|
Topic Name: Micrometer-sized solid-state lasers
Category: Optoelectronics
Research persons: Z.G. Xie, S. Götzinger, W. Fang, H. Cao and G.S. Solomon
Location: NIST-Boulder, MS 104.00, 325 Broadway, Boulder, Colo. 80305-3328, United States
Details
Physicists at the National Institute of Standards and Technology (NIST) 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’s 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—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. 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 “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*, the NIST-Stanford-Northwestern
team 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 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
“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.
Nevertheless, as they varied a
disk’s 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’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 quantum dot
effectively runs the show.
Funded:
NIST is a
non-regulatory federal agency within the U.S.
Commerce Department's Technology Administration. NIST's mission is to
promote U.S. innovation and industrial competitiveness by advancing measurement
science, standards, and technology in ways that enhance economic security and
improve our quality of life.
In Images
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 micrometers 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.
|
