Localized surface plasmons can induce optical field enhancement on roughened metal surfaces or noble metal nanoparticles. The resulting intense localized fields have a multitude of applications including surface enhanced Raman spectroscopy (SERS), chemical sensors, photovoltaics, medicine and photonic circuits. Specially designed metal nanostructures also exhibit a localized plasmonic response that can used for these applications or as a source for intense ultrafast electron pulses. We directly interrogate optical field enhancement on nanoparticles and nanostructures using femtosecond laser pulses and photoemission electron microscopy (PEEM). The enhancement in photoelectron yield from nanoparticles, nanogratings, and nanohole arrays, illuminated with 400 and 800 nm femtosecond pulses are found to be factors of 102 to 103 times greater than that produced by flat metal substrates. For particles and nanostructures, multi-photon PEEM images reveal that the greatest enhancement in photoelectron yield is localized at distinct regions whose magnitude and spatial extent is dependent on laser polarization. In conjunction with correlated scanning electron microscopy (SEM), nanoparticles are found to exhibit irregular spatial distributions in the multi-photon PEEM images that are correlated with the unique nanostructure or particle topology. When metals are coated with thin films of insulating metal oxides, a hybrid material, with unique new properties, is created. Hybrid materials often display dramatic reductions in work function, increases in quantum yield, and changes in electron angular distribution, when compared to bare metal structures. The mechanism enhanced electron yield and potential ultrafast electron source applications will be discussed.

Photoemission electron microscopy of plasmonic nanostructures

246th National Meeting (2013)

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