Acoustic surface plasmon on Cu(111) (Vol. 41, No. 5)

An acoustic surface plasmon (ASP) is a novel collective electronic excitation at metal surfaces. This new mode has a linear (or acoustic-like) dispersion, i.e. it can be excited at very low energy and wavelength, allowing it to participate in many dynamical processes, such as chemical reactions and nano-sensors at surfaces and sub-wavelength optics and photonic devices as well as new microscopy techniques.

After the original discovery of an ASP on the close-packed surface of beryllium it now has also been excited and detected on Cu(111). Thus, the ASP is indeed a general phenomenon on metal surfaces that support a partially occupied surface state within a wide bulk energy gap. Non-local screening of the surface electrons due to bulk electrons creates the ASP.

Of particular interest is the interaction of the ASP with light: nm-size objects at surfaces, such as atomic steps or molecular structures, can provide coupling between light and ASPs of much lower wavelength than conventional SPs. In this way, the new mode can serve as a tool to confine light in a broad frequency range up to optical frequencies on surface areas of a few nanometers, thus facilitating control of events at metal surfaces with both high spatial (nm) and high temporal (fs) resolution. Another consequence of the acoustic-like character of the ASP dispersion is that both phase and group velocities are the same, so signals can be transmitted undistorted along the surface. The theoretically estimated ASP decay lengths of 100 ∼ 1,000 nm for medium (100 meV) to far (10 meV) infrared are an appealing prospect for the field of nano-optics.