(Nanowerk News) Fast switching and modulation of light are at the heart of, among other things, modern data transfer, in which information is sent over fiber-optic cables in the form of modulated beams of light. It’s been possible for several years now to miniaturize light modulators and integrate them onto chips, but the light sources themselves – light-emitting diodes (LEDs) or lasers – still pose a problem for engineers. A research team at ETH Zurich led by Prof. Lukas Novotny, together with his colleagues at EMPA in Dübendorf and at ICFO in Barcelona, have now devised a new mechanism by which small but efficient light sources can be produced in the future.
The results of their research were recently published in a scientific journal Natural Ingredients (“Exciton-assisted electron tunneling in van der Waals heterostructures”).
Try the unexpected
“To achieve this, we first had to try the unexpected,” says Novotny. For several years he and his co-workers had been working on a miniature light source based on the tunnel effect. Between two electrodes (made of gold and graphene in this case) separated by an insulating material, electrons can tunnel according to the rules of quantum mechanics. Under certain circumstances – that is, if the tunneling process is inelastic, meaning that the energy of the electrons is not conserved – light can be created.
“Unfortunately, the output from the light source is rather poor because the emission of radiation is very inefficient,” explains postdoc Sotirios Papadopoulos. This emission problem is well known in other fields of technology. In cell phones, for example, the chip that generates the microwaves needed for transmission is only a few millimeters across. In contrast, the microwave itself has a wavelength of about 20 centimeters, which makes it a hundred times larger than a chip. Compensating for this difference in size requires an antenna (which in modern cell phones is actually no longer visible from the outside). Similarly, in the Zurich researchers’ experiment the wavelength of light is much greater than that of the light source.
Semiconductors outside the tunnel junction
“So one might think that we are consciously looking for an antenna solution – but in reality we are not”, says Papadopoulos. Like other groups before it, the researchers are investigating layers of a semiconductor material such as tungsten disulfide with a single atom thickness sandwiched between tunnel junction electrodes to create light in this way. In principle one would assume that the optimal position should be between the two electrodes, maybe a bit closer to one than the other. Instead, the researchers tried something completely different by placing a semiconductor over graphene electrodes – completely outside the tunnel junction.
Surprising antenna action
Oddly enough, this seemingly absurd position works surprisingly well. The researchers discovered the reason by varying the voltage applied to the tunnel junction and measuring the current flowing through it. This measurement shows a pronounced resonance, which matches the so-called exciton resonance of a semiconductor material. Excitons are made of positively charged holes, which correspond to the missing electrons, and electrons that are bound by those holes. They can be aroused, for example by shining a light. Exciton resonance is a clear sign that the semiconductor is not directly excited by charge carriers – after all, no electrons are flowing through it – but rather absorbs the energy created at the tunnel junction and then re-emits it. In other words, it acts a lot like an antenna.
Applications in nanoscale light sources
“For now, this antenna is not very good because it is inside a semiconductor that a so-called dark exciton is made, which means that not much light is emitted”, Novotny admits: “Improving this will be our homework in the near future”. If the researchers succeed in making light emission by semiconductors more efficient, it may be possible to create light sources that are only a few nanometers in size and, thus, a thousand times smaller than the wavelength of the light they produce. Since no electrons flow through the semiconductor antenna, there are also no unwanted effects that normally occur at the limit and can reduce efficiency. “After all, we have opened the door for new applications”, said Novotny. Trying unexpected things turned out to be fruitful.