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Phosphor Emits Two-for-One Photons

Photonics Spectra
May 1999
Aaron J. Hand

Researchers at the University of Utrecht's Debye Institute have made a promising leap in the efficiency of fluorescent phosphors, creating a material that absorbs one UV photon and gives off two red photons. Combining this achievement with similar success in the green and blue region could enable safer and more widely applicable fluorescent lamps.

In a typical fluorescent lamp, a gas discharge generates UV light, which excites phosphors that coat the inside of the tube. Red, green and blue emissions combine to create white light. The new phosphor is a step to overcome roadblocks to using xenon gas in fluorescent lights instead of toxic mercury. Besides the environmental benefits, xenon discharges immediately, enabling previously unattainable applications such as automobile brake lights, or light sources for copy or fax machines. The increased efficiency also could improve the viability of plasma displays, noted Andries Meijerink, a researcher on the project.

Two ions better than one

The Dutch researchers joined the decades-long search for a photon-doubling phosphor about 31/2 years ago, studying how lanthanide ions -- which provide the luminescence -- lose energy after being excited by UV light. Where others had failed, Meijerink and his colleagues succeeded by using two lanthanide ions instead of one. With this setup, a UV photon hits a gadolinium ion, which transfers the energy to two europium ions, each of which emits a red photon.

Although the phosphor could be commercially viable within five years, Meijerink predicted, its success relies on finding a workable green quantum cutter. "We have a second quantum cutter, giving green and blue light, but the efficiency is only 120 percent," he said, noting that the green light -- an important fraction of white light -- needs to reach a 200 percent efficiency like the red. "Further research will involve other pairs of lanthanides to obtain new downconversion routes."

The researchers have tackled the higher-efficiency requirement. But to bring the technology closer to commercial feasibility, Meijerink said, the team also needs to improve the VUV absorption and to investigate the stability of the materials under VUV excitation.

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