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Using a focused ion beam generated in the FEI 235 dual-beam microscope in Lehigh’s Nanocharacterization Laboratory, Gan fabricated a series of nanogratings with grooves 150nm wide and 475nm apart.
The milling time for each successive groove was increased gradually to achieve a grating with a linearly graded groove depth. A nanoslit was fabricated 13 microns from the edge of the graded nanograting to allow a beam of white light to launch surface plasmon modes on the top surface containing the nanogrooves.
Under normal circumstances, observing a beam of light that is tightly confined at the metal surface would be impossible with an optical microscope.
“Fortunately for us, imperfections in the surface of the nanograting cause some of the light to be scattered into the far-field,” says Bartoli, “and it is this scattered light that we hoped to observe with our optical microscope.”
To simplify matters, Bartoli and Gan tested the grating on incident light comprised of two colors, red and green. This was done by first passing the incident beam of white light through a filter with transmission bands centered at red and green wavelengths.
“To our delight, we observed a colorful emission of red and green light at different positions along the grating,” says Bartoli. “This is an unambiguous experimental demonstration of ‘rainbow trapping’ in plasmonic nanostructures.”
“Now that we’ve managed to trap these electromagnetic waves, the next step is to find a controllable way of releasing them.”
Other collaborators in this project are Yujie Ding, professor of electrical and computer engineering, and Dmitri Vezenov, assistant professor of chemistry in the College of Arts and Sciences.
Improving the performance of LEDs
According to the U.S. Department of Energy, Americans spend more than $37 billion a year on residential and commercial lighting, accounting for almost 22 percent of total national electricity consumption. The government hopes to phase out incandescent lighting in favor of fluorescent, but the search is on for better alternatives.
The most promising contenders are gallium nitride-based light-emitting diodes (GaN-based LEDs). These small solid state devices contain several extremely thin layers (15-500nm) of gallium nitride, aluminum-gallium nitride and indium-gallium nitride deposited onto a sapphire substrate using metal-organic chemical vapor deposition. The wavelength of light they produce can be controlled by changing the ratio of In or Al to Ga in each layer, providing access to the whole spectrum of visible light.
Several challenges stand in the way of widespread adoption of LEDs. Improvements are needed to the internal quantum and light-extraction efficiencies of GaN-based LEDs, and manufacturing costs must come down.
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Most commercial GaN-based LEDs are grown at high temperature, typically 1,000 degrees C, on a highly polished sapphire substrate.
“Direct growth of GaN on sapphire at such temperatures often results in a poor quality film containing numerous defects known as threading dislocations that grow right through the GaN layer,” says Helen Chan, professor of materials science and engineering. “This is due to stresses generated in the film which arise from the large lattice mismatch (16 percent) between the crystal structures of GaN and sapphire.”
To overcome this, manufacturers deposit a buffer layer of GaN at lower temperature prior to growing the high temperature GaN layer. This process usually adds 30 to 45 minutes to the processing time and has a significant impact on the cost of fabrication.
Chan, Richard Vinci, professor of materials science and engineering, and Nelson Tansu, associate professor of electrical and computer engineering and a faculty member in the Center for Optical Technologies, have developed a cost-effective method of fabricating GaN-based LEDs on nanopatterned sapphire substrates.
