Scientists at the U.S. Department of Energy's Los Alamos National Laboratory and Massachusetts Institute of Technology have demonstrated that nanoscale semiconductor particles called "nanocrystal quantum dots" offer the necessary performance for efficient emission of laser light. The research appears in the October 13 issue of Science.
The demonstrated performance opens the door for developing novel optical and optoelectronic devices, such as tunable lasers, optical amplifiers, and light emitting diodes, from assemblies of these invisibly small particles.
"Our results provide a proof-of-principle and should motivate the development of nanocrystal quantum-dot-based lasers and amplifiers," said Los Alamos' Victor Klimov, who led the research effort.
Quantum dots are so small that quantum mechanical effects come into play in controlling their behavior. Quantum mechanics apply in the microscopic realm but its effects are largely unseen and unfelt in our macroscopic world.
Quantum dot lasers work like other semiconductor lasers, such as those found in home-audio compact disc players. Just as in the semiconductor laser chip in a CD player, the goal of a quantum dot laser is to manipulate the material into a high energy state and then properly convert it to a low energy state. The result is the net release of energy, which emerges as a photon.
The challenge, however, is that there are competing mechanisms by which the energy can be released, such as vibrational energy or electron kinetic energy. In quantum dots, the electrons are confined within a very small volume that forces them to strongly interact with each other. These strong interactions can lead to deactivation of the dot through the so-called "Auger process," preventing it from emitting a photon.
The Los Alamos-led researchers examined quantum dots formed of several types of crystalline material. They showed that the quantum dots exhibit sufficiently large optical gain for stimulated emission to overcome the nonradiative Auger process. Stimulated emission, or lasing, was only possible, however, when the dots were densely packed in the sample.
Quantum dots offer this performance over a range of temperatures, making them suitable for a variety of applications, and also can be "tuned" to emit at different wavelengths, or colors. The emission wavelength of a quantum dot is a function of its size, so by making dots of different sizes scientists can create light of different colors.
The quantum dot material Klimov and his colleagues worked with is easily manipulated through well-established chemical synthesis methods. Fabricating densely packed quantum dot arrays should be a straightforward material processing challenge.
In addition to Klimov, the Los Alamos team of researchers included postdoctoral fellows Jennifer Hollingsworth, Alexander Mikhailovsky, Su Xu and student researcher Anton Malko. The MIT team was led by Professor Moungi Bawendi.
In Los Alamos, the quantum dot research was funded by the Laboratory Directed Research and Development Program.
The Los Alamos group conducted most of its research prior to this summer's Cerro Grande Fire, which hit the group very hard. Several postdoctoral researchers on the team lost data and equipment in the fire and much of Klimov's optics equipment was damaged.
Klimov joined Los Alamos in 1995. He received his master's and doctoral degrees from Moscow State University and has worked there and at the Moscow Institute of Geodesy, first as an assistant professor and later as an associate professor. In 1993, for his pioneering studies of semiconductor quantum dots he was awarded the highest Russian academic degree, Dr. of Sciences. Klimov was also awarded the 2000 Los Alamos National Laboratory Fellows Prize for his recent work on nanocrystal quantum dots.
Bawendi Quantum Dot Research
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New Composite Material Goes Negative On Physics
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