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* The “Century of the Photon”

Such technologies are possible, Schubert says, because of advances in photonics that are transforming society just as electronics revolutionized the world in recent decades. In fact, some have called this the “century of the photon.” North America’s optoelectronics market grew to more than $20 billion in 2003. The LED (light-emitting diode) market is expected to reach $5 billion in 2007, and the solid-state lighting market is predicted to be $50 billion in 15-20 years, Schubert says.

LEDs are specialized semiconductor devices that can potentially convert electricity to light, without the wasteful creation of heat. The color emitted is controlled in large part by the energy gap of the semiconductor and, in advanced structures, by the “photonic band gap” of the material, a term that describes a range of wavelengths that cannot travel through that particular substance. By suppressing certain wavelengths and enhancing others, the band gap determines the color.

LEDs are most familiar in such roles as indicator lights, displays on consumer electronics, exit signs, traffic signals, and roadwork signs. The first LEDs, which were made of GaAsP (gallium arsenide phosphide), emitted red light. New materials and technologies made amber, green, and blue LEDs possible. Now that several types of white LEDs have become available, manufacturers have begun to look beyond the specialized display market to the use of LEDs for general illumination, television monitors, and large-area displays.

Researchers are now exploring new organic materials (polymers) for the fabrication of organic light-emitting diodes (OLEDs). The goal is to use these light-emitting polymers to create thin, flexible sheets of light. Light-emitting wallpaper or even clothing would be possible.

A World-Class Team

Rensselaer’s goal of smart lighting will require revolutionary advances in fields such as materials science, device design, physics, and nanotechnology. To meet this interdisciplinary challenge, Rensselaer has invested heavily in attracting a world-class research team and providing state-of-the-art laboratories.

Rensselaer recruited Schubert in late 2002 to head the new Future Chips Constellation. Internationally known for his work on semiconductor doping and light-emitting diodes, Schubert holds 28 patents. He invented the resonant cavity LED that helped transform traffic signals and airport runway lighting, as well as the photon-recycling semiconductor LED, a promising new approach to the challenge of white LEDs.

Wetzel joined the team in March 2004. He is known for his work in materials physics and the chemistry of light emission. Since 1993, he has explored the use of gallium nitride compounds for LEDs, first in Berkeley and next in Japan in the lab of Isamu Akasaki, where gallium nitride materials were used to pioneer the fabrication of blue LEDs. With Uniroyal Optoelectronics in Tampa, Fla., Wetzel developed successful MOCVD (metal organic chemistry vapor deposition) techniques to produce very intense green LEDs.

At Rensselaer, Wetzel is tackling the challenges of making green LEDs even brighter and to vary their color into the yellow. He says the large band-gap energy in our LEDs gives them the ability to emit short-wavelength colors such as purple, blue and green. Since it is easier to lower the band-gap energy than to raise it, it should be possible to adjust the design to create the whole range of colors, he says.

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Rensselaer recruited Lin from Sandia National Laboratory, with the help of a $750,000 NYSTAR (New York State Office of Science, Technology and Academic Research) grant to support his research. Lin, who joined the Rensselaer faculty in July 2004, is known for his pioneering work in photonic crystals and optical waveguide devices, which are needed for optical signal routing. The waveguide built by Lin’s team from a photonic crystal was listed by Science Magazine as one of the 10 most important breakthroughs of 1999.

Photonic crystals, which are critical to Schubert’s vision of a tunable light source, were strongly advanced by Lin, who holds two patents on them. He was the first to demonstrate fabrication and testing of a 3-D photonic crystal at optical wavelengths. He demonstrated the use of photonic crystals of tungsten instead of the conventional dielectric materials, and he has already used VLSI (very large-scale integration) techniques to build a photonic crystal with enhanced emissions at a 1.5 micrometer wavelength. At Rensselaer, he plans to move on to a crystal with emissions in a visible wavelength. Reaching his goal of the emission of photons in a narrow wavelength of visible light with no radiation at other wavelengths would revolutionize lighting, making huge energy savings possible.

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