Artificial Muscles for Lifelike Color Displays

Adjustable diffraction gratings made of tiny artificial muscles could bring more lifelike colors to TVs and computer displays, physicists at the Swiss Federal Institute of Technology (Eidgenössische Technische Hochschule, or ETH) in Zurich, Switzerland show in the September 1 issue of Optics Letters.

In ordinary displays such as TV tubes, flat-screen LCDs, or plasma screens, each pixel is composed of three light-emitting elements, one for each of the fundamental colors red, green, and blue. The fundamental colors in each pixel are fixed, and only their amounts can change -- by adjusting the brightness of the color elements -- to create different composite colors. That way, existing displays can reproduce most visible colors, but not all. For example, current displays do not faithfully reproduce the hues of blue one can see in the sky or in the sea, says Manuel Aschwanden (aschwanden@nano.mavt.ethz.ch, +41-44-632-08-04).

Aschwanden and his colleague Andreas Stemmer figured that one can overcome such limitations by changing the fundamental colors themselves, not just their brightness, using a tunable diffraction grating.

In their setup, white light hits a 100-micron wide, gold-coated artificial muscle membrane that's been molded into a shape resembling microscopic pleated window shades. The artificial muscle is made of a polymer that contracts when voltage is applied. When white light hits a diffraction grating, different wavelengths fan out at different angles (see image at Physics News Graphics).

"It's like when you hold a CD in direct sunlight, and you rotate it," Aschwanden says. Like the microscopic tracks on a CD surface, the grooves on the artificial muscle split white light into a rainbow of colors. But instead of rotating the surface to obtain different colors, the ETH team adjusts the diffraction angle by applying different voltages to the artificial muscle. As the membrane stretches or relaxes, the incoming light "sees" the grooves spaced closer or tighter. All the angles of reflection change, so the entire fan of wavelengths turns as a whole. The desired color can then be isolated by passing the light through a hole: As the hole stays fixed, different parts of the spectrum will hit it and go through it.

To obtain composite colors, every pixel would use two or more diffraction gratings. By this method, a display could produce the full range of colors that the human eye can perceive, Aschwanden says.

Tunable diffraction gratings are routinely used in applications such as fiberoptic telecommunications and video projectors, but existing technologies are based on hard, piezoelectric materials rather than artificial muscles, limiting their stretchability to less than a percentage point. By contrast, artificial muscles can change their length by large amounts. Getting a full range of colors requires a source of "true" white light to begin with -- rather than a mere combination of red, green and blue that looks like white light to the human eye. For that purpose, the technology could exploit a new generation of white LED lights that have recently been developed, Aschwanden says (see PNU 772).

Aschwanden and Stemmer, Optics Letters, 1 September 2006
Contact Manuel Aschwanden, ETH
aschwanden@nano.mavt.ethz.ch, +41-44-632-08-04
Image at Physics News Graphics