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Controlling photons with InGaN quantum dots

By emitting photons from an indium gallium nitride quantum dot at the top of a GaN micropyramid, researchers have created a polarised light source. The device can be used in applications such as energy-saving computer screens and wiretap-proof communications
Polarised light - where all the light waves oscillate on the same plane - forms the foundation for technology such as LCD displays in computers and TV sets, and advanced quantum encryption.

Normally, this is created by normal unpolarised light passing through a filter that blocks the unwanted light waves. At least half of the light emitted, and thereby an equal amount of energy, is lost in the process.

A better method is to emit light that is polarised right at the source. This can be achieved with quantum dots - crystals of semiconductive material so small that they produce quantum mechanical phenomena. But until now, they have only achieved polarisation that is either entirely too weak or hard to control.

A semiconductive materials research group led by Per Olof Holtz, a professor at Linköping University, have now developed an alternative method.

The concept is based on InGaN QDs grown on top of elongated GaN hexagonal pyramids, by which the predefined elongation determines the polarisation vectors of the emitted photons from the QDs. This growth scheme should allow fabrication of ultra-compact arrays of photon emitters, with a controlled polarisation direction for each individual emitter.

With these, they have succeeded in creating light with a high degree of linear polarisation, on average 84 percent. The results are being published in the Nature periodical Light: Science & Applications.

“We’re demonstrating a new way to generate polarised light directly, with a predetermined polarisation vector and with a degree of polarisation substantially higher than with the methods previously launched,” Holtz says.

In experiments, quantum dots were used that emit violet light with a wavelength of 415 nm, but the photons can in principle take on any colour at all within the visible spectrum by varying the indium content.

Two ways of creating polarised light (Credit: Fredrik Karlsson, LiU)

“Our theoretical calculations point to the fact that an increased amount of indium in the quantum dots further improves the degree of polarisation,” says reader Fredrik Karlsson, one of the authors of the article.

The micropyramid is constructed through crystalline growth, atom layer by atom layer, of the semiconductive material GaN. A couple of nanothin layers where the metal indium is also included are laid on top of this. From the asymmetrical quantum dot thus formed at the top, light particles are emitted with a well-defined wavelength.

The results of the research are opening up possibilities, for example for more energy-effective polarised LEDs in the light source for LCD screens. As the quantum dots can also emit one photon at a time, this is very promising technology for quantum encryption, a growing technology for wiretap-proof communications.

The work is described in detail in the article, "Direct generation of linearly polarised photon emission with designated orientations from site-controlled InGaN quantum dots," by A. Lundskog et al in Science & Applications (2014) 3, e139, published online on 31st January 2014. doi:10.1038/lsa.2014.20

The project has been conducted within Nano-N consortium funded by the Swedish Foundation for Strategic Research.


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