Title: Direct imaging of the near field and dynamics of surface plasmon resonance on gold nanostructures using photoemission electron microscopy
Abstract: Localized surface plasmon resonance (LSPR) can be supported by metallic nanoparticles and engineered nanostructures. An understanding of the spatially resolved near-field properties and dynamics of LSPR is important, but remains experimentally challenging. We report experimental studies toward this aim using photoemission electron microscopy (PEEM) with high spatial resolution of sub-10 nm. Various engineered gold nanostructure arrays (such as rods, nanodisk-like particles and dimers) are investigated via PEEM using near-infrared (NIR) femtosecond laser pulses as the excitation source. When the LSPR wavelengths overlap the spectrum of the femtosecond pulses, the LSPR is efficiently excited and promotes multiphoton photoemission, which is correlated with the local intensity of the metallic nanoparticles in the near field. Thus, the local field distribution of the LSPR on different Au nanostructures can be directly explored and discussed using the PEEM images. In addition, the dynamics of the LSPR is studied by combining interferometric time-resolved pump-probe technique and PEEM. Detailed information on the oscillation and dephasing of the LSPR field can be obtained. The results identify PEEM as a powerful tool for accessing the near-field mapping and dynamic properties of plasmonic nanostructures. A novel form of microscopy has been used to map the field dynamics of surface plasmon resonances. Hiroaki Misawa and co-workers in Japan used high-resolution (10 nm) multiphoton photoemission electron microscopy (MP-PEEM) to image the field distribution of a variety of metallic nanostructures, such as arrays of gold rods, nanodisks and dimer pairs. MP-PEEM works by recording the electrons emitted from a sample in response to illumination with a series of ultrashort pulses. Illumination with near-infrared pulses caused excited plasmon resonances in the gold nanostructures to show up in the resulting MP-PEEM images as local hot spots, from which the field distribution was determined. The short duration of the pulses made it possible to probe the dynamics of the plasmon resonance field distribution on the femtosecond timescale.