Electron emission by femtosecond laser pulses from individual Au nanorods is studied with a time-of-flight momentum resolving photoemission electron microscope (ToF k-PEEM). The Au nanorods adhere to a transparent indium-tin oxide substrate, allowing for illumination from the rear side at normal incidence. Localized plasmon polaritons are resonantly excited at 800 nm with 100 fs long pulses. The momentum distribution of emitted electrons reveals two distinct emission mechanisms: a coherent multiphoton photoemission process from the optically heated electron gas leads to an isotropic emission distribution. In contrast, an additional emission process resulting from the optical field enhancement at both ends of the nanorod leads to a strongly directional emission parallel to the nanorod's long axis. The relative intensity of both contributions can be controlled by the peak intensity of the incident light.

We studied the excitation of large-area Au bow-tie nanoantenna arrays, which we have fabricated on indium-tin-oxide (ITO)-coated glass substrates using colloidal lithography with nanoscale polystyrene colloidal particles. Ultrashort (100 fs, 800 nm) laser pulses of a Ti-Sapphire laser resonantly excite electron emission from a few tens of nanometer wide-gap regions of the array. We investigated the near-field enhanced photoemission using time-of-flight momentum microscopy. The variation of the electron emission intensity as a function of kinetic energy, parallel momentum, power density, and polarization of the laser beam reveals two distinct emission mechanisms: a coherent multiphoton photoemission process from the optically heated electron gas and a field emission process resulting from the optical near-field enhancement at the nanoantenna tips. The analysis of the momentum-resolved kinetic energy spectra indicates a spatially inhomogeneous distribution of the electron gas temperature within the bow-tie resonators.

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