Fabrication and electromagnetic properties of metallic nano-helices

Abstract

Helically-shaped sytems are widely known in physics, biology or chemistry. Examples are some types of carbon nanotubes, DNA and collagen (double and triple helices, respectively), lipid bilayers, bacterial flagella (Salmonella and Esterichia Coli), coils of spirochetes bacteria, tendrils, etc. [1] Regarding practical applications, helices are widely studied and used at the macro- and micro scale. They are present in custom devices to store mechanical energy (springs, helical micro-electro-mechanical systems) [2] or parts of custom photonic crystals [3,4]. Furthermore, metallic helical structures are used as antennae in a wide range of applications covering the mid-infrared, microwave and radio frequency ranges of the electromagnetic spectrum [5, 6]. In addition, due to their particular shape lacking mirror symmetry, these structures are model systems to study chiral effects either in optical, magneto-optical or magneto-transport measurements [7, 8]. However, the performance of the helical antennae has not been investigated into the optical frequency range yet, primarily due to the required extremely small scale. Optical (or plasmonic) antennae demand fabrication accuracies better than a few nanometers, a precision not achieved up to date in helical nano-structures made from metals. Despite the associated difficulties, the production and understanding of these kinds of metallic nano-devices are highly desirable since they represent a novel type of antenna with unprecedented magneto-electric (chiral) behaviour. Therefore, the optical (plasmonic) helical antenna is an emerging opportunity for innovative optoelectronic devices given the unique properties rising from this particular non-centrosymmetric shape. The present thesis reports on the fabrication of a scalable production technique for nano-sized helically shaped metals (nano-coils) with a structural quality on the nanoscale far exceeding any prior art. Their fabrication accuracy allowed the observation of a localized surface-plasmon along the metallic nano-helix. This fact, together with the optical properties associated with their chiral shape determine the functionality of metallic nano-helices as antennae in the optical spectrum. In addition, the sample’s chirality is manifested in the magneto-optical and magneto-resistance measurements of these samples as well.