Researchers at ETH Zürich, Empa, ICFO and Japan's National Institute for Materials Science have created an antenna for light sources on a chip using an unusual placement of a semiconductor material. In the future, efficient nanoscale LEDs and lasers could be produced in this way.
Modern data transmission is based, among other things, on the fact that information can be sent quickly through optical fibers in the form of light beams.
The light sources are technologically challenging. The researchers at in the recent study have laid the foundations for tiny, efficient light sources. For their mini light sources, the researchers use rules of quantum mechanics and a surprising antenna solution.
The fast switching and modulation of light is at the heart of modern data transfer, in which information is sent through fibre optic cables in the shape of modulated light beams. It has been possible for several years now to miniaturize light modulators and to integrate them into chips, but the light sources themselves - light emitting diodes (LEDs) or lasers - can still be problematic for engineers. The team of researchers has found a new mechanism by which tiny but efficient light sources could be produced in the future.
"To achieve this, we first had to try the unexpected", says ETH Zurich's Prof. Lukas Novotny. For several years he and his coworkers have been working on miniature light sources that are based on the tunnel effect. Between two electrodes (made of gold and graphene) separated by an insulating material, electrons can tunnel according to the rules of quantum mechanics. Under particular circumstances - that is, if the tunnel process is inelastic, meaning that the energy of the electrons is not conserved - light can be created.
"Unfortunately, the yield of those light sources is rather poor because the radiative emission is very inefficient", explains postdoc Sotirios Papadopoulos. This emission problem is well-known in other areas of technology. In mobile phones, for instance, the chips that create the microwaves needed for transmission are only a few millimeters in size. By contrast, the microwaves themselves have a wavelength of around 20 centimeters, which makes them a hundred times larger than the chip. To overcome this difference in size an antenna is needed. Also, the wavelength of the light is much larger than the light source.
"One might think, then, that we were consciously looking for an antenna solution - but in reality we weren't", says Papadopoulos. Like other groups before them, the researchers were investigating layers of semiconductor materials such as tungsten disulfide with a thickness of a single atom sandwiched between the electrodes of the tunnel junction in order to create light in this way. In principle one would assume that the optimal position should be somewhere between the two electrodes, maybe a little closer to one than to the other. Instead, the researchers tried something completely different by putting the semiconductor on top of the graphene electrode - completely outside the tunnel junction.
Surprisingly, this seemingly illogical position worked very well. The researchers found out the reason for this by varying the voltage applied to the tunnel junction and measuring the current flowing through it. This measurement showed a clear resonance, which matched a so-called exciton resonance of the semiconductor material. Excitons are made of a positively charged hole, which corresponds to a missing electron, and an electron bound by the hole. They can be excited, for instance, by light irradiation. The exciton resonance was a clear sign that the semiconductor was not excited directly by charge carriers - after all, there were no electrons flowing through it - but rather that it absorbed the energy created in the tunnel junction and subsequently re-emitted it. In other words, it acted very much like an antenna.
"For now, this antenna is not very good because inside the semiconductor so-called dark excitons are created, which means that not much light is emitted", Novotny concedes: "Improving this will be our homework for the near future". If the researchers are successful in making the light emission by the semiconductor more efficient, it should be possible to create light sources that measure only a few nanometers and are, thus, a thousand times smaller than the wavelength of the light they produce. As there are no electrons flowing through the semiconductor antenna, there are also none of the unwanted effects that typically occur at boundaries and that can reduce the efficiency. "In any case, we have opened a door to new applications", says Novotny.