Optical metamaterials by block-copolymer self-assembly

• Main Author: Salvatore, Stefano
• Published: University of Cambridge 2014
• Subjects: 620.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.607582

This thesis explores the fabrication processes and the optical characterization of a metamaterials made by block copolymer self-assembly. Optical metamaterials are artificial systems designed to produce optical properties that may not be found in nature. They gain their properties not from their chemical composition but rather from their desig structure that affects the interaction with electromagnetic waves, producing an optical response different from the constituent material. The structural features have sub-wavelength size so that the metamaterial is perceived homogeneous by the incident waves and the electromagnetic response is expressed in terms of homogenized material parameters. In this work such structural features will be fabricated well below optical wavelengths by metal replica of a gyroid morphology generated by block copolymer self-assembly. Block copolymers consist of two or more chemically different polymers that are covalently tethered. Self-assembly in such systems is driven by enthalpy reduction through microphase separation that minimizes unfavourable interfaces, leading to a range of potential morphologies. The gyroid morphology consists of a continuous three dimensional structure with constant mean curvature and chiral directions. The gold gyroid effectively behaves as a metamaterial with its own distinct optical characteristics: a reduced plasma frequency and highly enhanced transmission, an hall mark of optical metamaterials. The optical characterization discussed in this dissertation showed good agreement with infinite difference time domain (FDTD) calculations and analytical models. The optical properties of the fabricated metamaterials were successfully tuned by modifying the structural dimensions and the surrounding mediums. The transmission efficiency was, then, further enhanced by fabricating an hollow gyroid structure with increased surface area. The hollow gyroid enabled also the fabrication of composite metamaterial employing the combination of different metals in the 3D continuous structure. Next, exible and stretchable metamaterial were fabricated by infiltrating an elastomer around the gyroid network, paving the way for practical applications. Finally, the gyroid metamaterial was used as vapour sensor, exploiting the regular pore size and the variation of the optical response with surrounding media.