Photonics to revolutionize the world

12 Jan 2004

To mark the new year Opto & Laser Europe takes a look at three revolutionary optical technologies that are likely to have a profound impact on society when they become commercially available.

From Opto & Laser Europe January 2004

Electronic paper THE DREAM To create a thin, flexible electronic display that offers superior contrast and resolution to a newspaper and consumes almost no electricity.

CURRENT STATUS Several firms have recently made significant progress. Last summer, E ink of the US announced what it claimed was the world's thinnest, lightest and most rugged active-matrix display. At just 0.3 mm thick, the firm's glass-free display was less than half the width of a credit card. It comprised a steel-foil transistor substrate with a thin, shatterproof plastic face containing electronically activated ink. Prototypes measure up to 3 inches in diameter and have a resolution of up to 96 dpi. Importantly, the display only consumes power from its battery when an image in the display is updated.

A spokesperson for E ink told Opto & Laser Europe that the company is now working hard to launch a high-resolution (160 dpi) version by mid-2004. The first commercial E ink displays will be monochrome (with four grey levels) and contain glass rather than plastic. Initial applications are likely to be in electronic books and other PDA devices. A couple of small batteries should be sufficient to power the display for at least several weeks, depending on how often a page is refreshed.

Another firm making good progress is ZBD Displays of the UK. The spin-out from UK defence agency QinetiQ has demonstrated displays that offer up to 16 grey levels and a resolution of up to 200 dpi. The devices are based on sophisticated liquid-crystal technology and, like E ink's, only consume power when an image is updated. Thanks to a partnership with Varitronix (a leading maker of LCDs), the first commercial versions of ZBD's technology should be available in early 2004.

FUTURE CHALLENGES The big challenge is to produce a display that is flexible, which involves creating driving electronics and backplanes that can cope with being flexed. The first step will probably be the advent of "conformal" displays, which instead of being a flat sheet are curved into a fixed shape.

TIMELINE Both ZBD and E ink say that commercial versions of their technologies will be available on the market in the first half of 2004. Their initial displays may well have the readability of paper and consume very little power, but unfortunately they are unlikely to be flexible. Their initial applications are likely to be electronic books, mobile phones and PDA-style devices.

Artificial retina THE DREAM To restore sight to people with retinal disease through use of an optoelectronic implant.

CURRENT STATUS Of the many groups working on retinal implants, Optobionics of the US is perhaps at the most advanced stage. The firm, which was founded by brothers Alan Chow (an ophthalmologist) and Vincent Chow (an electrical engineer), has developed a silicon implant that is currently undergoing clinical trials approved by the US Food and Drug Administration.

According to a spokesperson for the company, its artificial silicon retina (ASR) microchip is about 2 mm in diameter and 25 µm thick, and contains around 5000 microscopic silicon photodiodes.

When visible light strikes the ASR its photodiodes generate tiny electric currents that stimulate the healthy layer of cells behind the damaged retinal photoreceptors. The implant is completely passive and requires no electrical power supply or wires.

Over the past three years, 10 patients suffering from a retinal disease called retinitis pigmentosa (RP), which affects the photoreceptors in the eye, have received the implant. A formal report of the results of the trial is expected in a medical journal this year, and preliminary results suggest that the implant gives a distinct improvement in vision, with no signs of any rejection or detachment of the implant.

FUTURE CHALLENGES Several challenges still exist. The first is to properly assess the improvement in vision and long-term effects resulting from using the implant. A second challenge is to make implants that contain more photoreceptors.

Although an array of 5000 photodiodes may sound like a lot, it is only a tiny fraction of the total number of photoreceptors in the eye. The healthy human eye contains approximately five million cones (colour photoreceptors), of which 50,000 are located in the fovea, the most light-sensitive part of the retina.

TIMELINE Optobionics chose not to comment on when its implants may become available.

White LEDs THE DREAM To make commercial white LEDs that are sufficiently bright and cheap to compete with filament light bulbs.

CURRENT STATUS Gallium-nitride-based LEDs that emit white light are already on the market and are beginning to find their way into portable goods such as torches and bicycle lights. The big issue is how to fabricate brighter, less expensive versions (see "Future Challenges" below).

According to Bob Steele of US-based market analyst Strategies Unlimited, the efficiency of today's white LEDs already exceeds that of standard incandescent lamps. "White LEDs are already better than a whole array of incandescent-type light bulbs - by a factor of two in some low-wattage cases," he told Opto & Laser Europe. "The best commercially available LEDs emit 25 lm/W, while the incandescents come in at 17 lm/W."

However, fluorescent tubes are still way ahead, with efficiencies in excess of 60 lm/W. According to Steele, it is crucial that LEDs reach at least the same level of efficiency because lighting in many commercial applications currently uses fluorescent bulbs.

Three different approaches can be used to generate white light from the semiconductor sources. The first technique combines a blue LED with a yellow-emitting phosphor. The second approach involves coating the packaging surrounding a UV LED with a red-green-blue-emitting phosphor, whereas the final method is to directly combine the output of red, green and blue LEDs. Because it is as yet unclear which of these is the best technique to adopt, many firms are hedging their bets and working on more than one of the methods.

FUTURE CHALLENGES "The technology is proceeding very well," commented Steele. "The main stumbling block at the moment is cost. If you compare a white LED to a low-wattage fluorescent, the cost difference factor used to be 100, but it has come down to 30 or 40 now in terms of cost per lumen."

Steele says that the second hurdle to overcome will be to increase the brightness of an LED. A standard commercially available white LED will typically emit around 120 lm, but according to Steele an incandescent can produce 1700 lm and fluorescent tubes can emit anything up to 3000 lm. "In the not-too-distant future, it may be feasible to have LEDs emitting several hundred lumens, and maybe up to 1000 lm in a few years," he said.

So what is being done to deal with these challenges? According to Steele, researchers are concentrating on three areas: increasing the internal quantum efficiency of the LED itself; improving the light extraction from the chip; and driving the LEDs at higher current densities.

"Being able to drive the chips at higher current densities is a big deal," said Steele. "But to do that, thermal management becomes a crucial aspect of the problem. The harder you drive, the more heat you generate, and the shorter the lifetime of the device. I think thermal management is key to driving up the performance of these devices."

TIMELINE It seems clear that we are not going to see white LEDs challenging light bulbs overnight. "Realistically, I think it is 10 years before you will see a large and significant penetration of the white-light illumination market with LEDs. It will happen gradually over time," said Steele.

One of the first important emerging markets for high-power white LEDs could be applications in automotive headlights. "I think we will see white LEDs in car headlamps in 2006 and 2007, depending on technical issues," said Steele.