Progress Made Toward Single-Qdot Laser

A micrometer-sized solid-state laser has been built that, while not powered by a single quantum dot, effectively demonstrates that just one dot can play a dominant role in the device's performance. These highly efficient optical devices could one day produce the ultimate low-power laser for telecommunications, optical computing and optical standards.

The typical laser has a vast number of emitters -- electronic transitions in an extended crystal, for example -- confined within an optical cavity. Light trapped and reflecting back and forth in the cavity triggers the cascade of coherent, laser light. Researchers made the first quantum dot laser about 10 years ago.

Microdisk lasers used in experiments by NIST, Stanford University and Northwestern University are made by layering indium arsenide on top of gallium arsenide and etching out disks about 1.8-µm across on pillars of gallium arsenide. Scanning tunneling microscope image (inset) shows some of the approximately 130 "quantum dot" islands of indium arsenide in each disk. (Images: NIST)

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.

In recent experiments, a team of researchers at the National Institute of Standards and Technology (NIST) and Stanford and Northwestern universities made “microdisk” lasers by layering indium arsenide on top of gallium arsenide. The mismatch between the different-sized atomic lattices forms indium arsenide islands, about 25-nm across, that act as quantum dots, or qdots. The physicists then etched out disks, 1.8-µm across and containing about 130 qdots, sitting atop gallium arsenide pillars.

The disks are sized to create a “whispering gallery” effect in which infrared light at about 900 nm circulates around the disk’s rim. That resonant region contains about 60 qdots, and can act as a laser. It can be stimulated by using light at a nonresonant frequency to trigger emission of light. But the qdots 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 qdot -- and quite possibly none -- has its characteristic frequency matching that of the optical resonance.

Nevertheless, as they varied a disk’s temperature from less than 10 K to 50 K, the researchers always observed laser emission, although they needed to supply different amounts of energy to turn it on. At all temperatures, they said, some qdots 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.

It’s not quite a single-dot laser, but it’s a case where one qdot effectively runs the show, the researchers said.

The research is reported in Physical Review Letters. For more information, visit: www.nist.gov