Scientists make telecoms quantum dot laser breakthrough

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Scientists have managed to build an extremely small, very efficient solid-state laser that could pave the way for very-low power lasers for telecommunications and optical computing.

Physicists at the National Institute of Standards and Technology (NIST) and Stanford and Northwestern Universities have built micrometer-sized solid-state lasers in which a single quantum dot plays an important role in the device's performance.

When correctly tuned, these microlasers switch on at energies in the sub-microwatt range - which makes them very useful for transmitting signals in fibre optics and could be used in optical computing.

Typically, lasers have a vast number of emitters confined within an optical cavity. The light trapped in these optical cavities reflects back and forth triggering cascades of coherent, laser light. Around ten years ago, researchers made the first quantum dot laser.

Quantum dots are nanoscale regions in a crystal structure that can trap electrons and "holes", the charge carriers that transport current in a semiconductor. When a trapped electron-hole pair recombines, light of a specific frequency is emitted. Quantum-dot lasers have attracted attention as possible embedded communications devices not only for their small size, but because they switch on with far less power then even the solid-state lasers used in DVD players.

The NIST-Stanford-Northwestern team managed to produce "microdisk" lasers by layering indium arsenide on top of gallium arsenide. The mismatch between the different-sized atomic lattices forms indium arsenide islands, about 25nm across, that act as quantum dots. The physicists then etched out disks, 1.8 micrometers across and containing about 130 quantum dots, sitting on top of gallium arsenide pillars.

The disks create a "whispering gallery" effect in which infrared light at about 900nm circulates around the disk's rim. That resonant region contains about 60 quantum dots, and acts as a laser.

The laser can be stimulated by using light at a non-resonant frequency to trigger emission of light. But the quantum dots are not all identical. Each dot differs from the other and that means their emission frequencies are slightly different, and also change slightly with temperature as they expand or contract.

According to the researchers, at most one quantum dot has its characteristic frequency matching that of the optical resonance.

When the scientists varied the temperature of the disk from 10K (-263.15 degree Celsius) to 50K (-223.15 degrees Celsius), they always observed laser emissions, although they needed to supply different amounts of energy to turn it on.

According to the researchers, at all temperatures some quantum dots have frequencies close enough to the disk's resonance that laser action will happen. But at certain temperatures, the frequency of a single dot coincided exactly with the disk's resonance, and laser emission then needed only the smallest stimulation.

While the scientists admit that it is not quite a single-dot laser, they said it is "a case where one quantum dot effectively runs the show."

As reported by IT PRO, researchers working at Toshiba Europe made a breakthrough in quantum encryption which would alert users if a hacker managed to intercept security keys.

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