Second harmonic (SH) radiation can only be generated in non-centrosymmetric bulk crystals under electric dipole approximation. Nonlinear thin films made from bulk crystals are technologically challenging because of complex and high-temperature fabrication processes. In this work, heterostructures made of two distinct amorphous materials, namely SiO2 and TiO2, were prepared through plasma-enhanced atomic layer deposition (PEALD) with deposition temperature of 100 degrees C. By using the uniaxial dispersion model, we characterized the form birefringence of the deposited films, which can play a crucial role for the phase-matching condition in nonlinear waveguides or other nonlinear optical applications. By applying a fringe-based technique, we determined the largest diagonal component of the effective bulk second-order susceptibility, chi zzz(2) = 1.30 +/- 0.13 pm/V, at a wavelength of 1032 nm. Noteworthy, we observed strong SHG signals from two-component nanolaminates, which are several orders of magnitude larger than those from single layers. The SHG signals from our samples only require the broken inversion symmetry at the interface. Here, optical properties of nanocomposites can be precisely engineered using the promising PEALD technology.

Gravitational wave detectors (GWDs) rely heavily on low mechanical and optical loss mirror coatings to detect cosmic events happening in the universe. This work discusses optical losses through light absorption and scattering mechanisms in silicon nitride (SiNx) and silicon oxynitride (SiOxNy) thin films deposited by the plasma-enhanced chemical vapor deposition technique. We report an efficient and repeatable procedure to tune the refractive index of both SiNx and SiOxNy thin films, while preserving a low optical absorption and scattering in the range of ppm. Finally, we demonstrate the design, fabrication, and characterization of an SiNx/SiOxNy multi-layer stack composed of 20 layers with a total thickness of 3.9 µm, achieving a reflectance above 99% (and a low absorbance of 0.259%) in the near-infrared region, which is a promising step toward meeting the optical requirements of the third generation of GWDs.

A systematic exploration of amorphous ABC heterostructures revealed that nanoscale morphological modifications markedly improved their artificial bulk second-order susceptibility. These amorphous birefringent heterostructures were fabricated using plasma-enhanced atomic layer deposition of three oxides, thereby breaking centrosymmetry at the nanoscale. We observe that the optical nonlinearity depends on the thickness variation of the three constituent materials, SiO2, TiO2, and Al2O3, ranging from tens of nanometers to the atomic scale, and where the thin films exhibit second-order susceptibility at their interfaces. Our findings reveal that the enhancement of nonlinear optical properties is strongly correlated with a high interface density and superior interface quality, where the interface second-order nonlinearity transitions to bulk-like second-harmonic generation. An effective bulk second-order susceptibility of chi zzz = 2.0 +/- 0.2 pm/V at 1032 nm is achieved, comparable to some conventional monocrystalline nonlinear materials. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

The Optical Interference Coatings Measurement Challenge represents a distinctive occasion for the optical coatings community to benchmark and verify their test equipment and methodologies. In the 2025 Measurement Challenge, optical losses of high-reflecting mirrors (R>99.99%) for 1064 and 355 nm were to be determined.

The reflectivity of gold layers is strongly dependent on their structural characteristics. Reflectivity tends to increase with larger average grain sizes, corresponding to a reduced grain boundary area. A decrease in the grain boundary area results in less restricted movement of free electrons, leading to higher reflectivity. Gold layers produced by evaporation demonstrate a larger average grain size and higher reflectivity compared to those produced by sputtering. The reflectivity of the evaporated layers could not be achieved by modifying the gas supply during sputtering. The addition of N2 to the Ar supply resulted in a reduction in reflectivity for the sputtered layers.

