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Quantum Dots Break 'Artificial Atom' Model

Photonics Spectra
Feb 2004
Daniel S. Burgess

A worldwide collaboration of scientists investigating the electronic states of quantum dots by the photons they emit has determined that the conception of the dots as "artificial atoms" is incomplete. The group included researchers from Ludwig Maximilians Universität in Munich, Germany; Heriot-Watt University in Edinburgh, UK; Ohio University in Athens; the Russian Academy of Sciences in Novosibirsk; Instituto de Microelectrónica de Madrid; and the University of California, Santa Barbara.

Quantum Dots Break 'Artificial Atom' Model
The photoluminescence spectra as a function of the magnetic field of a triply charged InAs quantum dot reveal that dots do not behave like "artificial atoms." The dot has three bound electrons. The colors represent the intensity of the photon emission, with blues being less intense and reds being more intense. At a magnetic field strength of zero, the emission spectra show two sharp lines. Above 2 T, oscillations appear that indicate a signature of the continuum of energies well above those of the bound states.

Khaled Karrai of the Center for NanoScience at Ludwig Maximilians and Richard J. Warburton of Heriot-Watt explained that semiconductor quantum dots have been regarded as artificial atoms because they display sharp, discrete spectral lines when the confined electrons and holes that are analogous to the quantum energy levels of atoms recombine. In earlier work, the team examined the behavior of dots with singly, doubly or triply charged excitons and discovered that they conformed to that of artificial hydrogen, helium and lithium atoms.

The researchers therefore were surprised to observe that when quantum dots with triply charged excitons are in a strong magnetic field, they display oscillating photoluminescence spectra indicative of electrons interacting with the continuum of states above the quantized energy levels. In atoms, such electrons are not bound to the nucleus and do not affect the optical transitions between the lower levels. Quantum dots thus seem to play by different rules.

"We did not expect that investigating the optical emission of an isolated quantum dot could tell us much about the continuum well above the bound quantized states," Karrai said. "Well, against all expectations, as it turned out, a spectacularly clear signature of the energy spectrum of the continuum was observed in the optical emission of charged quantum dots."

To investigate the phenomenon, the scientists subjected a sample of 40-nm-across InAs quantum dots grown on GaAs to magnetic fields of –9 to 9 T at a temperature of 4.2 K. To observe the photoluminescence, they employed a miniaturized confocal microscope designed by the teams in Munich and Edinburgh. The microscope operates at temperatures between 1.6 and 300 K and is machined entirely from titanium to resist magnetically induced perturbations. Karrai noted that the instrument is so stable that it enables the investigation of a single quantum dot for months at a time without drifting out of focus. A standard, low-power 830-nm laser diode served as an excitation source, and a Roper Scientific back-illuminated, nitro-gen-cooled silicon CCD camera collected the response through a 300-mm-focal-length grating monochromator.

They theorize that after a triply charged dot is stimulated and emits a photon, it is left with three charges. Those charges may take two configurations, in one of which the charge experiences the continuum above the conduction electron levels, so that the energy states hybridize. In the absence of the magnetic field, Karrai said, quantum mechanics conspire to favor the configuration in which the charge does not explore the continuum, but the application of the magnetic field enables the dot to achieve this state.

In fact, there is nothing in the researchers' calculations that suggests this can only be true in quantum dots. "In principle, one could expect a similar effect from atoms in their excited state," Karrai said. The magnetic fields required to achieve this state, however, might be many orders of magnitude higher. "It turns out that the material parameters for quantum dots are much more favorable to the observation of this effect than in atoms."

The group next plans to explore the interaction of such a quantum dot and a continuum filled with electrons, which Karrai suggested might lead to the observation of new phenomena involving quantum interferences. Further work will examine whether the polarization of the photons from the excitation source may be used to control the spin of the electrons in the dots, enabling applications in spintronics.


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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