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NIST measures dipole moment of semiconductor quantum dots.

NIST researchers have developed a new measurement technique for the characterization of semiconductor quantum dots (QDs), which are used in quantum computing, optical communication, and quantum cryptography applications. Little detailed knowledge of the transition dipole moment of QDs previously had been available, although it is critical in determining gain for QD laser design and in implementing the coherent manipulation of the QD state in quantum logic gates that may be part of future-generation optical computers.

A NIST researcher developed a new technique to directly measure the dipole moment of QDs and has applied it to self-assembled InGaAs/GaAs dots. It relies on the measurement of pulses of light that exit an optical waveguide containing the QDs as the light reflects multiple times from the waveguide facets. Light that is coupled out of the waveguide is mixed with a variably delayed gating pulse in a non-linear crystal to time resolve the output. The absorption co-efficient is determined by comparing the energy of successive pulses and taking into account the measured waveguide facet reflectivity and background absorption, as determined from measurements of a waveguide that does not contain QDs. The dipole moment is derived from the ground state absorption and the QD areal density, which was determined from transmission electron micrographs generated by collaborators at the National Renewable Energy Laboratory. Quantum dot density variation in the samples proved to be the largest source of uncertainty in the measurement.

The NIST dipole moment measurement technique overcomes the large uncertainties of other measurement techniques which were based on threshold currents of laser diodes, had large background material absorption, or had difficulty estimating coupling efficiency into and out of the QD region. Techniques are now being developed to directly measure the absorption of a single, isolated quantum dot.

CONTACT: Kevin Silverman, (303) 497-7948;
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Title Annotation:News Briefs
Publication:Journal of Research of the National Institute of Standards and Technology
Date:Sep 1, 2003
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