Optimization of grating couplers for (Bloch) long range surface plasmons on metal stripe waveguides

Gratings capable of in-coupling an incident Gaussian beam into long-range surface plasmon polaritons (LRSPPs), propagating along a thin narrow Au stripe on a low-index cladding (fluoropolymer), or on a truncated 1D photonic crystal as a Bloch LRSPP, are investigated for potential biosensing application. The gratings are optimized, initially, through 2D modelling using the vectorial finite element method (FEM). Different 3D grating designs are then investigated through 3D modelling using the vectorial finite difference time domain (FDTD) method, including wide gratings on adiabatic and non-adiabatic flared stripes. Wide gratings on flared stripes have a larger area enabling size-matching and coupling to larger incident beams, which facilitates optical alignments. We also investigate the effects of changing the size of the beam incident on the structures. In-coupling efficiencies of up to 25% at λ0 = 1310 are predicted for optimal gratings - such coupling efficiencies are among the highest reported to date for a structure of this kind. Similar grating designs are also examined as out-coupling structures, capable of coupling (Bloch) LRSPPs into a perpendicularly-emerging Gaussian beam. The absorptance, reflectance and transmittance of the structure were also calculated. Structures were fabricated on a multilayer wafer supporting Bloch LRSPPs to verify design concepts.