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17 April 2007


Microdisk laser dominated by single quantum dot

Researchers at Stanford and Northwestern universities and the National Institute of Standards and Technology (NIST) in the USA have fabricated highly efficient micron-sized disk lasers in which a single quantum dot can play a dominant role in the device’s performance (Z.G. Xie et al, Physical Review Letters (2007) 98, 117401). When tuned correctly, the microdisk lasers switch on at energies in the sub-microwatt range, and could perhaps yield the ultimate low-power laser for fiber-optic telecoms, optical computing and optical standards, the researchers claim.

The NIST-Stanford-Northwestern team made microdisk lasers by growing 25nm-wide indium arsenide quantum dots on gallium arsenide, then etching out 1.8 micron-wide disks (containing about 130 quantum dots) atop GaAs pillars. The disks are sized to create a ‘whispering gallery’ effect, in which infrared light with a wavelength of about 900nm circulates around the disk’s rim. That resonant region contains about 60 quantum dots, and can act as a laser.

Picture: Microdisk laser. Scanning tunneling microscope image (inset) shows some of the 130 or so InAs quantum dots in each 1.8 micron-wide disk. Credit: NIST.

The microdisk laser can be stimulated by using light at a non-resonant frequency to trigger light emission. However, the quantum dots are not all identical. Variations between dots mean that their laser emission frequencies are slightly different and also change slightly with temperature as they expand or contract. So, at any one time, there is at most one quantum dot with a characteristic frequency matching the optical resonance frequency of the microdisk.

Nevertheless, as the researchers varied a disk’s temperature from less than 10K to 50K, at all temperatures some quantum dots have frequencies close enough to the disk’s resonance that laser action will occur, although different amounts of energy were needed to turn it on. At certain temperatures, the frequency of a single dot coincides exactly with the disk’s resonance, and laser emission then needs only slight stimulation. It’s not quite a single-dot laser, but one quantum dot effectively dominates operation, the researchers say.