EPFL reinvents the liquid crystal display

ON, OFF: liquid crystals microtubes from Demetri Psaltis' Lab have a much shorter response time than traditional displays.

ON, OFF: liquid crystals microtubes from Demetri Psaltis' Lab have a much shorter response time than traditional displays.

An EPFL laboratory has developed a new technology, based on optofluidics, to improve LCD screens and optical information processing systems. The scientists are targeting a refresh rate on the order of a kilohertz, ten times faster than the technology currently used in LCD television screens.

Hundreds of devices around us use liquid crystals. Screens of all kinds – telephones, computers, flat-screen televisions – are based on this well-established technology, which has been studied for more than a hundred years and was first put into practical use about a half-century ago.
The principle hasn’t changed much over the years. Each point on the screen (pixel) contains a liquid crystalline substance, and an applied voltage causes the crystals to polarize, or all to point in the same direction. If this direction is parallel to the orientation of the polarized glass covering them, light can travel through. If not, the light is blocked and the pixel becomes opaque.

While this is sufficient for many applications, the technique nonetheless has its limitations, stemming from a short “response time” . When the voltage is cut off or its orientation is changed, the crystals and screen lose their parallel orientation and light is blocked. But this reorganization takes a certain amount of time, about a hundredth of a second at best.

Targeting the kilohertz threshold
Using a fundamentally new approach, developed by scientists in EPFL’s Optics Laboratory (LO), in collaboration with the Liquid Crystals Laboratory at the University of Calabria (Italy), future LCDs might finally be able to overcome this limitation. “We have measured response rates under a thousandth of a second,” affirms EPFL scientist Andreas Vasdekis. To breach this “barrier,” the researchers used optofluidics, a field of investigation in which the LO and its director, Demetri Psaltis, who is also Dean of the School of Engineering, are recognized pioneers. An article outlining their method has been published recently in the journal Nature Photonics.

“Instead of applying a voltage to our crystals, we create flow in a microchannel, via mechanical pressure exerted on the walls of the microchannel,” explains Andreas Vasdekis. This so-called “peristaltic” technique is comparable to the mechanism by which food is propelled through the esophagus.

Go with the flow
When the flow advances or retreats in these tiny (just a few microns in diameter) channels, the crystals change orientation slightly in order to align themselves with the current, thus permitting light to pass through. “It’s a more direct method than using voltage. It’s mechanical, which allows us to reduce the reaction time in the crystals to much lower values,” the scientist explains.

For the moment, this research is at the proof of concept stage. The researchers have succeeded in generating a 1 kHz cadence in the micro-tubes, or 1,000 oscillations per second. “We anticipate specialized applications, such as optical information processing systems, but there is also the potential for applications that would reach a broad public, such as new generation flat screens, or three-dimensional screens,” he continues. There is also the added advantage that the technology can be applied using industrial processes that are already established and inexpensive, such as soft lithography. Nanotechnological “pumps” controlled by high-frequency electrical impulses can also be exploited.

Commercial opportunities are certain to follow, because “traditional” liquid crystal displays are so widely used. “The approach that we have developed is a major advance on a technology that is already mature and well established,” comments Psaltis. “That will allow us to rapidly develop applications, possibly within a five-year time frame.”

Author: Emmanuel Barraud
Source: EPFL