Photonic crystal displays its true colors

Optical materials

September 14, 2007

Flat panel displays based on photonic crystal technology have taken a significant step closer to the market thanks to research carried out at the University of Toronto in Canada and the University of Bristol in the UK. The team have fabricated a proof-of-principle device that shows bright reflective colors in red, green, and blue [Arsenault et al., Nat. Photonics (2007) 1, 468].

Reflective displays should outperform backlit or emissive displays in a range of environments, especially bright sunlight. Several technologies could fit the bill, but photonic crystals are attractive because they can be tuned to different wavelengths. Unlike every other display on the market today, this technology would not require expensive color filters nor would suffer the loss of light from filters or spatially modulated color schemes.

“The most important result is that we now have a material that on its own can produce the whole spectrum of colors. The potential to simplify manufacturing is tremendous,” claims André C. Arsenault of the University of Toronto and Opalux Inc., a spin-out company founded to commercialize the technology. The material is dubbed ‘photonic ink’.

The active material in the display is a nanocomposite comprising an ordered array of silica microspheres in a cross-linked matrix of polyferrocenylsilane (PFS), a metallopolymer. When a solvent is absorbed into the polymer matrix, it swells and increases the spacing between the silica microspheres, thus changing the reflected color. This swelling is controlled by applying a voltage to the silica-PFS composite inside an electrochemical cell.

“This is a beautiful piece of work,” agrees Yadong Yin of the University of California, Riverside. “The system has a significantly improved tuning range and long-term bistability, and promises easy integration on flexible substrates.” But Yin points out that the slow switching speed of a few seconds will mean that photonic crystals will not be replacing liquid crystal displays in your computer or TV any time soon.

More likely applications include outdoor signage, portable electronics, and full-color electronic paper. There is still plenty of work to do, such as overcoming the viewing angle dependence, scaling the process for production, and, for a full-color display, how to control the brightness and saturation of each pixel.

Pauline Rigby