About LEDs LEDs are semiconductor diodes, electronic devices that permit current to flow in only one direction. The diode is formed by bringing two slightly different materials together to form a PN junction. In a PN junction, the P side contains excess positive charge ("holes," indicating the absence of electrons) while the N side contains excess negative charge (electrons). When a forward voltage is applied to the semiconducting element forming the PN junction, electrons move from the N area toward the P area and holes move toward the N area. Near the junction, the electrons and holes combine. As this occurs, energy is released in the form of light that is emitted by the LED. The material used in the semiconducting element of an LED determines its color. The two main types of LEDs presently used for lighting systems are aluminum gallium indium phosphide (AlGaInP, sometimes rearranged as AlInGaP) alloys for red, orange and yellow LEDs; and indium gallium nitride (InGaN) alloys for green, blue and white LEDs. Slight changes in the composition of these alloys changes the color of the emitted light. Light emitting diodes (LEDs) were first developed in the 1960s, but only in the past decade have LEDs had sufficient intensity for use in more than a handful of lighting applications, and specifiers are confronted with an increasing number of lighting products that incorporate LEDs for certain applications. Primarily, these applications have taken advantage of the characteristics of LEDs that have made them most suitable for indication, not illumination. from Lighting Answers: LED Lighting Systems, published by NLPIP, the National Lighting Product Information Program. Established by the Lighting Research Center in 1990, NLPIP team members are LRC researchers — leading experts in efficient lighting, human factors, and technology transfer. The NLPIP product testing laboratory is one of only three non-manufacturer, NVLAP-accredited labs in the US.

The Future Chips researchers are working on developing the light source for these technologies. “The computers are already very smart. They are waiting on us to provide the data,” says Christian Wetzel, the Wellfleet Career Development Constellation Professor, Future Chips, and associate professor of physics.

Schubert, Lin, and Wetzel — all recognized photonics pioneers — form the nucleus constellation, which also includes Thomas Gessmann, a research assistant professor of electrical, computer, and systems engineering; Jong Kyu Kim and Theeradetch Detchprohm, two postdoctoral researchers; and a number of doctoral students and three undergraduate students.

In addition, other Rensselaer research centers, including the Center for Advanced Interconnect Systems Technologies, the Interconnect Focus Center-New York, the Rensselaer/IBM Center for Broadband Data Transfer Science and Technology, the NSF Nanotechnology Science and Engineering Research Center, and the Lighting Research Center, provide a broad range of expertise, potential collaborations, and facilities for work in this emerging field.

A major focus of the constellation, smart lighting is a revolutionary new field in photonics based on efficient light sources that are fully tunable in terms of such factors as spectral content, emission pattern, polarization, color temperature, and intensity.

“The research program in the Future Chips Constellation aims at nothing less than transforming many sectors of the economy, including communications, medicine, defense, entertainment, and the environment,” Rensselaer President Shirley Ann Jackson says. “We are delighted to have these stellar individuals join our dynamic research environment, working to develop next-generation technology in semiconductor design and performance.”

Schubert says smart lighting will not only offer better, more efficient illumination, it will provide “totally new functionalities.” For example:

Studies have shown that spectral (color) variations in light have profound effects on the human circadian and visual systems. Controlling the amount of red, yellow, and blue in white light has implications for sleep in Alzheimer’s patients, growth of premature infants, seasonal depression, jet lag, and the well-being of night-shift workers. Some researchers have suggested that inappropriate lighting can upset the body chemistry and even lead to certain types of cancer.

In live-cell biological imaging, smart lighting could make it possible to coordinate intensity, wavelength, and polarization with image scanning to reveal a new wealth of features. Using this revolutionary cellular microscopy technique, for example, researchers could observe and analyze multiple single cells in real time as they react to a drug or infectious agent.

		Shawn Yu-Lin, who completed the team this summer, is known for his pioneering work in photonic crystals and optical waveguide devices, which are needed for optical signal routing.

Smart lighting could address the world’s pressing need for increased food production because light plays a pivotal role in plant growth and photosynthesis. Adjustable smart-light sources predominately made up of blue, red, and infrared wavelengths — the portions of the spectrum best absorbed by plants — could provide a low-cost, energy-efficient way to grow crops off-season.

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 to 20 years, Schubert says.

		E. Fred Schubert, who leads the Future Chips Constellation, holds 28 issued patents and invented the resonant cavity LED that helped transform traffc signals and airport runway lighting.

LEDs commonly are used for 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 are looking beyond the specialized display market to the use of LEDs for general illumination, television monitors, and large-area displays.

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

		Christian Wetzel, who joined the team last spring, is known for his work in materials physics and the chemistry of light emission. Since the early 1990s, he has explored the use of gallium nitride compounds for LEDs.

Wetzel, who joined the team last spring, is known for his work in materials physics and the chemistry of light emission. Since the early 1990s, he has explored the use of gallium nitride compounds for LEDs, first at 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.