A new organic LED (OLED) structure emits single photons and could improve security for data transmission. Such single-photon-emitting light sources are a basic requirement for new encryption technologies, said Jens Pflaum, a professor at the Institute of Physics of the University of Würzburg. Pflaum developed the device with physicists from the universities of Stuttgart and Ulm. Currently, malicious parties can “fish for” data during transmission without being detected because conventional light sources such as lasers emit large quantities of identical photons. The new light source comprises standard OLED materials, is relatively easy to manufacture and can be controlled electrically. Most importantly, it operates at room temperature. Comparable optical components manufactured from semiconductor materials, such as gallium arsenide, function only at temperatures far below freezing. The new component is made of an electrically conductive layer applied to a glass-plate substrate. Next, an organic plastic matrix, with the individual color molecules embedded, is added to the substrate. The matrix is fitted with electrical contacts, which can be connected to a battery. This induces a flow of electrical current to the color molecules, stimulating them to fire single photons continuously. The physicists demonstrated this with photon-correlation measurements. The innovative component with which single photons can be produced at room temperature (red arrow) is schematically represented in the diagram (left) and shown in action (right). Electric current passes through the circular contacts, stimulating the underlying color molecules to light up. The optically active area of the component is about 2 mm in diameter. Courtesy of Benedikt Stender. There were three crucial components in the device’s success, according to Maximilian Nothaft of the University of Stuttgart. The first was selection of the right color molecules. The molecules have chemical structures in which three organic complexes are grouped around one central iridium atom. Second, the physicists had to distribute the color molecules properly within the matrix. Molecules too densely packed would have interacted, no longer emitting single photons. And third, the interface between the electrical contacts and the matrix had to be precisely designed to enable the required electrons to be injected into the polymer matrix. In this instance, the physicists used an aluminum/barium double-layer contact. Their findings appeared in the Jan. 17 issue of Nature Communications (doi: 10.1038/ncomms1637). The device could someday be used, for instance, to increase the security of online payment systems. “There are of course other fascinating examples for possible applications of single-photon emission one might think of, like quantum information processing and quantum metrology,” Nothaft said. “Another very interesting feature of phosphorescent molecules is the increased splitting of triplet sub-levels which might facilitate nanoscale sensing of magnetic fields.” The researchers will next try to reduce background emission, thereby increasing the signal-to-noise ratio of electrically driven single molecules, Nothaft said. They also hope to demonstrate triggered single-photon emission.