NIST measures dipole moment of semiconductor quantum dots.NIST (National Institute of Standards & Technology, Washington, DC, www.nist.gov) The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration (www.technology. researchers have developed a new measurement technique for the characterization of semiconductor quantum dots (QDs), which are used in quantum computing quantum computing Experimental method of computing that makes use of quantum-mechanical phenomena. It incorporates quantum theory and the uncertainty principle. Quantum computers would allow a bit to store a value of 0 and 1 simultaneously. , optical communication, and quantum cryptography applications. Little detailed knowledge of the transition dipole moment The Transition dipole moment or just Transition moment, is a term usually denoted  .An oscillating electric or magnetic moment can be induced in an atom or molecule by an electromagnetic wave. 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 Dipole moment A mathematical quantity characteristic of a dipole unit equal to the product of one of its charges times the vector distance separating the charges. 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 waveguide, device that controls the propagation of an electromagnetic wave so that the wave is forced to follow a path defined by the physical structure of the guide. 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 re·flec·tiv·i·ty n. pl. re·flec·tiv·i·ties 1. The quality of being reflective. 2. The ability to reflect. 3. 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 The National Renewable Energy Laboratory (NREL), located in Golden, Colorado, as part of the U.S. Department of Energy, is the United States' primary laboratory for renewable energy and energy efficiency research and development. . 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; silverma@boulder.nist.gov. |
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