Microplasma arrays promise to revolutionize lighting

Ashley N. Paddock, ashley.paddock@photonics.com

A new plasma-based lighting system comprising microcavity arrays produces inexpensive, wafer-thin, flexible sheets of light – and its creators say it will revolutionize illumination.

Just as in a fluorescent light, a microcavity array is energized by an applied voltage. The new technique, developed by Dr. Gary Eden and Dr. Sung-Jin Park of the University of Illinois at Urbana-Champaign, involves confining plasma in parallel rows of microcavities within thin-sheet materials and introducing electricity.

The discovery began to take shape in 1996, when two graduate students approached Eden with a block of silicon and asked if they could drill a small hole in it to try to produce a plasma inside the hole. They did this with a hole about 400 µm in diameter, which Eden said was a crude forerunner of the current microcavity array (MCA) lighting system.

Arrays that confine plasma in parallel rows of microcavities within thin-sheet materials result in inexpensive, wafer-thin and flexible sheets of light. Courtesy of Eden Park Illumination Inc.

The issue of space and pressure intrigued Eden. A fundamental rule for stable, steady-state plasma is pressure times diameter scaling: the smaller the plasma dimensions, the higher the pressure can be. The very high pressures that can be obtained in microplasmas give rise to unique lighting and other properties.

The microcavities in the flexible sheets are key to these light arrays. In one of the most important in-development implementations, a sheet of aluminum foil is placed in an anodizing bath. By controlling the bath parameters – temperature and time of anodizing – large arrays of microcavities can be formed with near-optimum shape and with automatically placed interconnecting aluminum electrodes. The largest array so far contains 250,000 luminous microcavities. Thin laminated films on the surface of the wafer contain the electrical power interconnects that feed the individual cavities. When AC power is supplied through the almost-invisible grid, the array bursts to life.

Currently, the largest arrays produced are 6 sq. in. These can be tiled together, in different colors if desired, to make larger arrays; much larger arrays can be made, too, limited only by the size of the anodizing bath. Aluminum foil with a thickness of 125 µm is used. The cavities are then sealed in very thin sheets of glass, resulting in an array that is 1 to 2 mm thick. The plasma arrays are ruggedized to a certain extent and have an ultimate thickness of about 4 mm; the resulting wafer weighs less than 200 g.

Compared to contemporary lighting, the MCA lighting has much greater efficiency. Because the array is flat, it does not lose the lumens per watt that a standard light does. The MCA lighting lasts 20,000 hours before failure and is more environmentally friendly because it does not contain mercury, and its plastic, glass and aluminum contents are easily repurposed and recyclable.

Eden and Park founded Eden Park Illumination Inc. in 2007 to bring their new technology to market. The lighting technology company is a close affiliate of the Laboratory of Optical Physics and Engineering at the university. The research was funded by the Air Force Office of Scientific Research.

Because of the rate of production, the MCA is not cost-competitive with current lighting options, but since Congress mandated that incandescent lights be phased out in 2014, the research team believes that there will be a much greater acceptance of MCA technology in the future.

It is suitable for light production and is ideal for on-chip special chemistries. By using the linear microchannel design, as employed in an MCA lighting panel, a large number of parallel receptors can be placed within a small area to perform a variety of sensory activities, depending upon the chemical composition within the cavities.