Best Lasers May Be Misanthropes
Stephanie A. Weiss
ALBUQUERQUE, N.M. -- Quantum dots are antisocial, and that's good news for Sandia National Laboratory researchers who want to make better lasers.
Quantum dots, atomic islands of material that appear on semiconductor thin films, can produce laser light. Their very small size and 3-D nature modify the fundamental photonic properties of the film. Packing more dots into a small area makes a more intense light source, and shrinking the dots reduces their wavelength into some spectral regions that semiconductor lasers can't easily reach otherwise.
The problem with quantum dots is that it's hard to create dots of uniform size and composition in orderly rows. Sandia scientist Jerry Floro has been working with researchers at the Univer-sity of Illinois at Urbana-Champaign and at Brown University in Providence, R.I., to better understand how and why dots form as they do.
Using a patented probe, the researchers "watched" while they deposited atomic layers of silicon germanium. The first 10 layers went down smoothly, but as they added more layers, pyramids buckled up from the substrate. With additional layers, the pyramids organized themselves into evenly spaced rows, occasionally "coarsening," or absorbing, neighboring dots.
The process revealed an ideal quantum dot repellant: another quantum dot. The self-organization step resembles what happens when strangers meet in an elevator: They line up in rows, each standing a little taller and thinner than usual to avoid touching a neighbor. If too many people enter this quantum elevator, individuals steadfastly maintain their space -- by killing and eating their neighbors.
Floro acknowledged that silicon germanium is not a good laser material; the group's next step is to determine how well its general physics will apply to laser materials such as indium gallium arsenide.
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