Propagation length of surface plasmon polaritons determined by emission from introduced surface discontinuities

Flexible far-field microscopy methods suitable for directly measuring surface plasmon polariton propagation along optically thick or buried waveguides are introduced. The methods monitor the local intensity of surface plasmon polaritons by imaging the light scattered when the plasmons encounter discontinuities in the form of (i) the terminal end of the guide, (ii) randomly dispersed nanoparticles, and (iii) nanoholes drilled through the guide. Measurements by these three methods give consistent values to within ∼15% of 39 μm for the propagation length along 5-μm-wide Au-stripe waveguides deposited on an oxidized silicon wafer and excited at a wavelength of 860 nm. This range is due to varying losses associated with the introduction of the nanoholes and nanoparticles. These losses are quantified and could be reduced with realistic experimental improvements. Finite-element computations find that propagation in these optically thick (107 nm) guides is intrinsically limited not only by Ohmic losses, but also by radiation emitted into the substrate from the stripe edges. The radiative loss depends on the slope of the edge sidewall and on the wafer oxide thickness, both of which must be considered when reconciling experiment with theory.