Liquid crystal and meta-surface devices for enhancing light manipulation from visible to THz regime

Abstract

Liquid crystals lie at the heart of many light-switching devices. Convectional devices switch as a result of the imposition of an external voltage, leading to an electric field inside the bulk. In this thesis I investigate bistable liquid crystal-based devices with switching between states controlled by light. Such an enhanced, photoactive response can be realised by integrating liquid crystals with photoalignment or photoconductive alignment layers. The resulting liquid crystal cells, asymmetric by design, need to be monitored for the stability and uniformity, especially in the regions exposed to light. In this work, I report on an integrated, versatile model and technique to characterise their core parameters as well as more subtle effects, such as the strength of anchoring energy. The snapshot method (the so-called ‘OMPA’) also provides two dimensional maps of the cells’ thickness, pretilt angle and uniformity. A dynamic, optically addressed, waveplate forming rewriteable twisted liquid crystal structures is then presented. The cell is bistable, with switching between states controlled by one-step illumination of a single PAAD azobenzene alignment layer with visible light. There is no requirement for electrodes or an applied field to control the dynamic behaviour. The photo-alignment properties of the layer enable reversible switching between two perpendicular alignment states at the cell surface, resulting in controllable polarization manipulation of visible and infrared light with an efficiency greater than 90%. Towards controlling longer wavelengths i.e., THz radiation exploiting the low optical anisotropy of liquid crystals in this range, is one of the main challenges. Although, there have been several solutions incorporating semiconductors or planar metamaterials, there is still the need to incorporate external stimulus (voltage, bulky magnets, temperature). This is a considerable technological challenge, since new applications impose compact, and thin devices with the less possible complexity in their systems and experimental setups. All-optical control of liquid crystals exploiting the THz itself is one of the main challenges nowadays. Enhanced localization of THz electric field in the vicinity of metamaterials may drive liquid crystal molecules and thus control their optical properties without the need of external stimulus. Here, I present an experimental and theoretical study of liquid crystal loaded metamaterials devices capable of shifting the narrowband resonant response of metamaterials by 45 GHz when switching between two planar liquid crystal states for both Babinet complementary patterns. My study suggests that this is due to the orientational optical nonlinearity of nematic liquid crystals induced in the vicinity of metamaterials near-field ‘hotspots’, which prompt liquid crystal molecules to spatially re-orient along the localized electric field lines. I envisage that these findings can directly lead to the increase of the efficacy of THz modulators and other active optical components exploiting the enhanced nonlinear light-matter interactions in liquid crystal-metamaterials hybrid structures.

More information

Identifiers

ORCID for Vasileios Apostolopoulos: orcid.org/0000-0003-3733-2191

Catalogue record

Export record