Researchers at Chalmers University in Sweden have developed a technique to build an ultra-thin metasurface lens and other optical components using standard photoresist from chip making.
The technique exposes the metasurface of nanostructures in the resist using electron beam lithography (EBL), dramatically cutting the processing time and cost as well as reduces safety hazards compared to other nanotech processes. This reduces the fabrication process to a single lithography step and removes any subsequent need for material deposition, lift-off or etching. This allows lenses that are a thousand times thinner than glass to be built on a flat, flexible plastic substrate.
“We put a thin layer of this plastic on a glass plate and using electron-beam lithography we can draw detailed patterns in the plastic film, which after development will form the metasurface. The resulting device can focus light just like a normal camera lens, but it is thousands of times thinner – and can be flexible too,” said Daniel Andrén, a PhD student at the Department of Physics at Chalmers and first author of the scientific article published in ACS Photonics.
The team has built a positive lenses with a diameter of 1cm, gratings with anomalous reflections, and cylindrical metalenses on flexible plastic substrates using the technique.
Building the large 1cm lens was possible by using high EBL writing speeds, partly by the use of a large beam current and partly by the relatively high sensitivity of the ma-N 2410 resist, which can be exposed with low doses. Even shorter writing speed could be achieved with other resists that are up to 100 times more sensitive (such as the UV–N series).
The design stage requires files of sizes larger than 50 GB potentially poses some memory limitations for designing larger metasurfaces, but this could be avoided by clever design and processing algorithms already used for large-scale metasurfaces.
The advantages of using the resist methodology go beyond simplicity and scalability. In particular, the method can be adapted to practically any substrate and requires processing temperatures as low as 90 °C. This is a significant advantage compared to established techniques, which rely on high-temperature material growth such as metallo-organic chemical vapour deposition or the use of nonconventional chemicals such as atomic layer deposition techniques. The team built metasurfaces on flexible plastic substrates polyethylene terephthalate (PET) and on metal mirrors
This opens up new avenues for combining flexible electronics with flat optical devices, for example, creating tunable attenuators by inducing specific deformations of the substrate. Reflective hybrid metallic/dielectric metasurfaces have also been proposed as a platform for new optical techniques.
The metasurfaces that create the lens effect are stable over time and can be left in ambient conditions without any noticeable degradation for at least six months, withstanding temperatures at least as high as 100 °C and relatively high light intensities up to 5 W/cm2 generated by a 532 nm continuous laser source without any apparent reduction in the metasurface performance.
This could open up whole new areas of development for optical component design as the technique is accessible also to laboratories and researchers that lack access to the advanced tools for material growth, deposition, and etching, typically required for metasurface fabrication. The method can also be adapted to any kind of resist, positive or negative.
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