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Bio-inspired helicoidal multilayers for photonic innovation

Periodic Reporting for period 1 - HELICOID (Bio-inspired helicoidal multilayers for photonic innovation)

Reporting period: 2017-11-15 to 2019-11-14

Natural structures that interact with light, in both nano- and micron-scales, constitute an invaluable source of “blueprints” for innovation in bio-inspired technological devices that can make an impact in the technology-based economy. These micro and nano- structures affect the motion of photons and can create particular optical effects such as the intense blue color of the Morpho butterfly. One of the natural structures having an artificial counterpart with huge economic impact are optical thin films. In industry for example, single and multi-layers are used for anti-reflective coatings, filters and mirrors. The natural world has created complex multilayer systems with added functionalities, such as polarized iridescence or color-mixing properties. In some cases, nature has also introduced infrastructures within the multilayers, leading to optical phenomena such as the circular-polarization (CP) selective response found in some scarab beetles. This property arises from a helicoidal structure, where parallel nano-fibrils of chitin form stacked planes, with varying angle of fibril orientation in the sequential layers, leading to the selective reflection of the polarization handiness of light. Fabricating such structures can be highly attractive for applications, for example as wavelength-tuned chiral micro-mirrors for optical devices, or as security labels for the anti-counterfeiting industry. Up until this project, the fabrication of thin film multilayered structures embedding custom designed nanostructures within the multilayers was mainly limited to multi-step electron beam lithography approaches, which can be cost and time consuming for structures requiring several layers.
This project aimed at exploring a novel thin film fabrication technique to enable the fabrication of artificial multilayers with such infrastructures embedded within the layers, which could lead to novel optical devices. Specifically, inspired by the helicoidal structure found in scarab beetles, a roll-up thin film self-assembly technique was explored to generate multilayer stacks from pre-patterned thin films on a single step. Such an approach would require significantly less fabrication steps than the state-of-the-art multilevel alignment approaches.
The main objectives of this action were 1) to take advantage of optical simulations to study and design multilayers with embedded nanopatterns and 2) to combine state-of-the-art nano-patterning techniques with a roll-up thin film technology to develop a multi-layer platform with engineered patterns at each layer of the stack.
During the project, for the proof-of-concept of this fabrication approach, the work focused on realizing self-rolled multilayers with embedded gold nanoparticles, which due to their strong plasmonic response, could enable us to achieve the selective circular polarization responses with few layer systems. The fabrication of such structures was successful and proved that such technique can be use to enable self-rolled multilayered metasurfaces in general, the main results have been published in a peer reviewed journal. Furthermore, we also explored the fabrication of other types of multilayered metamaterials such as hyperbolic multilayers. The results of the latter are in the process of preparation for a publication.
The development of these type of new fabrication approaches can benefit the scientific community to enable new avenues to create novel materials and components. In this context, the results from this work could lead to innovative photonic technologies with potential applications in security labels, optoelectronic devices, sensing or imaging systems.
The initial stage of the project focused on optical simulations using a commercially available software (Lumerical). Multilayered structures composed of a thin dielectric layered supporting gold nanoparticles were designed and studied. The simulations focused on the optical response from multilayers with varying nanoparticle designs and relative angles was studied. Optimal material parameters such as thin film thickness and particle dimensions were recovered and were the base for starting the next phase of the project concerned with sample fabrications.
The second stage of the project dealt with developing the fabrication protocols to enable the thin film self-rolling of dielectric layers with gold nanoparticle arrays on top. This stage took place at cleanroom nano/micro-fabrication facilities. Different techniques including thin film deposition, photolithography, etching and electron-beam lithography were implemented.
Along side the sample fabrication period, structural characterization was performed using scanning electron microscopy, while the structures were characterized optically via reflection spectroscopy using circularly polarized light. We could demonstrate structures with selective response of circularly polarized light.
Details on the developed fabrication protocols can be found in the open access publication, alongside the optical characterization for helicoidally stacked gold nanoparticle arrays. ( Besides the open access publication, the results were disseminated in several international conferences such as Living Light 2018 in Cambridge, United Kingdom (2nd place poster prize), International Workshop on Nanomembrane Origami Technology, Shanghai, China (invited talk), Gordon Conference in Plasmonics 2018 in Maine, United States, and the Surface Plasmon Photonics 9 in Copenhagen Denmark (best poster prize).
Up until this project, the fabrication of thin film multilayered structures embedding custom designed nanostructures within the multilayers was mainly limited to multi-step electron beam lithography approaches, which can be cost and time consuming for structures requiring several layers. The results obtained during this project made progress beyond such state-of-the-art as it demonstrated the capabilities of the thin film self-rolling technique to enable similar multilayered devices while relying on a reduced number of nano-lithography steps. We expect this technique to open new capabilities in terms of the potential designs and material combinations compatible with such approach when compared to the aforementioned approaches in the field. Opening the possibilities for fabricating new material systems can lead to innovative optical devices in futures works.
Conceptual figure of the fabrication approach developed during the project