Hafnium oxide (HfO2) is a high-index dielectric material of growing importance for optical coatings and meta-optical components operating from the ultraviolet (UV) to the visible spectral range. Its large bandgap, chemical stability, and compatibility with established semiconductor processes make it particularly attractive for applications requiring low absorption and precise thickness control. In this work, we investigate the optical material properties of HfO2 thin films deposited by plasma-enhanced atomic layer deposition (PEALD). The complex refractive index is experimentally determined over a broad spectral range extending from the vacuum ultraviolet (140 nm) to the visible (600 nm) by combining spectroscopic ellipsometry, spectrophotometry, and infrared ellipsometry. A comprehensive dispersion model is employed to extract consistent optical constants and thickness values. The results highlight the suitability of PEALD-grown HfO2 films for advanced optical and meta-optical applications.

Gravitational wave detectors (GWDs) rely heavily on low mechanical and optical loss mirror coatings to detect cosmic events happening in the universe. This work discusses optical losses through light absorption and scattering mechanisms in silicon nitride (SiN x ) and silicon oxynitride (SiO x N y ) thin films deposited by the plasma-enhanced chemical vapor deposition technique. We report an efficient and repeatable procedure to tune the refractive index of both SiN x and SiO x N y thin films, while preserving a low optical absorption and scattering in the range of ppm. Finally, we demonstrate the design, fabrication, and characterization of an SiN x /SiO x N y multi-layer stack composed of 20 layers with a total thickness of 3.9 µm, achieving a reflectance above 99% (and a low absorbance of 0.259%) in the near-infrared region, which is a promising step toward meeting the optical requirements of the third generation of GWDs.

Multifunctional saltwater-repellent coatings for optical ZnS components have been developed by optimizing nanorough structural properties. These coatings can open a path to sensor windows with improved durability and functionality in maritime and other challenging environments.

The progressive increase in microbial resistance to antibiotics is a global health threat that requires solutions for rapid and reliable determination of antibiotic susceptibility in order to select appropriate antibiotics and dosages prior to treatment. We have established a screening platform that enables the detection of cell growth after just a few cell divisions. Our methodological approach for a robust phenotypic antibiotic susceptibility testing is based on the innovative combination of three cutting-edge technologies: (i) a high-throughput microfluidic platform where individual bacterial cells are encapsulated in picoliter-sized droplets, (ii) a 2D angle-resolved light scattering sensor to perform label-free hourly screening of the droplets, and (iii) a computational image analysis approach based on convolutional neural networks to evaluate the dynamics of microbial growth in droplets. For the gram-positive Staphylococcus aureus and gram-negative Escherichia coli, we demonstrate that microbial growth in droplets can be successfully detected within one to two hours. Furthermore, the potential of this platform for rapid phenotypic antibiotic susceptibility testing is demonstrated as a proof-of-concept with the clinically relevant bacterium S. aureus under various concentrations of the antibiotic tetracycline. Notably, we reach a robust phenotypic decision regarding the sensitivity to this antibiotic within two hours.

W/Si multilayer mirrors are a promising candidate for soft X-ray applications at wavelengths below 2.4 nm. However, their optical performance is strongly affected by interface roughness and interlayer mixing, which limits reflectivity. One approach to improving interface quality is the application of BIAS voltage during deposition. In this study, W/Si multilayer mirrors with bilayer thickness of ~1.5 nm and 100 bilayers were fabricated using DC magnetron sputtering, with ion assistance of 75 V, 100 V, and 200 V applied during the deposition of silicon layers. Grazing incidence X-ray reflectivity (GIXR) measurements at Cu Kα (λ = 0.154 nm) showed that applying BIAS ≤ 100 V reduced interface roughness and increased reflectivity, with a maximum effect observed at 75 V. In contrast, at 200 V, strong diffusion intermixing reduced the bilayer thickness to 1.29 nm and nearly eliminated reflectivity. Soft X-ray reflectivity measurements at λ ~ 1.5 nm confirmed that ion assistance improved optical performance, increasing mirror reflectivity from ~1% (BIAS = 0 V) to ~2.3% (BIAS = 75 V). Atomic force microscopy (AFM) measurements also demonstrated a reduction in surface roughness from 0.22 nm to 0.11 nm due to using ion assistance. These results indicate that moderate ion assistance (<100 V) can enhance the optical quality of W/Si multilayer mirrors by reducing interface roughness, while excessive BIAS (>100 V) leads to diffusion intermixing and optical degradation. The novelty of this work lies in the direct application and variation in BIAS voltage during Si-layer growth, enabling detailed investigation of its influence on interface roughness and reflectivity. This approach provides a simple and effective tool for optimizing the performance of W/Si multilayer mirrors for soft X-ray applications.

