1st Nano-S&T - Congress of Nano-Science & Technology, Dalian, China, 10/23 - 10/26/2011

Acoustic Surface Plasmons - a Novel Low-Energy Excitation for Optical Transport at Metallic Interfaces

Karsten Pohl

Department of Physics and Materials Science Program, University of New Hampshire, Durham, NH, U.S.A.


The recent discovery of a fundamentally new sound-like plasmon on a bare metal surface of beryllium may introduce a new research direction in the area of plasmonics [1]. While conventional surface plasmons are optical modes and have a finite excitation energy of a few eV, the novel acoustic mode can be excited with very low energies of a few meV. This allows, in principle, for a coupling with visible light for signal processing and advanced microscopies as well as new catalysts on metallic surfaces. In order to show that this novel excitation is a general phenomenon on closed-packed noble metal surfaces, as predicted by our theoretical collaborators, we have measured the dispersion of the acoustic surface plasmon on Cu(111) by electron energy-loss spectroscopy for a parallel momentum-transfer range from 0 to 0.20 1/Å [2]. We can report that the dispersion is indeed linear (acoustic) with a slope (sound velocity) in good agreement with theory and energy values that extend up to 1 eV. We will show that the novel acoustic surface plasmon, ASP, is a general phenomenon on metal surfaces that support a partially occupied surface state within a wide bulk energy gap. It is caused by the non-local screening of the surface electrons due to bulk electrons. The adsorption of about 0.25 ML of oxygen on the surface removes the surface state on Cu(111) and indeed destroys the ASP.

[1] B. Diaconescu, K. Pohl, L. Vattuone, L. Savio, Ph. Hofmann, V.M. Silkin, J.M. Pitarke, E.V. Chulkov, P.M. Echenique, D. Farías, M. Rocca - Nature 448, 57 (2007).

[2] K. Pohl, B. Diaconescu, G. Vercelli, L. Vattuone, V.M. Silkin, E.V. Chulkov, P.M. Echenique, M. Rocca - EPL 90, 57006 (2010).


Dr. Pohl completed his doctorate in surface physics at the University of Pennsylvania in 1997 with Prof. E. Ward Plummer, by studying the electronic and geometric surface structure of hydrogen adsorption on beryllium surfaces with a combination of photoelectron spectroscopies and low-energy electron diffraction. After working with Drs. Robert Hwang and Norman Bartelt at Sandia National Laboratories in California on strain effects in ultrathin film self-assembly on metal surfaces as a post doctoral fellow, developing a novel scanning tunneling microscope in the process, he joined the Faculty of the University of New Hampshire in 2000, where he is an Associate Professor in the Department of Physics and the Materials Science Program. His group explores molecular and stress relaxation driven self-assembly processes yielding novel nano-materials and the electronic properties of reduced dimensional systems, involving experimental tools such as scanning probe microscopy, photoelectron spectroscopy, and low-energy electron microscopy or LEEM, as well as extensive numerical modeling. Dr. Pohl is a founding member of the NSF-Center for High-rate Nanomanufacturing at UNH and has received an Early Career Award from the National Science Foundation in 2002.