Master of Science (MSc)
Physics and Computer Science
Faculty of Science
Dr. Li Wei
Through surface plasmon polaritons (SPPs) that propagate along the interface between a metal and a dielectric material, plasmonic waveguides have the ability to confine light at subwavelength scale beyond the diffraction limit, which opens a promising platform to further downsize the active and passive photonic devices. The fields of the SPPs have maximum amplitude at the metal/dielectric interface and decay exponentially toward both media, where the penetration of the fields in the dielectric is very susceptible to the change in the refractive index of the dielectric. This makes surface plasmon resonance (SPR) a remarkable technique in sensor applications to investigate the medium near interface by exploiting the method of attenuated total reflection (ATR) to excite the SPP mode in plasmonic waveguides. In conventional SPR sensors, the most commonly used is Kretschmann’s configuration. The optical resonance of the ATR curve in Kretschmann’s configuration has been recognized from the excitation of the SPP mode supported by a two-layer (metal/dielectric) structure with the effective index of the SPP given by, where andare the permittivity of the metal and sensing medium. This has not been accurately interpreted because the coupling layer prism is not considered. On the other hand, conventional three-layer Kretschmann configuration based sensors exhibit very broad ATR lineshape resulting in poor performance of sensitivity. By using multilayer plasmonic waveguide structures, the sensitivities of SPR based sensors could be significantly enhanced. Thus, it is very important to study the SPP modes in plasmonic waveguides, which provides an insight into the origin of the optical resonance in the ATR curve and also facilitates the design of multilayer plasmonic waveguides for ultrasensitive SPR based sensor applications.
In this thesis, we theoretically study the SPP modes supported by three- and four-layer asymmetric plasmonic waveguides with taking account of the high-index prism layer. The dispersion equations for three- and four-layer plasmonic waveguides are derived, which are used to characterize the SPP modes, supported by three-layer symmetric and asymmetric, and four-layer asymmetric plasmonic waveguides. With the derived dispersion equations, we have analyzed the modal index and the propagation length for different plasmonic waveguides. The profiles of the electric and magnetic fields have been visualized by using COMSOL Multiphysics software. To explore the origin of the optical resonance in SPR sensors associated with the SPP mode excited in plasmonic waveguides, the ATR spectra of asymmetric three-layer Kretschmann configuration, and asymmetric four-layer plasmonic waveguides are investigated. Our results show that there are optimum thicknesses for the metal and the dielectric layer with the strongest optical resonance in ATR curves, which could be determined from the analysis of the SPP modes.
We also propose an ultrasensitive SPR sensor based on a multilayer plasmonic structure to generate Fano resonance in near-infrared, where the transparent conductive oxide - Cadmium Oxide (CdO) serves as a plasmonic material. It is formed by a six-layer prism/Teflon/CdO/Teflon/Si/analyte slab waveguide. A sharp asymmetric Fano resonance lineshape is successfully achieved from the mode coupling between the dielectric waveguide (DWG) mode supported by Teflon/Si/Analyte and the long-range SPP (LRSPP) mode supported by prism/Teflon/CdO/Teflon. The shape of the Fano resonance could be engineered by adjusting the structural parameters of the inner layers to enhance the intensity sensitivity. With optimized structural parameters, our proposed design can achieve a maximum intensity sensitivity of 19,904 RIU-1, which is 129-fold enhancement than that in conventional long-range SPR based scheme. The proposed highly sensitive CdO-based plasmonic sensor shall be useful for sensing applications that operate in the near-infrared region.
Khattak, Anum, "Plasmonic Slab Waveguides: Theory & Application for Sensors" (2020). Theses and Dissertations (Comprehensive). 2280.