How small can a laser get? Good question, and according to the physicists at the National Institute of Standards and Technology (NIST) the answer is very small indeed, one single quantum dot small perhaps.
To put this into some perspective, a typical microdisk laser of the type currently used in experiments by NIST and Stanford Univeristy is constructed of layers of indium arsenide on top of gallium arsenide, the resulting etched out diskj being 1.8 micrometers across and containing an average of 130 quantum dot islands of indium arsenide in each one. The newly developed micrometer-sized solid-state lasers see a single quantum dot take the dominant role and when correctly tuned, NIST claims, can switch on at energies in the sub-microwatt range.
Ok, so it’s not an actual one quantum dot laser, not quite, but that one dot does effectively run the whole show. The disks are sized to create a “whispering gallery” effect in which infrared light at about 900 nanometers circulates around the disk’s rim. That resonant region contains about 60 quantum dots, and can act as a laser. It can be stimulated by using light at a non-resonant frequency to trigger emission of light. But the quantum dots are not all identical. Variations from one dot to another mean that their emission frequencies are slightly different, and also change slightly with temperature as they expand or contract. At any one time, the researchers report, at most one quantum dot—and quite possibly none—has its characteristic frequency matching that of the optical resonance.
What’s a quantum dot is probably your next question, and a valid one at that. The answer being nanoscale regions in a crystal structure that can trap electrons and the charge carrying ‘holes’ that transport current in a semi-conductor. When a trapped electron-hole pair recombines, light of a specific frequency is emitted.
And why should anyone not wearing a lab coat and cola bottle spectacles care? Because one day it is precisely these kind of highly efficient optical devices that will provide the low-power lasers used in the telecommunications and optical computing devices of the future, that’s why. “Quantum-dot lasers have attracted attention as possible embedded communications devices not only for their small size” NIST explain “but because they switch on with far less power then even the solid-state lasers used in DVD players.”