A quantum dot (QD) is a nanometer-scale inclusion of one semiconductor inside a second semiconductor with a larger bandgap. Electrons and holes in the QD can occupy only a given set of states with discrete energies, as in an atom. QD's can thus be used to do “atomic physics” experiments in the solid state.; For example, the spontaneous emission rate from a QD can be enhanced by placing the dot inside a resonant optical cavity. We grew InAs/GaAs QD's by strain-driven self assembly in molecular beam epitaxy. Above and below the dots, we deposited reflectors consisting of alternating quarter-wavelength thick layers of AlAs and GaAs. The sample was then plasma etched into microscopic posts. Light in the posts is trapped laterally by total internal reflection and longitudinally by the reflectors. We have seen significant emission-rate enhancement for a single QD inside such a micropost microcavity.; A single QD can also serve as a source of single photons “on demand.” Electrons and holes are introduced into the dot by pumping with a pulsed laser. For each pump pulse, the last photon emitted from the dot has a frequency different from that of all the other photons. This photon can thus be isolated by spectral filtering. When the QD is coupled to a micropost microcavity, the majority of these photons are captured by the cavity mode. We have used this technique to demonstrate an efficient source of single photons. This device has potential applications to quantum cryptography, quantum computation, and other areas.
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