Optical circuits are from a utopia but very much real, alas it is still a long time before we will see any commercial products in the shapes of processors and other circuits that are optical. Researchers at NIST (National Institute of Standards and Technology) have now presented a solid-state laser that can be activated with less than a microwatt. The goal is to construct a laser that can be used in telecommunication, optical circuits and optical standards. The things that makes these lasers interesting for circuits is not just there size of them but also that they require very little power to be triggered. This kind of quantum dot lasers is quite similar to regular lasers though. They do what regular lasers do; they just do at a smaller scale.
They make these quantum dot lasers by adding a layer of indium arsenide on top of a surface of gallium arsenide and then cut out 1.8 micrometer discs of the gallium arsenide. Since the two atomic lattices are so different islands of indium arsenide will be formed on the surface, about 130 per disc each measuring 25nm in diameter.
The laser features comes from the fact that the crystal structure can trap electrons and ”holes,” charge carriers. When a trapped electron and a ”hole” meet and interact a specific wavelength of light will be emitted. The scientists have used the about 60 islands that are located close to the rim of the disc, when triggered light with a 900nm wavelength will circulate the rim of the disc. The actual trigger is light of a non-resonant wavelength.
The problem is that these quantum dots aren’t identical but all have small variations in size and are thus sensitive to different wavelengths and in response to that emit different wavelengths. At the same time they are very sensitive to temperature changes and will expand or contract if the temperature goes up or down, but as long as they were working between 10K and 50K they could observe laser light being emitted. Although, they needed varying amounts of energy to do so.
The article is published in Physical Review Letters 98, although it’s not free.
