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Electrowetting light valves shape up

21 Apr 2005

A US team produces a highly visible, low-cost display from an array of oil-filled cells.

A team from the Universiy of Cincinnati, US, and spin-off company Extreme Photonix has devised a low-cost electrowetting display technology that it feels could rival LCDs. The researchers' backlit display consists of high-transmission light valves containing a mix of chromophore doped oil and deionized water. (Appl. Phys. Lett. 86 151121)

"The real advantages of electrowetting valves are transmission efficiency and potential manufacturing cost savings," Jason Heikenfeld of Extreme Photonix told Optics.org. "[Unlike LCDs] electrowetting light valves transmit light at any view-angle or polarization."

Simpler to construct than LCDs, Heikenfeld explains that electrowetting valves should bring cost-savings when devices go into production. In addition, being more efficient means that expensive backlighting can be replaced with cheaper, low-power alternatives.

The light valves are fabricated on a glass substrate that is coated with an ITO film to give a series of transparent electrodes. Each electrowetting cell contains a small quantity of black oil and de-ionized water. To laterally confine the oil, the substrate is patterned with an oil repellent grid.

Applying a voltage across the electrowetting valve causes the oil film to bead up in the center of the cell, allowing light to pass through on either side. The transmissivity of the cell can be modulated from around 5% to more than 80% by increasing the voltage across the electrodes from zero to 30 V. Switching speed depends on the cell size and ranges from 10 ms for a 1 mm2 cell to 100 ms for a 3 mm2 cell.

As Heikenfeld explains, one of the most challenging elements of the work has been to reduce light leakage. By doping the film with around 1 wt. % of nonpolar organic chromophores the researchers have managed to create a film that is able to absorb with near-neutral optical density across the visible light spectrum.

Heikenfeld and Andrew Steckl, a professor at the University of Cincinnati, founded Extreme Photonix in 2001 to industrialize the technology. With its first products due for release in five years, the company plans to realize $10-20 million in revenues through a combination of sales and licensing activities by 2008.

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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