, evanescent polariton modes. Scattering-type near-field optical microscopy has matured as the prime tool for such studies, yet mostly the out-of-plane components of the optical near fields are probed, owing to the elongated geometry of the nanotip. Here, we demonstrate a complementary far-field nonlinear microscopy approach that allows selective probing of in-plane polariton field components. Accessing the full vector field is interesting when studying complex mode patterns such as hyperbolic polaritons or skyrmions, where the in-plane field components are typically only inferred from the out-of-plane component but not measured directly. To this end, we use nonlinear infrared-visible wide-field sum-frequency generation microscopy, where the short visible wavelength of the nonlinear signal provides the high spatial resolution to access evanescent modes in the infrared. The symmetry selection rules of the nonlinear process further enable polarization-selective imaging of both in-plane polariton field components through spatial interferometry. The concept is demonstrated experimentally using surface phonon polaritons at the AlN-air interface launched by a gold antenna. A simple, semi-analytical model reproduces the peculiar propagation patterns. Hyperspectral imaging with a tunable narrowband laser further gives access to the polariton dispersion. The wide-field methodology holds high promise for in-depth and high-throughput studies of infrared nanophotonic structures.

Van der Waals (vdW) heterostructures provide a versatile platform for multifunctional photonic and optoelectronic devices, yet their performance is often limited by interfacial inhomogeneities that perturb the local dielectric environment. This is particularly critical in polaritonic heterostructures, where the near-field optical response is extremely sensitive to buried defects. Here, we present a high-resolution microreflectance mapping technique for assessing interfacial quality in vdW heterostructures and enabling the fabrication of twisted polaritonic stacks. By combining hyperspectral imaging with transfer-matrix-method (TMM) fitting, we determine flake thickness directly on PDMS stamps and quantify buried air gaps and other interfacial nonuniformities in assembled stacks. Using h-BN and α-MoO3 heterostructures as benchmark systems, we detect buried air gaps down to 7 nm with ∼1 μm lateral resolution. Correlation with scattering-type scanning near-field optical microscopy (s-SNOM) confirms the quantitative accuracy of the method and shows that nanometric air gaps already produce measurable changes in phonon-polariton (PhP) propagation. Our results establish microreflectance mapping as an accessible far-field approach for quantitative interfacial control in polaritonic heterostructures, a key requirement for twistoptics applications. To facilitate implementation, we provide the analysis in an open-source graphical user interface that can be integrated into standard optical-microscope setups.

Whether at the molecular or cellular scale in organisms, cell–cell adhesion adapts to external mechanical cues arising from the static environment of cells and from dynamic interactions between neighboring cells. Cell–cell adhesion needs to resist detachment forces to secure the integrity and internal organization of organisms. In the past, various techniques have been developed to characterize adhesion properties of molecules and cells in vitro and to understand how cells sense and probe their environment. Atomic force microscopy and dual-pipette aspiration, where cells are mainly present in suspension, are common methods for studying detachment forces of cell–cell adhesion. How cell–cell adhesion forces are developed for adherent and environment-adapted cells, however, is less clear. Here, we designed the Cell–Cell Separation Device (CC-SD), a microstructured substrate that provides a step toward measuring both the intercellular forces and external stresses of cells toward the substrate. The device is based on micropillar arrays, originally developed for cell traction-force measurements. We designed PDMS micropillar-blocks, to which cells could adhere and be able to connect to each other across the gap. Controlled stretching of the whole substrate changed the distance between blocks and increased the gap size. This allowed us to apply strains to cell–cell contacts, eventually leading to cell–cell adhesion detachment, which was measured by pillar deflections. The CC-SD provided an increase in the gap between the blocks of up to 2.4-fold, which was sufficient to separate substrate-attached cells with a fully developed F-actin network. Simultaneously measured pillar deflections allowed us to address cellular response to the intercellular strain applied. The CC-SD thus opens up possibilities for the analysis of intercellular detachment forces and sheds light on the robustness of cell–cell adhesion against rupture in dynamic processes during tissue development.

Canalization is an optical phenomenon that enables unidirectional light propagation without predefined waveguiding designs. Recently demonstrated using phonon polaritons in twisted van der Waals (vdW) layers of α-MoO ₃ , it offers unprecedented possibilities for controlling light-matter interactions at the nanoscale. However, practical applications have been hindered by the complex sample fabrication of twisted stacks. In this work, we introduce a previously unexplored canalization phenomenon in a single-thin vdW layer (α-MoO ₃ ) interfaced with a substrate exhibiting a given negative permittivity. This enables a proof-of-concept application of polariton canalization: super-resolution nanoimaging (~λ ₀ /220). Canalization-based imaging transcends conventional projection constraints, allowing the super-resolution images to be obtained at any desired location in the image plane. This versatility stems from the synergetic manipulation of three key parameters: incident frequency, rotation angle of the thin vdW layer, and thickness. Our results provide insights into the properties of canalization and constitute a seminal step toward multifaceted photonic applications, including imaging, data transmission, and ultracompact photonic integration.

