Single-photon sources with near-unity light-extraction efficiencies (η) are ideally required for processes relying on the transfer of quantum information between two remote stationary quantum bits, such as quantum cryptography or the development of a quantum computer. Moving toward this goal, M.E. Reimer and co-workers, from the Delft University of Technology and M.A. Verheijen and E.P.A.M. Bakkers from the Eindhoven University of Technology, The Netherlands, have recently succeeded in developing a highly efficient method of collecting single photons. As reported in the March 13 issue of Nature Communications (DOI: 10.1038/ncomms1746), the researchers have fabricated a high-efficiency single-photon source based on a nanowire with controlled shape, which they suggest will pave the way toward a single-photon source with near-unity light-collection efficiency.
A stream of single photons is directed toward the tapered nanowire waveguide tip with high efficiency (42%). The single quantum dot positioned in the middle of the nanowire couples to the fundamental waveguide mode with efficiencies exceeding 95%, such that the photons are emitted along the nanowire axis. The nanowire diameter is approximately 200 nm.
The researchers designed a tapered nanowire as a waveguide where a single InAs0.25P0.75 quantum dot was precisely positioned on its axis such that it can collect photons as they are generated. This was achieved by growing InP nanowires using low pressure metal–organic vapor-phase epitaxy (MOVPE) methods, and then synthesizing the InAs0.25P0.75 quantum dot at a specific time such that it was incorporated at approximately half the nano-wire length. Finally, the temperature was raised to suppress axial growth and favor shell formation, which allowed the nanowire geometry to be shaped to an optimum nano-wire diameter of 160–220 nm at the quantum dot position, with a tapering angle toward the tip (∼2°) (see Figure). This design overcomes the principal drawback of existing devices where quantum dots are randomly positioned in the nano-wire, leading to a drastic reduction in the collection efficiency. Furthermore, with this very small nanowire taper the researchers also avoided unwanted reflections at the semiconductor–air interface.
A final important feature of this novel nanostructure design was the integration of a bottom mirror to reflect downward emitted photons back to the nanowire tip. The researchers integrated a gold mirror at the nanowire base by transferring the nanowires into a flexible and fully transparent polymer film and coating it with the metal by evaporation. Through this novel approach the researchers were able to achieve a 20-fold enhanced single-photon emission flux.
