MICRODISK LASERS: Single quantum dot controls micron-size solid-state laser

June 1, 2007

Physicists at the National Institute of Standards and Technology (NIST; Gaithersburg, MD), Stanford (Palo Alto, CA) and Northwestern (Evanston, IL) Universities have built micron-size solid-state lasers in which a single quantum dot can play a dominant role in device performance. Correctly tuned, these microlasers switch on at energies in the submicrowatt range, and might ultimately yield low-power lasers for telecommunications, optical computing and optical standards.

The NIST-Stanford-Northwestern research team made the “microdisk” lasers by layering indium arsenide on top of gallium arsenide. The mismatch between the different-size atomic lattices forms indium arsenide islands, about 25 nm across, that act as quantum dots. The physicists then etched out disks, each 1.8 microns across and containing about 130 quantum dots, atop gallium arsenide pillars-sized to create a “whispering gallery” effect in which infrared light at about 900 nm circulates around the rim of the disk. The resonant region contains about 60 quantum dots, and can act as a laser.¹

The quantum dots are not all identical, however. Variations from one dot to another yield slightly different emission frequencies that also change slightly as the dots expand or contract with temperature. At any one time, the researchers report, at most one quantum dot-and possibly none-is emitting at the optical resonant frequency of the microdisk. Nevertheless, as they varied a disk’s temperature from less than 10 K to 50 K, the researchers always observed laser emission, although threshold energies tended to vary. And at all temperatures, some quantum dots had frequencies close enough to the disk’s resonance that laser action occurred. At certain temperatures, the frequency of a single dot coincided exactly with the disk’s resonance, however, yielding a lower lasing threshold. So, while not quite a single-quantum-dot laser, the microdisk lasers are essentially controlled by a single quantum dot.