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Design and optimisation of integrated photonic waveguides and sensors

Ghosh, S. (2018). Design and optimisation of integrated photonic waveguides and sensors. (Unpublished Doctoral thesis, City, University of London)

Abstract

The study in this dissertation aimed to develop novel slot waveguide and resonator based compact integrated photonic sensors. Using guided photons as the probe for detection and measurement, and finally converting the signal magnitude from any domain to an electronic signal is one of the most effective approaches of sensing. Novel and efficient designs of hybrid and composite plasmonic horizontal slot waveguides and dielectric straight slot resonators are proposed and optimised to detect a small refractive index change. Practically, the concentrations of chemical liquid and gas or vapour are often expressed in terms of ‘grams per litre (g/l)’ and ‘parts per million (ppm)’, respectively. Thus, device sensitivities related to the detection of those substances may be expressed in amplitude or wavelength shift of the output optical signal per unit g/l or ppm. However, changes in the concentration and the chemical property of any liquid, gas, and chemical vapour result in refractive index variation of those substances. Therefore, we emphasised the detection of the refractive index change which, on the other hand, represents the concentration and/or chemical property change in the testing sample. Design, optimisations, and performance analyses of those waveguides are carried out by a direct divergence modified full-vectorial two-dimensional (2D) finite element method (FV-FEM). It provides an accurate spurious free better characterisation approach to handle all types of waveguides especially, the plasmonic and hybrid plasmonic waveguides where the guided mode is a complex mixture of the dielectric waveguide mode and surface plasmon polaritons (SPPs). Additionally, a full-vectorial three-dimensional (3D) FV-FEM dedicated to solve the 3D resonator problems is also developed and implemented. As an application of the 2D FV-FEM, first a metal nano-wire with identical and non-identical cladding conditions are considered and modal evolutions of its plasmonic fundamentals and complex supermodes are studied which also work as a benchmark of the direct divergence modified 2D FV-FEM code. Different mode effective area definitions are incorporated with this newly modified code and the low and high index contrast and hybrid plasmonic complex waveguides are simulated to determine the appropriateness of different effective area approaches for various waveguiding structures. Following this, the 2D FV-FEM is implemented in designing complex plasmonic slot based sensing waveguides. A horizontal slot composite plasmonic waveguide structure with a low index porous ZnO (P-ZnO) layer as slot material is reported and also incorporated in a compact symmetric Mach-Zehnder interferometer (MZI) to detect the presence of ethanol vapour in the environment. The waveguide is optimised to obtain a maximum slot confinement (41%) and overall a high phase sensitivity of the MZI device. A similar hybrid plasmonic horizontal slot waveguide is designed and optimised for detection of small refractive index change in the bio-layers (ssDNA and dsDNA) during DNA hybridisation. Next, a metal strip loaded horizontal slot hybrid plasmonic waveguide is designed for a high slot confinement and lower modal loss. The waveguide structure contains a suspended Si slab on top of an optimised thin metal layer (silver) to obtain a lower modal attenuation. It shows an enhanced 60% and 82% power confinement in the slot and sensing (slot+clad) sections, respectively with a small modal attenuation value of 0.036 dB/um. This waveguide is incorporated in an asymmetric Mach-Zehnder interferometer with an asymmetric power splitting scheme which results in an improved interferometric fringe visibility. This compact device exhibits a high temperature and chemical concentration sensitivity of 244 pm/±C and 437.5 nm/RIU, respectively. Beside these waveguides, a silicon-on-insulator (SOI) based vertically slotted straight resonator is also reported in this thesis. Due to its easy and straight structural design it is free from the bending losses and its fabrication steps are much easier compared to other complex devices such as ring, disk resonators, and grating based sensors. The slot cross-section is first optimised and then its length is calculated with those optimised parameters. The 3D straight resonator as a whole is then considered for bulk and surface sensing. Complete performance analyses and the resonating wavelength shift of the device due to small refractive index change during bulk and surface sensing applications are determined by using the newly developed 3D FV-FEM code. This straight resonator exhibits a 5.2 nm resonating wavelength shift for a 5 nm ultra-thin bio-layer and high bulk sensitivities of 820 nm/RIU and 683 nm/RIU for filled and empty slot conditions, respectively.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering
School of Mathematics, Computer Science & Engineering > Engineering > Electrical & Electronic Engineering
URI: http://openaccess.city.ac.uk/id/eprint/20954
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