Microscopy Probes Dielectric Constant of Nanostructured Films

Daniel S. Burgess

Using near-field scanning optical microscopy, researchers from Argonne National Laboratory in Illinois have imaged the interference patterns of surface plasmons on silver nanostructured thin films, enabling them to measure the local plasmon dispersion and decay length and thereby determine the frequency-dependent dielectric constant of the films. The disparity in the values of the dielectric constant as found in the study and as presented in optical handbooks underscores the importance of in situ experimental testing in the development of plasmonic devices for applications in sensing and information processing.

Using near-field scanning optical microscopy at various excitation wavelengths, researchers have imaged the interference patterns of surface plasmons on 75-nm-thick nanostructured silver films, enabling them to calculate the dielectric constant. Courtesy of Vitalii Vlasko-Vlasov and Ulrich Welp. Reprinted with permission. ©2006, American Institute of Physics.

Surface plasmons are collective oscillations of free electrons induced by an electromagnetic excitation that propagate along a metal/dielectric interface. Vitalii Vlasko-Vlasov, a principal investigator at the laboratory who is working closely on the project with fellow principal investigator Ulrich Welp, explained that they are of interest in the integration of photonics and electronics because they straddle the spatial scales of the two fields, confining excitations from micron-size photonic components in a form that can be guided along metal structures on the order of 100 nm in size and similar to those used in electronics.

Quantifying the local dispersion and decay length of surface plasmons on such structures is central to the design of a plasmonic circuit, he said. The problem, however, has been that these characteristics seem to be dependent on variables from the fabrication process, including the resulting porosity, grain size and surface roughness. Optical techniques such as attenuated total reflection can be used to determine dispersion from angular-dependent reflection measurements, but only on macroscopic samples.

The researchers thus turned to near-field scanning optical microscopy, employing an Aurora-2 system from Veeco Instruments Inc. of Woodbury, N.Y., to which they fitted a tunable laser. Their samples were 75-nm-thick silver films sputtered onto glass slides, into which they machined 140-nm-wide slits in 7.55-μm-diameter circular patterns. To produce interfering plasmons, they excited the films at wavelengths between 476 and 676 nm with a 643-AP-A01 krypton/argon laser from Melles Griot Inc. of Carlsbad, Calif.

Experimental emphasis

The investigators derived the surface plasmon wavelength from the observed interference periods and retrieved the dielectric constant using the calculated plasmon dispersion and the Drude formula. Although Vlasko-Vlasov cited previous findings that suggest that the dielectric constant of metal thin films is sample-dependent, he noted that the magnitude of the difference between the predicted and experimental values was surprising and that the physical mechanisms behind the disparity remain to be fully understood. Accordingly, the emerging field of plasmonics continues to require an experimental emphasis.

“This result underlines the need to obtain a careful characterization of the plasmonic properties of the individual sample and to establish sample fabrication procedures that will yield consistent material properties,” he said.

Near-field optical microscopy is particularly suited to such experimental work, Vlasko-Vlasov said, and offers a means of studying evanescent and confined electromagnetic waves in general. He opined that plasmonics would benefit from the development of turnkey microscope systems for this sort of materials analysis.

“Up to now, such a microscope has been a scientific tool rather than a standard test instrument,” he said.

Applied Physics Letters, April 24, 2006, 173112.