Plasmonic structures can slow light waves over a broad range of wavelengths, verifying the “rainbow” trapping effect, which had been predicted only recently in the theoretical studies of metamaterials. An experiment conducted by chemists at Lehigh University used focused ion beams to mill a series of increasingly deeper, nanosize grooves into a thin sheet of silver. By focusing light along this plasmonic structure, the series of grooves, or nanogratings, slowed each wavelength of optical light, capturing each color of the visible spectrum at various points along the grating. The findings hold promise for improved data storage, optical data processing, solar cells, biosensors and other technologies. Microscope images of graded gratings with different gradients. In this measurement, three filters are used: (a) multiband (Semrock, transmission bands centered at 542 and 639 nm); (b) green, centered at 546 nm (bandwidth 6 nm); and (c) red, centered at 655 nm (bandwidth 12 nm). (Image: Filbert J. Bartoli, Lehigh University) The research required the ability to engineer a metallic surface to produce nanoscale periodic gratings with varying groove depths. This alters the optical properties of the nanopatterned metallic surface, called surface dispersion engineering. The broadband surface light waves are then trapped along this plasmonic metallic surface, with each wavelength trapped at a different groove depth, resulting in a trapped rainbow of light. Through direct optical measurements, the team showed that light of different wavelengths in the 500- 700-nm region was "trapped" at different positions along the grating, consistent with computer simulations. To prepare the nanopattern gratings required "milling" 150-nm-wide rectangular grooves every 520 nm along the surface of a 300-nm-thick silver sheet. Although intrinsic metal loss on the surface of the metal did not permit the complete "stopping" of these plasmons, future research may look into compensating this loss in an effort to stop light altogether. "Metamaterials, which are man-made materials with feature sizes smaller than the wavelength of light, offer novel applications in nanophotonics, photovoltaic devices and biosensors on a chip," said Filbert J. Bartoli, principal investigator, professor and chairman of the Department of Electrical and Computer Engineering. "Creating such nanoscale patterns on a metal film allows us to control and manipulate light propagation. These findings present an unambiguous experimental demonstration of rainbow trapping in plasmonic nanostructures and represents an important step in this direction." "This technology for slowing light at room temperature can be integrated with other materials and components, which could lead to novel platforms for optical circuits. The ability of surface plasmons to concentrate light within nanoscale dimensions makes them very promising for the development of biosensors on chip and the study of nonlinear optical interactions," said Qiaoqiang Gan, who completed this work while a doctoral candidate at the university. He is currently an assistant professor in the Department of Electrical Engineering at State University of New York at Buffalo. The study was conducted by Bartoli, Gan, Yongkang Gao and Yujie J. Ding of the Center for Optical Technologies in the Department of Electrical and Computer Engineering, and by Kyle Wagner and Dmitri V. Vezenov of the Department of Chemistry, all at Lehigh. The study was funded by the National Science Foundation. It is published in the current issue of the Proceedings of the National Academy of Sciences. For more information, visit: www.lehigh.edu
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