Applications of plasmonics in silicon based photonic devices

Surface plasmon polaritons are highly confined electromagnetic waves which can be employed in developing miniaturised optical devices for bridging the size-mismatch between the nanoscale electronics and large diffraction-limited photonic devices. For this purpose, it is desired to develop silicon compatible plasmonic devices in order to achieve seamless integration with electronics on the silicon-on-insulator platform. Plasmonic devices such
as modulators, detectors, couplers, (de)multiplexers, etc, would possess the advantages of having a small device footprint, low cost, low power consumption and faster response time.

In this thesis, different silicon-based plasmonic devices were investigated using finite element simulations, including optical modulators, couplers and splitters. A metallised stub filled with SiGe/Ge multiple quantum wells or quantum dots in a silicon matrix, coupled to a dielectric waveguide was investigated. The modulation principles include ’spoiling’ of the Q factor and conversion of the electromagnetic mode parity, due to
variation of the absorption coefficient of the stub filling.

A CMOS compatible interference-based Mach-Zehnder modulator with each arm comprising a metal-insulator-semiconductor-insulator-metal structure, and a simpler single
arm variant, were considered for electro-optic and electroabsorption modulation respectively.
The electron density profiles in bias-induced accumulation layers were calculated with the inclusion of size-quantisation effects at the oxide-silicon interfaces. These
were then used to find the complex refractive index profiles across the structure, in its biased and unbiased states, and eventually the modulator insertion loss and extinction ratio, and their dependence on various structural parameters.

Finally, a silicon-based plasmonic nanofocusing coupler was investigated, which comprised symmetric rectangular grooves converging towards a central metal-silicon-metal
nano-slit at the apex of the structure. The structure was optimised to achieve maximum coupling of light incident from a wide input opening, and coherent excitation and focusing of surface plasmons as they propagate towards the nano-slit waveguide. Application of the nanofocusing structure to achieve simultaneous coupling and splitting was also investigated, whereby incident light was focused into two nano-slits separated by a metal gap region at the apex. Such a plasmonic coupler or splitter can be used for coupling light directly from a wide fibre grating opening into nanoplasmonic waveguides in future on-chip plasmonic-electronic integrated circuits, or into the two arms of a plasmonic Mach-Zehnder modulator.