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Photonic crystals offer time control

18 Aug 2004

A simple way to customise the rate at which quantum dots emit light is unveiled in Nature.

Dutch scientists have demonstrated that they can control the timing of light emission from semiconductor quantum dots by embedding them in a photonic crystal structure (Nature 430 654).

The news, reported in the 5 August issue of Nature, could open the way to more efficient solar cells and LEDs. It could also help create precise single-photon sources which are required for quantum cryptography and quantum computing.

The research was performed by Willem Vos and his colleagues from the University of Twente and the University of Utrecht in the Netherlands. They embedded 4.5 nm diameter nanoparticles of CdSe into a photonic crystal structure known as an inverse opal - a matrix of titania riddled with tiny spherical air holes.

By varying the separation between the air holes between 240 and 650 nm the team was able to slow down or speed up the rate of spontaneous emission from the CdSe dots. For example, an air-hole spacing of 420 nm yielded a radiative lifetime of 9.6 ns while a spacing of 500 nm increased this to 19.3 ns.

"These measurements are the first experimental demonstration of the use of 3D photonic crystals to control the radiative lifetime of emitters," say the researchers in their Nature paper. "They demonstrate a strong variation in the lifetime by a factor of two for photonic crystals with different lattice parameters."

The work also verifies a long-held theoretical prediction that was made by the "father of photonic crystals", American physicist Eli Yablonovitch, in 1987. His calculations made at Bellcore suggested that photonic crystals should be able to inhibit and control spontaneous emission.

Although much work has been performed on photonic crystals, to date it has concentrated on using the structures to guide light in a particular direction.

The Dutch research could also have practical benefits. Speeding up the emission rate could lead to more efficient light sources such as LEDs, while slowing it down could help create solar cells that do not let energy leak away as light.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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