Innovative Light Source Developed

A specially designed organic light-emitting diode structure that emits single photons has been fabricated that could make data transmission more secure.

Light sources that emit single particles of light are the basic requirement for the development of new encryption technologies, said Jens Pflaum, a professor at the Institute of Physics of the University of Würzburg. He was joined in the device’s development by physicists from the universities of Stuttgart and Ulm.

The innovative component with which single photons can be produced at room temperature (red arrow) is schematically represented in the diagram (bottom) and shown in action (top). 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. (Image: Benedikt Stender)

Suitably equipped components would ensure that data can no longer be “fished for” during transmission without such process being noticed. These components might be used, for instance, to increase the security of online payment systems because any data manipulation would be immediately detected so that relevant countermeasures could be directly implemented.

Current conventional light sources such as lasers make this process impossible because the light source always emits large quantities of identical light particles, or photons.

The light source consists of standard materials for organic LEDs, is relatively easy to manufacture and can be electrically controlled. Most notably, however, is that it works at room temperature. Comparable optical components manufactured from semiconductor materials, such as gallium arsenide, function only at temperatures far below the freezing point.

Chemical structure of the iridium-based molecule used by the scientists to produce single photons. (Institute of Physics, University of Würzburg)

The new component is composed of an electrically conductive layer applied to a substrate — in this case, a glass plate. Next, an organic plastic matrix, in which the individual color molecules are embedded, is added to the layer. The matrix is fitted with electrical contacts, which can be connected to a battery. When this happens, a flow of electrical current to the color molecules is induced, stimulating them to continually fire single photons. The physicists demonstrated this with photon correlation measurements.

Three crucial components contributed to the success of the device: first, selection of the right color molecules, according to Maximilian Nothaft of the University of Stuttgart. The molecules have chemical structures in which three organic complexes are grouped around one central iridium atom. Next, the physicists had to provide a proper distribution of the color molecules within the matrix. Molecules too densely packed would have interacted, no longer emitting single photons. And lastly, the interface between the electrical contacts and the matrix had to be precisely designed to enable the required electrons — the carriers of the electric charge — to be injected into the polymer matrix in the first place. In this instance, the physicists were successful with an aluminum/barium double-layer contact.

The physicists’ findings appeared in the Jan. 17 issue of Nature Communications.

For more information, visit: www.physik.uni-wuerzburg.de