Fabrication and assessment of polymer based photonic structures
Creators: name: Richardson, George
Publisher: name: Imperial College London
Year: 2016

description: Photonic structures manipulate the flow of light using nano and micro-structured spatial variations in refractive index. Periodic structures can give rise to photonic band gaps and diffractive behaviour depending on their exact architecture and the direction and wavelength of incident light. The fabrication and analysis of photonic structures is a longstanding field of research, as their properties are desirable for many technological applications and further understanding of optical phenomena. Conventionally, subtractive lithographic techniques have been used to pattern inorganic materials due to the availability of high index materials and ability to pattern a wide range of structures. However, in recent years numerous alternative patterning methods and materials have become available. Organic materials provide a route for creating photonic structures that are compatible with thin film optoelectronic technologies and use simple thermal or solution based methods of patterning. In this work, a method for patterning polymer based materials directly from solution is developed. This solution moulding is compared to the well established technique of hot embossing - a standard industrial route for producing surface relief structures in plastics. Diffraction gratings are fabricated in the commodity polymer polystyrene, using both techniques and are analysed structurally using scanning electron microscopy. Optical diffractometry is then employed as a non-destructive technique which yields diffraction patterns containing information about the structures. The diffraction spectra are explored in order to gain insight into the information they provide about the structures and subsequently to compare the fabrication methods. Many useful polymeric materials exhibit thermal degradation or cross-linking, making them unsuitable for hot embossing. The success of solution moulding opens new possibilities, by enabling the patterning of a wide range of materials at low temperatures, provided that they can be prepared in solution. Attention is therefore turned to the patterning of non-commodity materials. A high index, transparent titanium-poly(vinyl alcohol) (Ti:PVA) hybrid material is structured using solution moulding. Whilst its properties make it highly suitable for optical applications, the material cross-links quickly after deposition, meaning that it would be very challenging to hot emboss. Furthermore, a ferroelectric polymer, poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)), is structured to make simple memory devices. In this case, solution moulding is employed to achieve the specific structural configuration of the device, and allow for the filling of the initial ferroelectric structure with a semiconducting polymer, also from solution. With control over the fabrication of polymer gratings, novel photonic structures are fabricated by depositing a monolayer of polystyrene microparticles into grating channels, in a process referred to as `nano-pinballing'. This self-assembly route results in structures that resemble those found in nature, to provide structural colour, for example in opals and on the bodies of some animals. The templated microparticle layer creates a quasi-random lattice that contrasts with the grating structure to produce both order and disorder. The lattices fabricated are analysed statistically from optical micrographs, and two-dimensional optical diffractometry is employed, with an emphasis on the ability to detect both non-cumulative and cumulative disorder.