Issue Date: May 19, 2008
Polaritons, which are exotic, quantum mechanical particles that are part matter and part light, have spent most of the past 50 years as ideas in the minds of theoreticians or they have only been generated at extremely low temperatures. Now, Pavlos Savvidis and his colleagues at the University of Crete, in Greece, have harnessed physical versions of the hybrid entities to produce a light-emitting diode (LED) that can operate much closer to room temperature than previously demonstrated (Nature 2008, 453, 372). This work raises hopes that polaritons could yield new energy-efficient lasers or be applied in quantum computing.
"When I first encountered the idea of a polariton, its beauty stunned me," comments Benoît Deveaud-Plédran, from the Quantum Photonics National Center of Competence in Research, located in Lausanne, Switzerland. "I never expected that polaritons would make their way out of the laboratory."
Polaritons are a combination of three ingredients: an electron in an excited state, a positively charged hole that the electron leaves behind when it becomes excited, and a photon emitted when the same electron loses energy and drops back into the hole. When these components are surrounded by a highly reflective material that prevents the photon's escape, the photon is trapped and creates another electron-hole pair. Continuous recycling of photons and electron-hole pairs leads to the half-light and half-matter polariton.
Savvidis and colleagues created an electron-hole pair in indium gallium arsenide. The team then constrained the photon-electron-hole quasiparticle with highly reflective layers of gallium arsenide and aluminum arsenide. Finally, they applied an electrical current to excite this system to create a steady source of light at the relatively high temperature of –38ºC—balmy, given that polariton electroluminescence had previously been achieved only at –196 ºC.
Savvidis' team is now working to make the polariton light emission happen at room temperature. They are also trying to get the emitted light to be coherent to make a laser. Polariton lasers are expected to be far more energy efficient than conventional semiconductor lasers, Savvidis says. "A polariton laser would exhibit coherent light emission at two orders of magnitude lower power, compared with conventional semiconductor lasers," he notes.
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