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  • Tunable optical filter uses nanoantennas

Photonics Spectra
Apr 2012
Ashley N. Paddock, ashley.paddock@photonics.com

CAMBRIDGE, Mass. – A new tunable color filter based on optical nanoantennas can precisely control color output, enabling its use for display and bioimaging applications and for marking currency.

By precisely controlling the shape of the nanoantennas, engineers at Harvard School of Engineering and Applied Sciences (SEAS) 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 SEAS. The investigators dubbed the filter a “chromatic plasmonic polarizer.”


To demonstrate their work, Kenneth B. Crozier and his colleagues created a plate of chromatic plasmonic polarizers that spells “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.


Conventional RGB filters that are used to create color in televisions and monitors have one fixed output color and create a broader palette of hues through blending, an unnecessary step for these filters.

“Instead of changing the polarization of light to control the intensity of each of the spatially separated red, green and blue parts of the pixel, we will change the polarization to get the desired output color,” Ellenbogen said.

To achieve this, a separate mechanism to control the brightness, such as a white LED, is needed for each pixel, he explained.

“Using chromatic plasmonic polarizers, we can mix two colors into one nanoantenna placed at a single location,” Ellenbogen said. “Therefore it can potentially reduce the size of the pixel and eventually lead to better display resolution.”


The color output of a new type of optical filter depends on the polarization of the incoming light. Courtesy of Tal Ellenbogen, Harvard School of Engineering and Applied Sciences.


Because the color output of chromatic plasmonic polarizers is sensitive to the polarization of input light, he said it holds potential for bright-field polarization imaging applications, for medical, biological and general purposes.

“Some tissues, like muscle tissues, for example, can change the polarization state of the light that is transmitted through them,” he said. “This shows as a color change at the output of the chromatic plasmonic polarizer. Defects in tissue, such as cancerous tumors, for example, modify the polarization of transmitted light in different ways and, therefore, can potentially be detected by this method.”

To demonstrate the technology’s capabilities, the engineers used nanoparticles to make the letters “LSP,” short for localized surface plasmon. With unpolarized light or with light that is polarized at 45°, the letters are invisible. In polarized light at 90°, 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.


Chromatic plasmonic polarizers show polarization rotation by a plastic film.


“So far, we have demonstrated the concept of chromatic plasmonic polarizers based on 2-D nanoantenna structures,” Ellenbogen said. “We have ideas for using the third dimension to extend the functionality of the devices and to create chromatic plasmonic polarizers based on metal apertures or more complex nanostructures.”

The researchers have filed a provisional patent for their work, which appeared in the February issue of Nano Letters (dx.doi.org/10.1021/nl204257g).


GLOSSARY
polarization
With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarization direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forth). The polarization vector is always in the plane at right angles to the beam direction. Near some given stationary point in space the polarization direction in the beam...
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