| Nov 03, 2011 |
Researchers efficiently extract photons from single semiconductor quantum dots directly into an optical fiber
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(Nanowerk News) Researchers from the NIST Center for Nanoscale Science and Technology have led the development of a new technique for efficiently out-coupling photons from epitaxially-grown quantum dots directly into a standard single-mode optical fiber ("Efficient quantum dot single photon extraction into an optical fiber using a nanophotonic directional coupler").
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Single epitaxially-grown semiconductor quantum dots are potentially bright and stable sources of "on-demand" single photons for many applications in quantum information processing and communications. However, because these quantum dots are embedded in a high-refractive index semiconductor, total internal reflection limits the photon flux escaping the semiconductor to a small fraction (< 1 %) of the original emitted light.
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The team of researchers from NIST, the University of Maryland, the University of Regensburg, and the University of Rochester has developed an approach that circumvents this limitation, resulting in a collection efficiency of 6 % into a single-mode optical fiber, with broadband spectral operation over tens of nanometers. Each quantum dot is embedded in a suspended GaAs channel waveguide with a width and thickness of about 200 nm and a length of a few micrometers.
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Rather than collecting the small amount of emission from a selected quantum dot that escapes this waveguide vertically into free-space, an optical fiber taper waveguide captures some of the larger fraction of that quantum dot's emission which is trapped in the semiconductor. The taper waveguide is a standard 125 µm-diameter, single mode optical fiber that is gradually tapered to a diameter of about 1 µm along an approximately 1 cm-long section. The two waveguides form a directional coupler, a common device used in lightwave systems to transfer power between adjacent waveguides through evanescent coupling.
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Photon correlation measurements confirm the single photon nature of the out-coupled quantum dot emission. By using single mode optical fibers typical to lightwave systems, the new technique is designed to be compatible with many quantum information processing applications.
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Finally, detailed simulations predict that the collection efficiency can be improved by an additional factor of 5 if the location of the quantum dot can be precisely controlled.
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