Solution-processed photonics for light and heat management

Abstract

Printed photonic devices have drawn increasing interest during the last decade thanks to the prospect of low-cost, high-throughput and large-area manufacturing. However, the realisation of novel solution-processed technologies requires the development of new advanced and functional materials that combine the often-superior properties of inorganics with the straightforward, low-cost fabrication features of organics. This thesis is based on a printable molecular hybrid composed of titanium oxide hydrates and the commodity polymer poly(vinyl alcohol) exhibiting high refractive index, low optical losses, pristine film quality and ease of production. After confirmation of the structural organisation leading to these properties by optical and material characterisations, thermal patterning of the refractive index within a thin-film was found possible. This was thanks to the short time required to anneal the material, measured by time-resolved optical spectroscopy. The local change in refractive index and the versatile tunability of the hybrid allowed the straightforward fabrication of multiple coplanar refractive indices and large gradients by thermal stamping. Furthermore, a novel microcontact photo-annealing method enabled sharper patterns, crucial for advanced planar photonic structures such as lightguides for solar concentration. The high index and excellent quality of hybrid films were then applied in multilayer photonic structures. Combined for the first time with the commodity polymer poly(methyl methacrylate), distributed Bragg reflectors were produced in close agreement with optical modellings using a transfer-matrix method. Large-area (100 cmxcm) devices as well as cylindrical dielectric mirrors were fabricated through dip-coating methods. Finally, broadband reflectors were modelled and produced by geometrical chirping of the multilayer structure. Extended reflection band of 400 nm was enabled across the whole visible range while still transparent to infrared. Similarly, a 81-layer chirped design was used to reflect the solar near-infrared responsible for heating within buildings. This metal-free omnidirectional heat reflector was shown to be equivalent to commercial silver-based window glazings. A brief energy analysis confirms its suitability for low-solar heat gain by blocking most of the solar radiation, both in UV and infrared, while being transparent to visible light. This, to the best knowledge of the author, is the first demonstration of a solution-processed dielectric mirror for low-solar heat gain window glazing.