Scientists at MIT and the US Department of Energy's Los Alamos National Laboratory have demonstrated that nanoscale semiconductor particles called "nanocrystal quantum dots" offer the necessary performance for efficient emission of laser light. The research appeared 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 at Los Alamos. The MIT team was led by Moungi Bawendi, professor of chemistry.
The quantum dots used in the study were developed and synthesized at MIT and provided to the Los Alamos team. The stimulated emission was observed at both Los Alamos and MIT. The main thrust of Professor Bawendi's research is to understand the electronic, magnetic and vibrational properties of nanocrystallites, either isolated or in organized structures, using modern laser-based condensed matter optical and magneto-optical techniques.
Quantum dots are so small that quantum-mechanical effects come into play in controlling their behavior. Quantum mechanics applies in the microscopic realm but is 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 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 used by the researchers is easily manipulated through well-established chemical synthesis methods. Fabricating densely packed quantum dot arrays should be a straightforward material processing challenge, the researchers noted.
A version of this article appeared in MIT Tech Talk on October 25, 2000.