For some smart lighting applications, “design tunability” (fabrication of LEDs that have the desired color, polarization, etc.) will be adequate, Schubert says. Structural control can be provided by the use of photonic crystals, reflectors, and cavities resonator structures with deliberate defects that control photons.
Lin completed the team in the summer when he joined Rensselaer from Sandia National Laboratories. He 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.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 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.
Real-time tunability will be required, however, for such applications as lighting, imaging, and communications. Schubert plans to achieve this by designing devices that include multi-channel electrical and optical controls. Because such factors as temperature and voltage can affect the properties of the light emitted, it is possible to tune the light by controlling these factors. The controls also could be used to activate various portions of a photonic crystal.With incandescent lights independent tunability of color and intensity is not possible, Lin says, because increasing intensity raises the heat and shifts the color spectrum. With LEDs, the color is largely determined by the material, which is deposited in layers. Color shifts must be obtained by using several different colors of LEDs in an array. Photonic crystals, however, are created by lithography, the technology used to fabricate computer chips. One chip could be designed to carry several colors or intensities, making very precise tunability possible.Much basic science and computer modeling is still needed to understand what light characteristics are best for specific applications and how to design tunable LED arrays and photonic crystals that produce the desired light. A number of research projects are under way to develop the necessary technology, particularly in materials, photonic crystals, and improvements to LED devices.
One challenge, for example, is that current systems do not extract all of the light from an LED, both because of poor reflectivity and because polymers used to encapsulate the working structure lose transparency in reaction to the light, particularly in short-wavelength and high-power emitters.
Schubert has developed an omni-directional reflector (ODR) that makes it possible to extract more than 99 percent of the light from both inorganic and organic LEDs. This reflector consists of a semiconductor layer, a dielectric (insulating) layer perforated by an array of micro-contacts, and a metal layer.
“The ODR for LEDs will accelerate the replacement of conventional lighting used for a multitude of applications, such as lighting for homes, businesses, museums, airports, and on streets,” he says.
The photonics revolution already has provided tremendous benefits in energy savings, improved lighting, versatile new display technologies, and optical communications. Yet, the science and technology are still not available for the full control of many basic characteristics of light. As technical problems are overcome and new photonic products reach the marketplace, smart lighting will revolutionize such fields as illumination, imaging, display, and communications. Improved LEDs and new photonic crystal light emitters, moreover, are expected to greatly reduce the world’s consumption of energy and diminish harmful environmental impacts.
“Many roadblocks remain, but we are very optimistic,” says Schubert. “We believe that our multidisciplinary team focused on a common goal will change the world and make it a better place by advancing smart lighting from concept to reality.”
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