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With Emission, Building a Better Liquid Crystal Display

Photonics Spectra
Oct 2007
Hank Hogan

Researchers at Brown University in Providence, R.I., may have come up with a solution to a fundamental problem with LCDs. Many such displays have a backlight that shines through the liquid crystal material. As a result, the displays are dimmer and less energy efficient.

The investigators’ possible solution arises from a novel use of the liquid crystal material. “The technological leap we have made is to use the periodic liquid crystal and polymer medium, doped with a laser dye, to generate patterned emission,” said Gregory P. Crawford, dean of engineering.

LEDPattern_Fig1.gif

This schematic illustrates an array of lasing posts that provide red, green and blue emissions once pumped with a flashlamp. Such an array could provide a combination backlight and color filter for a conventional LCD, or it could form the basis of a three-color emissive display. Images reprinted with permission of the Journal of the Society for Information Display.


Unlike the cathode-ray tubes found in old televisions and computer monitors, liquid crystal displays are not emissive. They work either by reflecting ambient light or by implementing a backlight — the configuration found in laptops and flat panel televisions. As the name implies, a backlight is a light source that sits behind the liquid crystal cells. By selectively blocking the light, the liquid crystal material forms pixels that can be seen. With the addition of a dye or some other color mechanism, the display can produce colors through the combination of red, green and blue pixels.

One problem with these displays is that it is difficult to make a truly black pixel because some of the backlight may bleed through a pixel that is supposed to be off. Another is that every component of an LCD that sits between the viewer and the backlight robs the display of a bit of intensity.

LEDPattern_Fig2.gif
Building a patterned emissive array begins with prepolymer droplets (a) that are exposed to two interfering beams (b), resulting in the first (red) series of posts. The process is repeated (c and d) to create posts of different colors and with different pitches.


An emissive display, on the other hand, does not have a passive light blocker sitting in the optical path. Lasers are emissive and, noted graduate student Scott J. Woltman, have been used before in displays. Moreover, he added, liquid crystal laser research is ongoing. Such lasers fall into a category known as distributed feedback dye lasers, and the theory of their operation has been established for decades.

Exploiting these concepts, the researchers demonstrated that a patterned liquid-crystal laser film could be used for a multicolor emissive technology. They built a two-color prototype, with pixels patterned to emit green and red.

They used organic dye-doped reflection-mode holographic-polymer-dispersed liquid crystal lasers (H-PDLCs). They fabricated the lasers in an array using a mask to stamp polymer posts onto a glass substrate and exposing the masked substrate to interfering beams from an Nd:YAG continuous-wave laser operating at 532 nm and made by Coherent Inc. of Santa Clara, Calif. They created red-reflecting posts, then repeated the process to create green-reflecting posts. All posts were ∼0.5 mm in diameter.

LEDPattern_Fig3.gif
The transmission spectrum of a two-color array shows that the green (dashed line) and red (dash-dot) posts combine to produce an overall transmission curve (black).


They tested the lasing capabilities of the array using a pulsed Nd:YAG laser from Quantel SA of Les Ulis, France, for a pump and a fiber optic spectrometer made by Ocean Optics Inc. of Dunedin, Fla., to measure the results. They found, as expected, two peaks, one centered in the green at 559.3 nm and the other in the red at 609.5 nm.

An actual display would require the addition of a blue array and the use of a pump at a lower wavelength. Simulations by the researchers indicated that the color gamut possible with this approach would be larger than that of a traditional display. However, because lasers are involved, the viewing angle would not be large. Thus, such a display probably would be best suited for projection applications.

Before the prototype developed by the scientists can be transformed into a display, however, several problems must be solved. One is the development of a continuous-wave liquid-crystal laser, which, Woltman noted, is something of a holy grail for researchers in the field. Another concern is the overall efficiency of the device.

A final issue involves the pitch of the array and the uniformity of the posts. The prototype fabrication techniques cannot approach the 50- to 100-μm pixel sizes found in today’s liquid crystal displays. Crawford indicated that, for this problem, unlike some of the others, a solution already exists.

“We believe a greater accuracy in post sizes can be made using a combination of ink-jet printing and multiple-exposure holographic methods,” he said.

Journal of the Society for Information Display, August 2007, pp. 559-564.


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