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Title: Nanoplasmonic grating coupler for transducer applications
Author: Iqbal, Tahir
Awarding Body: Queen's University Belfast
Current Institution: Queen's University Belfast
Date of Award: 2013
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This thesis investigates the excitation of surface plasmon polaritons (SPPs) on a one dimensional (ID) grating etched in a gold film on a (high-index) gallium phosphide (GaP) substrate. The experimental component of the work addresses the design and building of an optical set-up for the far-field characterization of the spatially confined (-20 x 20μm) gratings in transmission, with better than 2° angular resolution, and the commissioning and use of a WiTec scanning near-field optical microscope (with bespoke modification) for near-field analysis. The purpose of the latter is to track the propagation of SPPs on the planar gold film region beyond the grating launch site. Both the far- and near-field experimental data are interpreted with reference to modelled results generated by using COMSOL, a commercial, finite-element analysis software package. Coupling efficiency of the incident light to the SPP is studied as a function of slit width where the grating periodicity and the (optimized) gold thickness remain constant. Defining a simple and convenient measure of the coupling efficiency the optimum slit width is found to be between 1/3 and 1/2 of the grating period. Such grating devices support only the fundamental mode and offer less radiative scattering of SPPs. The experimental results are in agreement with the far-field modelling results and the optimum slit-width range in the grating devices is close to that suggested by near-field analysis. The (maximum) propagation length, Lspp, just outside the grating device is found to be -13.33 ± 0.13μm under excitation by laser light of 785 nm wavelength. The Lspp is associated with the full width at half maximum (FWHM) of the SPP resonance. The grating on high refractive index substrate described above has intended application in heat assisted magnetic recording where it is envisaged that the focussing of optical energy to nanoscale dimensions will take place using SPPs generated by using a grating coupler integrated on a semiconductor laser chip. In addition, an optimal 1 D grating was designed and fabricated on a 50 nm Au film on a (low-index) glass substrate for application as a highly sensitive biosensor. The sensitivity was found to be 524 nm per refractive index unit (RIU) which is highly competitive with many other plasmon-based biosensors while being of simpler geometrical structure and more economical design.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available