[Clockwise from top left] Circular grating for extracting single photons from a quantum dot.
For optimal performance, the quantum dot must be located at the center of the grating.
Image taken with the camera-based optical location technique. A single quantum dot
appears as a bright spot within an area defined by four alignment marks. Electron-beam
lithography is used to define a circular grating at the quantum dot's location. Image of the
emission of the quantum dot within the grating. The bright spot appears in the center
of the device, as desired. Credit: NIST
(August 11, 2015) Life may be as unpredictable as a box of chocolates, but ideally, you always know what you’re going to get from a quantum dot. A quantum dot should produce one, and only one, photon—the smallest constituent of light—each time it is energized. This characteristic makes it attractive for use in various quantum technologies such as secure communications. Oftentimes, however, the trick is in finding the dots.
“Self-assembled, epitaxially grown” quantum dots have the highest optical quality. They randomly emerge (self-assemble) at the interface between two layers of a semiconductor crystal as it is built up layer-by-layer (epitaxially grown).
They grow randomly, but in order for the dots to be useful, they need to be located in a precise relation to some other photonic structure, be it a grating, resonator or waveguide, that can control the photons that the quantum dot generates. However, finding the dots—they’re just about 10 nanometers across—is no small feat.
Always up for a challenge, researchers working at the National Institute of Standards and Technology (NIST) have developed a simple new technique for locating them, and used it to create high-performance single photon sources.
This new development, which appeared in Nature Communications,* may make the manufacture of high-performance photonic devices using quantum dots much more efficient. Such devices are usually made in regular arrays using standard nanofabrication techniques for the control structures. However because of the random distribution of the dots, only a small percentage of them will line up correctly with the control structures. This process produces very few working devices.