Spatial distribution and near-field coupling of surface plasmon polariton Bloch modes

The Bloch nature of surface plasmon polaritons in a one-dimensional plasmonic crystal is directly observed using ultrafast and phase-sensitive near-field microscopy. Individual Bloch harmonics within one Bloch mode are separated and visualized. Different harmonics within the same Bloch mode turn out to have a different lateral amplitude distribution. The coupling between the different Bloch harmonics and a near-field probe is studied by comparing the measured energy distribution over the different harmonics to the theoretically expected distribution. It is found that higher order harmonics couple more efficiently to the probe than lower order harmonics.