High transparent thermoplastics have the capability to put glass out of business, especially in everyday life's optics. Their diverse nature gives rise to different antireflection principles. The reduction of surface reflection losses in polymethylmethacrylate (PMMA) is demonstrated by means of argon/oxygen plasma treatment. Since the presented reduction of reflection occurs in a wide spectral range, the technique may be applied for omnidirectional devices or curved substrates. The etching process creates a self-organized stochastic subwavelength structure at the substrate itself. The decrease in reflection is described by effective medium theory (EMT), converting the surface topology into a depth-dependent filling factor profile. In a second step this nano-scaled structure is used as the initial point for a broadband absorber by coating it with a nontransparent metal layer. A high-efficient absorber can be obtained, if the metal acts as backside coating of the double-sided plasma-treated substrate and steady-going transitions between the materials eliminating the Fresnel reflections. In practice, the magnitude of absorption depends on depth of structure as well as on the complex refractive index of the metal.
Stochastic, self-organized nanostructures are produced by a low-pressure plasma treatment on the polymer polymethylmetacrylate (PMMA). The phenomena obtained by plasma treatment (structure formation and antireflective effect) are investigated regarding surface modifications, structure growth, and chemical modifications. Optically, the structure acts like a gradient layer with decreasing effective refractive index towards air, which is suitable for antireflection of PMMA.
Antireflection (AR) properties are required for optical surfaces to avoid disturbing reflections as well as to improve the transmission of optical systems. The common method to reduce the reflection on optics is vacuum deposition of interference coatings. However, special efforts are required for each type of plastic to develop polymer-capable vacuum coating processes due to the manifold chemical and physical properties of optical polymers. An alternative procedure for the antireflection of polymers is the generation of surface structures that decrease the index of refraction in a surface region. In this paper, the suitability of the miscellaneous thermoplastic polymers for plasma-ion assisted deposition processes is evaluated. This comprises the study of damage effects caused by the contact with plasma and high-energy radiation as well as the development of special coating designs and of suitable process conditions. Coating properties achieved are discussed for PMMA and poly-cycloolefines. The same ion source arrangement as used for coating has been applied for etching an antireflective sub-wavelength surface structure into PMMA. In summary, the paper shows the practical application fields for both technologies.
We have developed a novel approach to design ultra-hydrophobic surfaces with optical quality. The nanostructure necessary for the functional effect is realized through enhanced nanoroughness of optical thin films. At the same time, the optical appearance must not be disturbed by scattering from the roughness. We have found that through wide-scale roughness analysis, applying white light interferometry, AFM and STM, and subsequent data reduction the roughness characteristics can be directly related to the wetting properties. As, on the other hand, vector scattering theories connect the roughness properties with scatter losses, a formalism has been established, where both the wetting properties and scattering behavior can be expressed within the same "language". Using this tool, the optical thin film design and the surface nanoroughness can be tailored to fulfill demands on wetting properties as well as on sufficient low scatter levels. For the deposition of high index single layers on Borofloat 33 substrates, qualified substrate-film-combinations are predicted by "virtual" coating simulations. Experiments with single oxide layer as test coatings yielded surfaces with a high water contact angle and light scatter losses below defined scatter thresholds.