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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.

Propagating surface plasmon polaritons (SPPs) enable ultrafast energy transport at metal surfaces on the nanoscale, 1–3 facilitating, for example, plasmonic nanocircuits, 4–7 plasmonic vortices, 8, 9 and nanofocusing. 10–15 Conical tapers represent the quintessential structure for adiabatic nanofocusing, 10, 11, 16–19 harnessing the efficient …