Fig. 5. Sequence of emission profiles through the nanofocus (see inset) for increasing delay times up to 50 fs.

The plasmon waves counterpropagate from the edges into the central position 0 and superpose constructively and destructively with each other and the electric field of the pump laser pulse. At the focal spot, within an optical cycle, one emission maximum and one minimum occur. From the slope of the dashed yellow line, the phase velocity of SR-SPPs can be calculated, which is 0.22c.