City Research Online

Abstract

The recent advances in nanotechnology have created the need for the development of materials and devices with unique properties, suitable for applications in related areas such as nano-electronics, nano-photonics and high sensitivity optical sensing. Nanowires are becoming good candidates for such applications, however, most of the ongoing research is still at the early stage and therefore all the effort in this field is to improve the fabrication techniques, as well as to increase the performance of such devices, by optimizing their key parameters, in order to bring them into the production line. Ormocomp and silicon nanowire waveguides (NWs) are studied and developed as optical waveguides intended to be used in optical sensing applications. The NWs are designed and developed as part of integrated optical devices by having tapered and feed waveguides connected at both the ends. The ormocomp and the silicon NWs are theoretically investigated using a full-vectorial H-field Finite Element Method (FEM). The aim is to obtain high power confinement in the sensing area, which is considered to be the core/cladding interface for the ormocomp NWs and the slot region (low-index area) for the silicon NWs. The modal field and the power confinement of the guided modes that contribute to the enhancement of the sensitivity in the corresponding sensing area of the NWs are studied, with respect to the variation of the refractive index of the cladding material and the operating wavelength. The structure parameters including the width and the height of the NWs are optimized to achieve the maximum possible sensitivity. A biosensor structure incorporating the silicon NWs with horizontal slot structure is also studied theoretically using the full-vectorial H-field FEM. It is designed to detect DNA hybridisation through the change of the effective index of the NW structure. The key parameters, such as power confinement, power density, change in effective index and sensitivity of the fundamental guided optical modes are presented, by optimizing the device parameters of the slot waveguide. Experimental characterisation of the integrated ormocomp NWs is also demonstrated. The integrated optical ormocomp NWs are used to measure the change of effective index when there is a change of refractive index of the material. An evanescent wave coupling technique is exploited for the ormocomp NW to be used as optical sensors. The evanescent field that exists at the core/cladding interface of the NWs can be enhanced by introducing surface plasmon resonance (SPR). The SPR is introduced by coating the integrated ormocomp NWs with a thin gold layer with a thickness of around 50-100 nm. The optical power at the output, the attenuation coefficient of the NWs, the SPR peak wavelengths and their shifts are experimentally extracted over three different cladding materials. The redshift of the supermode coupling between the dielectric mode and the anti-symmetric supermode is observed with higher cladding index and larger metal thickness. The power confinement in the sensing region with the SPR effect is improved by a factor of ten compared to the performance obtained by the un-coated ormocomp NWs.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments:	Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses School of Science & Technology School of Science & Technology > Engineering

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