Advanced software helps designers create thin-film coatings with precise spectral qualities for almost any design challenge. However, these computer-generated designs are not always easily manufactured with current coating equipment and monitoring techniques.
To address this, software developers are creating algorithms that automatically produce practical design solutions and specialized tools for particular design problems. These advancements aim to ensure designs can be fabricated while maintaining excellent spectral performance. Research at OptiLayer Software focuses on developing design methods suitable for specific monitoring and deposition processes.
Experienced engineers often rely on their own methods to simplify filter designs, especially for broadband blocking filters. Near-quarter-wave stacks are favored for manufacturing simplicity in short-pass, long-pass and Fabry-Perot filters. These designs benefit fabrication systems limited in depositing very thin layers and allow optical monitoring without wavelength changes. OptiLayer has developed a Sensitivity-Directed Refinement (SDR) algorithm that rapidly generates near-quarter-wave designs.
The SDR algorithm refines an initial quarter-wave stack design by selectively adjusting layer thicknesses based on sensitivity analysis. An automated version of this SDR algorithm allows for multiple design solutions by varying parameters such as the number of adjusted layers. For example, an SDR-designed blocking filter using Schott B270 glass, tantalum pentoxide, and silicon dioxide, achieved targeted transmittance levels across different spectral ranges while maintaining a near-quarter-wave structure.
Metal-dielectric composites are gaining interest due to their unique optical properties arising from surface-plasmon resonance. These composites, readily fabricated with standard equipment, allow for sophisticated spectral performance beyond traditional dielectric coatings. Unlike dielectrics, the optical properties of metal layers in these composites are influenced by both layer thickness and surrounding materials. Therefore, conventional coating design methods based on thickness variations alone may not be suitable.
OptiLayer suggests a design approach using a fixed-thickness metal-dielectric “sandwich” as a starting point. Gradual evolution techniques, adding layers near the substrate or environment while keeping the sandwich core fixed, can be applied. Constrained optimization can further refine the design. For instance, coatings exhibiting different reflected colors from the front and back surfaces, while maintaining over 50% transmittance, were designed. A SiO2/Ag/SiO2 sandwich structure was used to achieve orange front and violet back reflection. Similar color-shifting designs using gold have been successfully produced by researchers at the Institut Ruđer Boskovic.
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