Patterned Metal Films Focus Surface Plasmon Polaritons
Daniel S. Burgess
A team of investigators at the University of California, Berkeley, has demonstrated that circles and ellipses milled in silver films can focus surface plasmon polaritons. The development may enable the creation of ultrasmall integrated optoelectronics and have applications in ultrahigh-density data storage and biomolecular sensing.
Xiang Zhang, an associate professor in the department of mechanical engineering at the university, said that the exotic behavior of surface plasmon polaritons — electron oscillations coupled to optical excitations that exist at the interface between metals and dielectrics — has been of interest to researchers for years. Surface plasmon polaritons can carry the energy of an optical photon in packets 10 to 100 times smaller than the photon’s free-space wavelength over tens of microns in a noble metal, suggesting the feasibility of using them in tiny counterparts to optical systems.
With the development of techniques that enable the fabrication of nanoscale structures, the field of plasmonics, in which surface plasmon polaritons are actively controlled to some end, is emerging. “People now have the ability to create shapes to manipulate surface plasmon polaritons, to guide light or diffract light beyond the free-space diffraction limit,” Zhang said, citing surface plasmon polariton mirrors produced at Karl Franzens Universität Graz in Austria (see “Nanoscale Optics Unhindered by Diffraction Limit,” Photonics Spectra, November 2002, page 100).
Zhang and his colleagues are contributing to the fledgling field with their creation of plasmonic lenses. “Our motivation is to develop a new kind of tool for the control of surface plasmons,” said Zhaowei Liu, a graduate student on the Berkeley team.
In their demonstration, the scientists used a focused ion beam system to cut roughly 280-nm-wide slits into 150-nm-thick silver and 70-nm-thick aluminum films deposited on quartz, forming circular and elliptical shapes microns across. To study the effect of the milled structures, they used near-field scanning microscopy and a lithographic exposure technique to record the generated electromagnetic near-field distributions parallel and perpendicular to the surface of the metals.
A Spectra-Physics argon-ion laser operating at 514 nm served as the excitation source for the near-field scanning microscopy measurements, and a 365-nm mercury lamp was the source for the lithography experiments.
They confirmed that the films established interference patterns that guided the surface plasmon polaritons toward the focal points of the lenses. For the circular structures, the measured intensity at the center increased linearly with diameter.
The work is qualitative at this point, but the researchers are working to establish the efficiency of the focusing and to improve it. They also are investigating potential uses of plasmonic lenses.
“This is only the first step,” Liu said. “We have demonstrated the concept. Now we are beginning to look at applications.”
Nano Letters, online July 29, 2005, doi:10.1021/nl051013j.
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