Color Nanofilter Advances Displays

In a step toward more efficient, smaller and higher-definition display screens, a University of Michigan professor has developed a color filter made of nanothin sheets of metal with precisely spaced gratings.

An optical microscopy image shows seven color filters illuminated by white microscope light. (Images: Jay Guo)

The gratings, sliced into metal-dielectric-metal stacks, act as resonators. They trap and transmit light of a particular color, or wavelength, according to Jay Guo, an associate professor in the department of electrical engineering and computer science. A dielectric is a material that does not conduct electricity.

“Simply by changing the space between the slits, we can generate different colors,” Guo said. “Through nanostructuring, we can render white light any color.”

A paper on the research, Plasmonic nano-resonators for high resolution color filtering and spectral imaging, was published Aug. 24 in Nature Communications.

Guo’s team used the technique to make what they believe is the smallest color U-M logo. At about 12 X 9 µm, it is about 1/6 the width of a human hair.

Conventional LCDs are inefficient and manufacturing-intensive to produce. Only about 5 percent of their backlight travels through them and reaches our eyes, Guo said. They contain two layers of polarizers, a color filter sheet, and two layers of electrode-laced glass in addition to the liquid crystal layer. Chemical colorants for red, green and blue pixel components must be patterned in different regions on the screen in separate steps.

This optical microscopy image of a U-M logo that was produced with the new color filter process. The image is a bit blurry because it has been magnified about 5000 times.

Guo’s color filter acts simultaneously as a polarizer, eliminating the need for additional polarizer layers. In Guo’s displays, reflected light could be recycled to save much of the light that would otherwise be wasted.

Because these new displays contain fewer layers, they would be simpler to manufacture, Guo said. The new color filters contain just three layers: two metal sheets sandwiching a dielectric. Red, green and blue pixel components could be made in one step by cutting arrays of slits in the stack. This structure also is more robust and can endure higher-power light.

Red light emanates from slits set ~360 nm apart; green from those ~270 nm apart and blue from those ~225 nm apart. The differently spaced gratings essentially catch different wavelengths of light and resonantly transmit through the stacks.

“Amazingly, we found that even a few slits can already produce well-defined color, which shows its potential for extremely high resolution display and spectral imaging,” Guo said.

The pixels in Guo’s displays are about an order of magnitude smaller than those on a typical computer screen. They are about eight times smaller than the pixels on the iPhone 4, which are about 78 ?m. He envisions that this pixel size could make the technology useful in projection displays as well as in wearable, bendable or extremely compact displays.

The research is supported in part by DARPA and the US Air Force Office of Scientific Research. The university is pursuing patent protection for the intellectual property and is seeking commercialization partners to help bring the technology to market.

For more information, visit: www.engin.umich.edu