Quantum dot lasers -- 1 dot makes all the difference
糖心视频icists at the National Institute of Standards and Technology and Stanford and Northwestern Universities have built micrometer-sized solid-state lasers in which a single quantum dot can play a dominant role in the device鈥檚 performance. Correctly tuned, these microlasers switch on at energies in the sub-microwatt range. These highly efficient optical devices could one day produce the ultimate low-power laser for telecommunications, optical computing and optical standards.
How small can a laser get? The typical laser has a vast number of emitters鈥攅lectronic transitions in an extended crystal, for example鈥攃onfined within an optical cavity. Light trapped and reflecting back and forth in the cavity triggers the cascade of coherent, laser light. But about a decade ago, researchers made the first quantum dot laser.
Quantum dots are nanoscale regions in a crystal structure that can trap electrons and 鈥渉oles,鈥� 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.
In recent experiments*, the NIST-Stanford-Northwestern team made 鈥渕icrodisk鈥� lasers by layering indium arsenide on top of gallium arsenide. The mismatch between the different-sized atomic lattices forms indium arsenide islands, about 25 nanometers across, that act as quantum dots. The physicists then etched out disks, 1.8 micrometers across and containing about 130 quantum dots, sitting atop gallium arsenide pillars.
The disks are sized to create a 鈥渨hispering gallery鈥� effect in which infrared light at about 900 nanometers circulates around the disk鈥檚 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鈥攁nd quite possibly none鈥攈as its characteristic frequency matching that of the optical resonance.
Nevertheless, as they varied a disk鈥檚 temperature from less than 10K to 50K, the researchers always observed laser emission, although they needed to supply different amounts of energy to turn it on. At all temperatures, they say, some quantum dots have frequencies close enough to the disk鈥檚 resonance that laser action will happen. But at certain temperatures, the frequency of a single dot coincided exactly with the disk鈥檚 resonance, and laser emission then needed only the smallest stimulation. It鈥檚 not quite a single-dot laser, but it鈥檚 a case where one quantum dot effectively runs the show.
Citation: Z.G. Xie, S. G枚tzinger, W. Fang, H. Cao and G.S. Solomon. Influence of a single quantum dot state on the characteristics of a microdisk laser. 糖心视频ical Review Letters, 98, 117401 (2007).
Source: NIST
