Multiphoton Photoemission Microscopy of High-Order Plasmonic Resonances at the Ag/Vacuum and Ag/Si Interfaces of Epitaxial Silver Nanowires

Understanding the physics of surface plasmons and related phenomena requires knowledge of the spatial, temporal, and spectral distributions of the total electromagnetic field excited within nanostructures and their interfaces, which reflects the electromagnetic mode excitation, confinement, propagation, and damping. We present a microscopic and spectroscopic study of the plasmonic response in single-crystalline Ag wires grown in situ on Si(001) substrates. Excitation of the plasmonic modes with broadly tunable (UV-IR) femtosecond laser pulses excites ultrafast multiphoton photoemission, whose spatial distribution is imaged with an aberration-corrected photoemission electron microscope, thereby providing a time-integrated map of the locally enhanced electromagnetic fields. We show by tuning the wavelength, polarization, and k-vector of the incident laser light that for a few micrometers long wires we can selectively excite either the propagating surface plasmon polariton modes or high-order multipolar resonances of the Ag/vacuum and Ag/Si interfaces. Moreover, upon tuning the excitation wavelength from the UV to the near-IR spectral regions, we find that the resonant plasmonic modes shift from the top of the wires to selvedge at the Ag/Si interface. Our results, supported by numerical simulations, provide a better understanding of the optical response of metal/semiconductor structures and guidance toward the design of polaritonic and nanophotonic devices with enhanced properties, as well as suggest mechanisms for plasmonically enhanced photocatalysis.