====Surface Plasmon Polaritons====

Surface Plasmon Polaritons, SPPs, are electromagnetic waves trapped along a interface between two materials: a metal and a dielectric. In the bulk of both materials the wave decay exponentially as they move away from the interface. Thus it can be thought of as a two dimensional waveguide. The energy is focused along the interface. As a result the effects of small surface defects are greatly amplified.

The goal of this research is to use this sensitivity to surface structure to amplify the effects of the SAWs created by our already functioning transducers. Both the SPP and the SAW are surface phenomenon and thus provide ideal power overlap.

SPPs can be created through prism coupling and (after we figure it out) grating coupling. In both cases, the momentum vector of the light is increased through the coupling medium to match the fundamental SPP frequency. That couples the energy into the SPP in the same way that light is coupled into a proton exchanged waveguide. The SPP then travels along the interface until losses in the metal deplete it of energy. See the figures below.

{{::spp_prism_coupling.png?300|}}
{{::spp_grating_coupling.png?300|}}

Our end design will use the SPP as the waveguiding material. This should provide wavelength and spacial filtering that eliminate much of the noise from the current waveguides. It also promises theoretical efficiencies of up to 60%. It should look something like this:

{{::spp_waveguides_with_ao_modulation.png?|}}