Hybrid Microscopy Reveals Surface Plasmon Dynamics

Daniel S. Burgess

By combining the techniques of interferometric time-resolved two-photon emission and photoelectron emission microscopy, scientists at the University of Pittsburgh have generated movies of the dynamic behavior of surface plasmons on a silver grating. With a temporal precision and a spatial resolution of 330 as/frame and 50 nm, respectively, the approach offers investigators the ability to further probe the way plasmons propagate, decohere and interfere with other plasmons and their role in energy dissipation and concentration.

Researchers at the University of Pittsburgh have combined interferometric time-resolved two-photon emission and photoelectron emission microscopy to study the dynamics of surface plasmons. Movies are available as .avi files in the supporting information accompanying the journal article at http://pubs.acs.org. Courtesy of Atsushi Kubo.

Surface plasmons are fluctuations in the charge density of a metal that can be excited by optical fields and are of particular interest in applications such as near-field optical microscopy and subwavelength photolithography. To date, explained Atsushi Kubo, a postdoctoral researcher at the university’s department of physics and astronomy, scanning near-field optical microscopy has been the method of choice for studying plasmons, but it has limitations in terms of data acquisition and temporal resolution that make it impractical for the production of high-resolution movies.

The Pittsburgh researchers, led by Hrvoje Petek, thus developed the hybrid approach. In their setup, a homebuilt Ti:sapphire laser operating at 90 MHz generates surface plasmons using 10-fs pump pulses of 400-nm light with an average power of 100 mW. Exposure to probe pulses that are delayed by a Mach-Zehnder interferometer induces two-photon photoemission, enabling them to use an electron microscope to image the two-dimensional distribution of the photoelectron current mediated by the plasmon excitations.

In a demonstration of the setup, the scientists produced a movie of localized plasmon dynamics on a silver grating with 100-nm-wide slits. They also demonstrated that they could selectively excite plasmons in particular localized modes by specifying the delay time. Such control, Kubo suggests, might have applications in the control of plasmon fields on the nanometer scale in plasmonic integrated ultrafast switches or optical signal modulators.

The team plans to use the imaging technique to observe propagating plasmons. Further upgrades to the setup might include the incorporation of time-of-flight photoelectron energy analysis as an additional dimension for the imaging of electron dynamics in solid nanostructures, Kubo said.

Nano Letters, online May 17, 2005, doi:10.1021/nl0506655.