  We apply terahertz (THz) near-field streaking in a nanofocusing geometry to
investigate plasmon polariton propagation on the shaft of a conical nanotip. By
evaluating the delay between a streaking spectrogram for plasmon-induced
photoemission with a measurement for direct apex excitation, we obtain an
average plasmon group velocity, which is in agreement with numerical
simulations. Combining plasmon-induced photoemission with THz near-field
streaking facilitates extensive control over localized photoelectron sources
for time-resolved imaging and diffraction.
