In an interferometric time-resolved photoemission electron microscopy (ITR-PEEM) experiment, the near-field associated with surface plasmon polaritons (SPP) can be locally sensed via interference with ultrashort laser pulses. Here, we present ITR-PEEM data of SPP propagation at a gold vacuum interface recorded in a counter-propagating pump-probe geometry. In comparison to former work this approach provides a very intuitive real-time access to the SPP wave packet. The quantitative analysis of the PEEM data enables us to determine in a rather direct manner the propagation characteristics of the SPP.

The excitation of coherent phonons provides unique capabilities to control fundamental properties of quantum materials on ultrafast time scales. Recently, it was predicted that a topologically protected Weyl semimetal phase in the transition metal dichalcogenide Td-WTe2 can be controlled and, ultimately, be destroyed upon the coherent excitation of an interlayer shear mode. By monitoring electronic structure changes with femtosecond resolution, we provide here direct experimental evidence that the shear mode acts on the electronic states near the phase-defining Weyl points. Furthermore, we observe a periodic reduction in the spin splitting of bands, a distinct electronic signature of the Weyl phase-stabilizing non-centrosymmetric Td ground state of WTe2. The comparison with higher-frequency coherent phonon modes finally proves the shear mode-selectivity of the observed changes in the electronic structure. Our real-time observations reveal direct experimental insights into electronic processes that are of vital importance for a coherent phonon-induced topological phase transition in Td-WTe2.

Plasmonic wave packet propagation is monitored in dielectric-loaded surface plasmon polariton waveguides realized from para-hexaphenylene nanofibers deposited onto a 60 nm thick gold film. Using interferometric time resolved two-photon photoemission electron microscopy we are able to determine phase and group velocity of the surface plasmon polariton (SPP) waveguiding mode (0.967c and 0.85c at λ(Laser) = 812nm) as well as the effective propagation length (39 μm) along the fiber-gold interface. We furthermore observe that the propagation properties of the SPP waveguiding mode are governed by the cross section of the waveguide.

No abstract

No abstract