The OIC Measurement Challenge is a unique opportunity for the optical coatings community to benchmark and verify their test equipment and their test procedures for almost 20 years now. As always, a special task is given, which should be challenging but at the same time also meaningful. Moreover, the problem should be accessible to a broad range of potential participants with both, standard equipment and specialized tools. This time, the goal of the Measurement Challenge (MC) is the sensitive and precise measurement of the optical losses of high-reflecting (HR) mirrors at a wavelength of 1064 nm and/or 355 nm, and an AOI of 0°.

Recent advancements in photonics have intensified the performance requirements for optical systems and present significant challenges for optical coating technologies. Conventional interference coating systems often prove to be insufficient, especially in applications requiring large angles of light incidence or a wide wavelength range. Nanostructures, which consist of an air material mixture, offer promising alternatives. In this work, silica nanostructures are manufactured by the AR-plas2 method, in which first an organic layer is evaporated onto a substrate. This organic layer forms self-organizing nanostructures by a plasma etching step, which are subsequently coated with silica. Finally, the organic residues are removed by additional plasma etching and heat treatment steps, which results in hollow silica structures. The work examines the optical and functional properties of these structures designed for 355 nm to demonstrate their use as anti-reflective coatings for advanced optical systems.

Atomically thin heterostructures deposited by atomic layer deposition show strong form-birefringence as determined by ellipsometry close to theoretical values. The form-birefringence depends on the period of the heterostructures indicating partial intermixing of the constituent layers.

Gravitational wave detectors rely on low mechanical and optical loss mirror coatings. This work discusses optical losses through absorption and light scattering mechanisms in SiN x and SiO x N y coatings deposited by plasma-enhanced chemical vapour deposition.

Understanding the influence of topography on wettability is essential for improving the modeling of superhydrophobic surfaces. Moreover, wetting predictions can foresee corrosion, biological contamination, self-cleaning properties, and all phenomena related to wetting. In this context, this research work reports the experimental corroboration of a novel theoretical model for stochastic surfaces that relates the static contact angle for the heterogeneous wetting of surfaces to the root mean square (RMS) slope of the surface structures, allowing wetting prediction through topography. For this study, hydrophobic and superhydrophobic alumina thin films with gradual roughness were constructed. The films were deposited on glass using the dip-coating technique, textured with boiling water, and functionalized to achieve low surface energy using Dynasylan F-8815. Surface wettability was characterized using the sessile drop technique, and the RMS slope of the alumina surfaces was quantified using the atomic force microscopy (AFM) technique. The model, presented here for the first time, fits the experimental data, allowing wetting prediction for hydrophobic and superhydrophobic surfaces considering static contact angles. As expected, topography plays a fundamental role in achieving superhydrophobicity. Therefore, defining a topographic criterion, as performed here, for obtaining superhydrophobic surfaces is highly relevant to reduce the production costs of these surfaces and also enable new production processes and designs.

The performance of an optical component or surface might quicky be limited by light scattering induced by the surface and coating roughness, as well as imperfections and contaminations. On the other hand, the scattered light contains valuable information about its source, which makes scattering based techniques powerful characterization tools for these important features. A major advantage is the fast, robust, and contact free measurement approach enabling even close-to process applications. Based on several examples we demonstrate the potential of light scattering characterization during the fabrication process up to even in-situ coating inspection.