NanoSpectroscopy of Quantum Dots

9/2/1997 - 47-97r
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Scientists in the Electronics Science and Technology Division at the Naval Research Laboratory (NRL) have developed a way to measure, with great clarity, individual quantum dots formed of the semiconductor gallium arsenide, with diameters of about 10 nanometers. A quantum dot is a crystal structure so small that its internal energy can only take on certain discrete values and is often described as the solid-state analog of an atom.

Current research on quantum dots is largely a continuation of work on higher dimensional quantum wells and quantum wires -- work that began in the 1970s and continues today. Recently, considerable research has been conducted on the optical properties of quantum dots. The current surge is fueled both by the continuing anticipation that quantum dots will improve the performance of devices such as lasers, electro-optical devices and optical memories, and recent improvements in growth techniques. Unfortunately, all current types of quantum dots suffer from relatively large fluctuations in size. Size fluctuations lead to spectral line broadening, known as inhomogeneous broadening, which blurs the spectra and severely limits the information one can obtain from optical spectroscopy. This severe inhomogeneous broadening problem has motivated the development of a new research field -- that of single quantum dot spectroscopy. Within the last three years, there has been an explosion of effort to resolve and study individual quantum dots optically.

NRL recently sponsored (July 21 and 22, 1997) a workshop entitled, "Recent Advances in the Physics of Single Quantum Dots," which brought together over 80 scientists who are active or interested in this new field. Single quantum dots are resolved spatially and spectrally using nanoscopic spectroscopic techniques such as optical near-field spectroscopy, sensitive detectors, and resonant laser excitation. By measuring the spectra of individual quantum dots instead of ensembles, it is possible to sharpen the spectra enormously. Decreases in spectral line widths of over two orders of magnitude have been recorded. Such extraordinary improvements in resolution make possible the observation of a number of phenomena for the first time.

In 1996, the NRL researchers, led by Dr. Daniel Gammon of the Electronic Materials Branch, reported the discovery of an unexpected fine structure in the spectral lines that they have interpreted as a splitting of the spin degeneracy. They went on to report hyperfine shifts in the spectral lines due to interactions between the spin of the electrons with the spins of the underlying nuclei. These discoveries again are reminiscent of the early days of atomic spectroscopy at the turn of the last century as improvements in equipment and techniques led to the measurement of fine and hyperfine structure in the spectra of atomic gases. All previous measurements in single quantum dots have measured the electronic spectra. However, the electronic properties of quantum dots, and semiconductors in general, represent only part of the picture. The nuclei that make up the underlying lattice of the quantum dot are also important. Unfortunately, conventional nuclear spectroscopy is much less sensitive than the optical spectroscopy whereby the electronic properties are usually probed.

However, now, in a paper that has just been published in the journal Science, the NRL group reports the demonstration of nuclear spectroscopy of individual quantum dots. Both the nuclear spin and the nuclear vibrations were measured at the single quantum dot level in novel examples of Raman and nuclear magnetic resonance spectroscopies. These very challenging experiments represent an improvement in sensitivity of five orders of magnitude over previous semiconductor measurements. Such nanospectroscopy opens up the possibility of measuring lattice properties, such as strain and composition, on the 10-nanometer scale with a spectral sharpness that is well beyond ensemble measurements. According to Dr. Gammon, "NRL and the Navy anticipate that advanced nanostructured materials, such as quantum dots, will lead to improvements in a variety of technologies ranging from communications to remote sensing."

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