February 24, 2012 — Harvard researchers developed a new kind of tunable color filter that uses optical nanoantennae to control color output. A single active filter under exposure to different types of light can produce a range of colors.

Figure 1. Kenneth Crozier and his colleagues created a plate of chromatic plasmonic polarizers that spells out the acronym LSP. Under light of different polarizations, the letters and the background change color. The image at far right shows the antennas themselves, as viewed through a scanning electron microscope (SEM). Images courtesy of Tal Ellenbogen

Kenneth Crozier, associate professor of electrical engineering at the Harvard School of Engineering and Applied Sciences (SEAS), and colleagues engineered the size and shape of metal nanoparticles so that the color they appear depends on the polarization of the light illuminating them. Harvard has named the nano structures chromatic plasmonic polarizers.

Figure 2. The color output of the new type of optical filter depends on the polarization of the incoming light.

By controlling the optical nanoantennas shape, the engineers created a controllable color filter, tuned to react differently with light of different colors and different polarizations, said co-author Tal Ellenbogen, a postdoctoral fellow at SEAS. Conventional RGB filters have one fixed output color and create a broader palette of hues through blending. Each pixel of the nanoantenna-based filters is dynamic and able to produce different colors when the polarization is changed. This can create a pixel with a uniform color or complex patterns with colors varying as a function of position.

“The chromatic plasmonic polarizers combine two structures, each with a different spectral response, and the human eye can see the mixing of these two spectral responses as color,” said Crozier. “We would normally ask what is the response in terms of the spectrum, rather than what is the response in terms of the eye,” added Ellenbogen.

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To demonstrate the technology’s capabilities, the researchers used nanoparticles to spell out LSP, short for localized surface plasmon (Figure 1). With unpolarized light or with light that is polarized at 45 degrees, the letters are invisible (gray on gray). In polarized light at 90 degrees, the letters appear vibrant yellow with a blue background, and at 0 degrees the color scheme is reversed. Rotating the polarization of the incident light makes the letters’ colors change, moving from yellow to blue.

Seeing the color effects from current fabricated samples requires magnification, but large-scale nanoprinting techniques could be used to generate samples big enough to be seen with the naked eye. Building a television display with the nanoantennas is "absolutely feasible," say Crozier and Ellenbogen.

The researchers have filed a provisional patent for their work.

Potential applications include television displays and biological imaging, or invisible security tags to mark currency. The findings appear in the February issue of Nano Letters.

Kwanyong Seo, a postdoctoral fellow in electrical engineering at SEAS, also contributed to the research. The work was supported by the Center for Excitonics at MIT, an Energy Frontier Research Center funded by the Office of Basic Energy Sciences in the U.S. Department of Energy’s Office of Science; and Zena Technologies. In addition, the research team acknowledges the Center for Nanoscale Systems at Harvard for fabrication work.

Learn more at www.harvard.edu.

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