Luisa Coriand

Modeling of light scattering in different regimes of surface roughness.

The light scattering of rough metallic surfaces with roughness levels ranging from a few to several hundred nanometers is modeled and compared to experimental data. Different modeling approaches such as the classical Rayleigh-Rice vector perturbation theory and the new Generalized Harvey-Shack theory are used and critically assessed with respect to ranges of validity, accuracy, and practicability. Based on theoretical calculations and comparisons with Rigorous Coupled Wave Analysis for sinusoidal phase gratings, it is demonstrated that the approximate scatter models yield surprisingly accurate results and at the same time provide insight into light scattering phenomena. For stochastically rough metal surfaces, the predicted angles resolved scattering is compared to experimental results at 325 nm, 532 nm, and 1064 nm. In addition, the possibilities of retrieving roughness information from measured scattering data for different roughness regimes are discussed.

Definition of roughness structures for superhydrophobic and hydrophilic optical coatings on glass

With specific modeling, measurement, and analysis procedures, it is possible to predict, define, and control roughness structures for tailored wetting properties of optical coatings. Examples are given for superhydrophobic and hydrophilic sol-gel layers on glass substrate.

Wear-Resistant Nanostructured Sol-Gel Coatings for Functional Applications

Improvement of the wear resistance of functional surfaces is crucial in order to facilitate a variety of practical applications, such as self-cleaning or anti-fogging. This especially holds for functional surface nanostructures, whose tops can easily get worn off when exposed to even low abrasion forces. Thus, our work addresses the enhancement of the wear resistance of such fine-scale structures. We present an efficient manufacturing procedure for generating long-term durable surfaces with simultaneously tailored wetting behavior and high optical quality. Our approach is based on a sol-gel coating that consists of an alumina layer with specific nanoroughness yielding the function-relevant surface structure, and a protective thin smooth silica film providing the mechanical robustness without influencing that functional structure. The roughness of the alumina layer can be systematically adjusted, thus enabling us to achieve desired wetting effects all the way up to superhydrophilicity and, after application of an additional thin hydrophobic top coat, to superhydrophobicity. To demonstrate the enhanced robustness of these coatings we perform abrasive wear tests and investigate the impact of abrasion cycles on the wetting effects and optical properties of the coatings. Furthermore, the durability of the structures is directly revealed by advanced roughness characterization procedures based on Atomic Force Microscopy followed by power spectral density function (PSD) analysis.

Defined wetting properties of optical surfaces

Abstract Optical surfaces equipped with specific functional properties have attracted increasing importance over the last decades. In the light of cost reduction, hydrophobic self-cleaning behavior is aspired. On the other side, hydrophilic properties are interesting due to their anti-fog effect. It has become well known that such wetting states are significantly affected by the surface morphology. For optical surfaces, however, this fact poses a problem, as surface roughness can induce light scattering. The generation of optical surfaces with specific wetting properties, hence, requires a profound understanding of the relation between the wetting and the structural surface properties. Thus, our work concentrates on a reliable acquisition of roughness data over a wide spatial frequency range as well as on the comprehensive description of the wetting states, which is needed for the establishment of such correlations. We will present our advanced wetting analysis for nanorough optical surfaces, extended by a vibration-based procedure, which is mainly for understanding and tailoring the wetting behavior of various solid-liquid systems in research and industry. Utilizing the relationships between surface roughness and wetting, it will be demonstrated how different wetting states for hydrophobicity and hydrophilicity can be realized on optical surfaces with minimized scatter losses.

Roughness, optical, and wetting properties of nanostructured thin films

Roughness structures are essential for a variety of functional surfaces, for example surfaces with extreme wetting behavior like superhydrophobicity or superhydrophilicity. On the other hand, roughness also gives rise to light scattering, and thus limits the usability of such surfaces for optical applications. Our approach is based on using small-scale intrinsic roughness components of thin film coatings to achieve the desired functional properties while keeping the light scattering at acceptable levels. A comprehensive measurement and analysis methodology for effectively predicting, defining and controlling the structural and wetting properties of stochastically rough superhydrophobic surfaces is presented. Power Spectral Density (PSD) functions determined from atomic force microscopy data are used for thorough roughness analysis as well as to predict the wetting and light scattering properties. Dynamic contact angle analysis is performed by measuring advancing, receding, roll-off, and bounce-off angles. Examples of natural and technical superhydrophobic surfaces like the Lotus leaf and thin film coatings with stochastic nanoroughness are given. These surfaces reveal high advancing contact angles, low contact angle hysteresis, low roll-off angles, and, consequently, the effect of self-cleaning.

Light scattering-based measurement of relevant surface roughness

Light scattering measurements are used to determine surface PSD functions and application-specific roughness values in different relevant spatial frequency ranges. A new method is presented for area covering investigations of the high-spatial frequency roughness.