Self-assembly and nanotechnology within an optical fibre fFor improved evanescent field sensing

Abstract

Abstract: The self-assembly of TiO 2 nanoparticles is used to create a high index layer within a structured optical fibre. We show both experimentally (using a novel porphyrin probe) and theoretically that this approach leads to more than order of ma gnitude enhanced localisation of the optical field at the layer-air interface of the hole, both through edge localisation and through novel resonance localisation as a result of a ring resonator whispering gallery modes. Nanotechnology, nanophotonics, sensing, interfaces, self -assembly, self-assembled photonic s, porphyrins, optical fibers, waveguides, photonic crystal, structured waveguides. 1. INTRODUCTION Nanophotonics involves in many cases exploitation of the evanescent field to enhance particular functionality. In optical sensing, evanescent field spectroscopy (EFS) has become one the key methods for detection and diagnostics using optical waveguides, particularly optical fibres, either conventional [1], or structured [2]. It is an extremely powerful tool, both in terms of chemical sensor applications and as well as fundamental mo lecular studies of layers and films that circumvents the need for high finesse resonant spectroscopy, including cavity ring down spectroscopy, where temporal responses and lifetimes may need special consideration, for ultra high sensitivity. EFS using conventional fibres has some drawbacks since interaction with the outside usually requires exploitation of the cladding mode evanescent fields; coupling to and from the core travellin g mode is, for example, achieved via phase matching through long period gratings [1]. Whilst there are other configurations, structured fibres have channels where the core traveling modes directly overlap with the channels and therefore core mode evanescent fields are exploited. By having arbitrary long interaction lengths using structured optical fibres, for example, unprecedented direct sensitivity is, in principle, possible. Indeed, we recently reported the observation of a near IR band associated with charge transfer between dichloro[5,10,15,20-tetra(heptyl)porphyrinato]tin(IV) [Cl i Sn(THP)-Cl] a porphyrin molecu le and the inner silica surface of the channels of a structured optical fi bre over 90 cm in length [2], previously only postulated. For many applications, particularly in distributed chemical and biochemical, but also in novel optoelectronic devices that rely on overlap with the evanescent field, these lengths become impractical. Solutions involve ring waveguide configurations that are essentially integrated resonant spectrometers and the use of bandgap fibres often in co mbination with surface plasmon resonances, for example [3]. Bandgap waveguides can be extremely sensitive to perturbations, such as temperature and strain, which shift the bands (or dispersion) and affect interactions. They are also difficult to couple to standard telecommunications fibres, making remote sensing challenging. In any case, for many applications it is the field at the interface, particularly for selective surfaces, which is important and EFS remain s the preferred interrogation tool. The ability to perhaps integrate resonators into such fibres would be ideal. In this work, we propose a novel alternative that exploits advances in self-assembly, nanostructures and optical waveguides to enhance the evanescent field interactions within the holes of the structured optical fibre. The approach uses high index layers on the inside of the holes of the