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June 2012

Enhancing Electron Photoemission with Nanopillar Array

Figures: An array of nano-sized gold pillars, (a), creates a plasmonic surface resonance. (B), photoelectron kinetic energy spectrum for electrons ejected from the nanopillar array, showing significant increases compared to a flat gold surface.

Working with the Molecular Foundry's Bruce Harteneck, researchers in the Lab's Ultrafast Materials and Chemical Sciences programs have verified and measured a boost in photoemitted-electron energies when assisted by the plasmonic field of a gold nanopillar array. Such enhancement provides a way to investigate plasmon dynamics, important for possible coupling to active electronics.

Groups of electrons on a metal surface can oscillate coherently, a phenomenon known as a surface plasmon resonance. When excited by ultrafast laser pulses under particular conditions, these collective excitations can create a plasmonic field that boosts the energy of electrons photoemitted from a surface.

Nanostructured surfaces can simplify the process, allowing researchers to excite the plasmon resonances directly. Here the researchers investigated this effect with an array of gold nanopillars developed in the Molecular Foundry's Nanofabrication Facility. They found that their nanopillar architecture allowed for nonlinear ionization and generated surface fields about thirty-times higher than the excitation laser field alone, in turn propelling photoemitted electrons to much greater energies.

The strong field enhancement provided by the nanopillars may help scientists study plasmon dynamics in the future with minimal interference from the laser field.

"Surface Plasmon Assisted Electron Acceleration in Photoemission from Gold Nanopillars" Phillip M. Nagel, Joseph S. Robinsonc, Bruce D. Harteneckd, Thomas Pfeifera, Mark J. Abel, James S. Prella, Daniel M. Neumark, Robert A. Kaindl, Stephen R. Leonea
Chemical Physics, Available online 2 May 2012, ISSN 0301-0104, 10.1016/j.chemphys.2012.03.013.
Time-of-flight studies were supported by the NSF Division of Chemistry under Award #CHE-0742662 [PMN, SRL] and by the Materials Sciences and Engineering Division of the U.S. Department of Energy's Office of Basic Energy Sciences (DOE-BES) under Contract No. DE-AC02–05CH11231 [JSR, RAK]. Simulations were supported by the DOE-BES Chemical Sciences, Geosciences, and Biosciences Division, while nanopillar synthesis was performed under a user proposal at the Molecular Foundry, both under the above DOE contract. Few-cycle instrumentation development was supported by the AFOSR-MURI program under Award #FA95500410242.