Whether at the intramolecular or cellular scale in organisms, cell-cell adhesion adapt to external mechanical cues arising from the static environment of cells and from dynamic interactions between neighboring cells. Cell-cell adhesions need to resist detachment forces to secure the integrity and internal organization of organisms. In the past, various techniques have been developed to characterize adhesion properties of molecules and cells in vitro, and to understand how cells sense and probe their environment. Atomic force microscopy and dual-pipette aspiration, where cells are mainly present in suspension, are common methods for studying detachment forces of cell-cell adhesions. How cell-cell adhesion forces are developed for adherent and environment-adapted cells, however, is less clear. Here, we designed the Cell-Cell Separation Device (CC-SD), a microstructured substrate that measures both intercellular forces and external stresses of cells towards the matrix. The design is based on micropillar arrays originally designed for cell traction-force measurements. We designed PDMS micropillar-blocks, to which cells could adhere and be able to connect to each other across the gap. Controlled stretching of the whole substrate changed the distance between blocks and increased gap size. That allowed us to apply strains to cell-cell contacts, eventually leading to cell-cell adhesion detachment, which was measured by pillar deflections. The CC-SD provided an increase of the gap between the blocks of up to 2.4-fold, which was sufficient to separate substrate-attached cells with fully developed F-actin network. Simultaneously measured pillar deflections allowed us to address cellular response to the intercellular strain applied. The CC-SD thus opens up possibilities for the analysis of intercellular force detachments and sheds light on the robustness of cell-cell adhesions in dynamic processes in tissue development.

Electric cell-substrate impedance spectroscopy (ECIS) enables non-invasive and continuous read-out of electrical parameters of living tissue. The aim of the current study was to investigate the performance of interdigitated sensors with 50 μm electrode width and 50 μm inter-electrode distance made of gold, aluminium, and titanium for monitoring the barrier properties of epithelial cells in tissue culture. At first, the measurement performance of the photolithographic fabricated sensors was characterized by defined reference electrolytes. The sensors were used to monitor the electrical properties of two adherent epithelial barrier tissue models: renal proximal tubularLLC-PK1cells, representing a normal functional transporting epithelium, and human cervical cancer-derivedHeLacells, forming non-transporting cancerous epithelial tissue. Then, the impedance spectra obtained were analysed by numerically fitting the parameters of the two different models to the measured impedance spectrum. Aluminium sensors proved to be as sensitive and consistent in repeated online-recordings for continuous cell growth and differentiation monitoring as sensors made of gold, the standard electrode material. Titanium electrodes exhibited an elevated intrinsic ohmic resistance in comparison to gold reflecting its lower electric conductivity. Analysis of impedance spectra through applying models and numerical data fitting enabled the detailed investigation of the development and properties of a functional transporting epithelial tissue using either gold or aluminium sensors. The result of the data obtained, supports the consideration of aluminium and titanium sensor materials as potential alternatives to gold sensors for advanced application of ECIS spectroscopy.

Over the last years, polymers have gained great attention as substrate material, because of the possibility to produce low-cost sensors in a high-throughput manner or for rapid prototyping and the wide variety of polymeric materials available with different features (like transparency, flexibility, stretchability, etc.). For almost all biosensing applications, the interaction between biomolecules (for example, antibodies, proteins or enzymes) and the employed substrate surface is highly important. In order to realize an effective biomolecule immobilization on polymers, different surface activation techniques, including chemical and physical methods, exist. Among them, plasma treatment offers an easy, fast and effective activation of the surfaces by micro/nanotexturing and generating functional groups (including carboxylic acids, amines, esters, aldehydes or hydroxyl groups). Hence, here we present a systematic and comprehensive plasma activation study of various polymeric surfaces by optimizing different parameters, including power, time, substrate temperature and gas composition. Thereby, the highest immobilization efficiency along with a homogenous biomolecule distribution is achieved with a 5-min plasma treatment under a gas composition of 50% oxygen and nitrogen, at a power of 1000 W and a substrate temperature of 80 °C. These results are also confirmed by different surface characterization methods, including SEM, XPS and contact angle measurements.