Tunable Optical Filter Uses Nanoantennas

A new kind of tunable color filter that uses optical nanoantennas to obtain precise control of color output could be used to create invisible security tags to mark currency, and for display and biological imaging applications.

By precisely controlling the shape of the optical nanoantennas, engineers at Harvard have created a controllable color filter that is tuned to react differently depending on the color and polarization, said Tal Ellenbogen, a postdoctoral fellow at Harvard School of Engineering and Applied Sciences (SEAS). Conventional RGB filters used to create color in current TVs and monitors have one fixed output color and create a broader palette of hues through blending.

The color output of a new type of optical filter depends on the polarization of the incoming light. (Images: Tal Ellenbogen)

By contrast, 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 uniform color or complex patterns with colors varying as a function of position. The researchers dubbed these filters “chromatic plasmonic polarizers.”

“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 Kenneth Crozier, associate professor of electrical engineering at Harvard SEAS.

“We would normally ask what is the response in terms of the spectrum, rather than what is the response in terms of the eye,” Ellenbogen added.

To demonstrate their work, Crozier and his colleagues created a plate of chromatic plasmonic polarizers that spells out the acronym “LSP.” Under the 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.

To demonstrate the technology’s capabilities, the engineers used nanoparticles to spell out the acronym LSP, short for localized surface plasmon. With unpolarized light or with light that is polarized at 45 degrees, the letters are invisible. 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 colors of the letters change, shifting 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 TV display with the nanoantennas would take a great deal of advanced engineering, but it is “absolutely feasible,” Crozier and Ellenbogen said.

The researchers have filed a provisional patent for their work, which appeared in the February issue of Nano Letters.

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For more information, visit: www.seas.harvard.edu