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Utilizing laser interference lithography to fabricate hierarchical optical active nanostructures inspired by the blue Morpho butterfly

The famous non-iridescent blue of the Morpho butter by is caused by a `Christmas tree' like nanostructure which is a challenge for common fabrication techniques. Here, we introduce a method to fabricate this complex morphology utilizing dual beam interference lithography. We add a reflective coating below the photoresist to create a second interference pattern in vertical direction by exploiting the back reflection from the substrate. This vertical pattern exposes the lamella structure into the photosensitive polymer while the horizontal interference pattern determines the distance of the ridges. The photosensitive polymer is chosen accordingly to create the Christmas tree' like tapered shape. The resulting artificial Morpho replica shows brilliant non-iridescent blue up to an incident angle of 40. Its optical properties are close to the original Morpho structure because the refractive index of the polymer is close to chitin. Moreover, the biomimetic surface is water repellent with a contact angle of 110.