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  • New color filter could spell “grate” success for future displays

Photonics Spectra
Nov 2010
Marie Freebody,

ANN ARBOR, Mich. – Scientists are seeking to commercialize a new type of color filter that they claim could be a step toward more efficient, smaller and higher definition display screens. The filter can be integrated into current LCD technologies to increase brightness and power efficiency and, what’s more, the high resolution offered makes it ideal for use in compact wearable displays.

If you could peer inside any LCD screen, you would find that each pixel is made up of several layers, each with a specific job to do. Among the layers is a pair of polarizing filters that operate in conjunction with the liquid crystal to either allow light through to create a picture, or to block it completely to produce a black spot on the screen. And red, blue and green filters are needed to render white light into the huge variety of colors that make up a picture.

But what if you could use just a single color filter that produces different colors based on a physical effect rather than the traditional colorant-based light filters? That’s precisely what Jay Guo, an associate professor in the University of Michigan’s department of electrical engineering and computer science, has done in his new design.

Shown is an optical microscopy image of seven color filters illuminated by white microscope light. Images courtesy of Jay Guo.

The filter is composed of two metal gratings sandwiching a dielectric. The device selectively scatters light to produce various colors. By simply changing the space between the slits, different colors can be generated.

“We demonstrated color filtering elements capable of high resolution and with efficiency greater than current colorant-based filters,” Guo said. “Arbitrary colors can be produced simply by changing the period of the nanograting so there is no need to build different color filters in separate steps.”

Given the grating-based design of the filter, it can also function as a polarizer simultaneously, eliminating the need of a separate polarizer sheet in the LCD. The design formulated by the researchers is outlined in a Nature Communications paper published on Aug. 24, 2010, and was applied to construct what they believe is the smallest color University of Michigan logo.

This optical microscopy image shows a 12 x 9-μm University of Michigan logo produced with a new color filter process.

“The device uses a metal grating to selectively scatter the incoming wave to couple to the arrays of plasmonic nanoresonators,” Guo said. “A second metal grating is then used to scatter the trapped light to the other side of the structure, thereby achieving transmission color filtering. The light not resonantly coupled to the resonator is simply reflected. Incoming light polarized along the grating direction is also reflected.”

Because this reflected light can also be recycled, the device could save much of the light that otherwise would be wasted.

Guo and colleagues used one level of lithography patterning to form all of the different colors and used no chemical solvent in the structure fabrication. They found that red light emanates from slits set around 360 nm apart, green from those about 270 nm apart and blue from those set approximately 225 nm apart.

To further improve the commercial appeal of their filter, the scientists hope to show that the color filter elements can be fabricated in large areas by using nano-imprint lithography. This, they say, would help to reduce production costs. In addition, they are currently working on making reflective color filters using a similar principle.

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