One-step optical device manufacturing process based on replicating the photonic structures of butterfly wings
A novel one-step process for manufacturing optical devices may be introduced by replicating the complex photonic structures of a butterfly.
The beautiful colours of a butterfly’s wings are produced by a combination of pigments and reflection from their photonic structure. It is the latter that has intrigued materials scientists at the Georgia Institute of Technology in the USA. ‘If you examine the wing scale, you see all of the intricate microscale and nanoscale features. The fine structure is periodic rib-like [acting] as a photonic crystal,’ says Zhong Lin Wang, Regent’s Professor at the School of Materials Science and Engineering. This provides the potential for developing optical wave guides and splitters.
In attempting to create a material that mimics these wing scales, the researchers chose the bright blue Morpho peleides species as their specimen, which is native to the rainforests of Central and South America. The insect’s wings have ‘arrays of vertically aligned, net-like, skeleton structures, which are a perfect template’, explains Wang.
An exact replica was produced by depositing layers of alumina onto the wing structure – one Ångstrom per cycle – using the atomic layer deposition (ALD) process at lowtemperatures (80-100ºC). It took two hours to grow 50nm of material. Once completed, the coated scales were heated to 800ºc to crystallise the alumina and burn off the organic skeleton. The resulting creation is threedimensional, retaining the photonic structure of the original.
Wang adds, ‘There are two reasons for choosing alumina. First, [it] is a dielectric material. Its refractive index (-1.6) is different enough from that of air and can form a periodic change in the structure with air – the prerequisite of a photonic crystal. Second, once crystalised, alumina is one of the hardest ceramic materials and can hold the delicate structure.’ Although Wang envisages that other materials can be employed to create alternative biomimetic structures using ALD.
The team foresees potential for this manufacturing method in industry. ‘The complex nature of the structures would be impossible to create with any other process [without it being labour intensive]. Nature is the best manufacturer, with the highest reproducibility and accuracy,’ explains Wang.
Two-dimensional photonic crystal based waveguides and photonic integrated circuits are promising, but the fabrication technique is still at laboratory scale and largely limited by the cost and method of scaling up. This [onestep biotemplate] method will boost the development of such valuable optical devices.’ However, further refining of the lamella is required to ensure it completely traps the light inside it.
Researchers have also found that by varying the thickness of the alumina coating, which controls the size and periodicity of the photonic structures, the wavelength and hence colour of the reflected light also shifts. This property enables the material to be tuned to a desired wavelength for a particular application.
Zhong Lin Wang, email: firstname.lastname@example.org.