ETH researchers have developed an antenna for a light source using a rare material of semiconductor material. In the future, nanoscale LEDs and lasers can be successfully manufactured in this way. The rapid change and change of light is at the heart, among other things, of modern data transmission, which sends information over fiber optic cables in the form of flexible light. It has been possible for many years to reduce light modulators and put them on chips, but the light sources themselves – light-emitting diodes (LED) or lasers – are still a problem for engineers. A team of researchers from ETH Zurich led by Professor Lukas Novotny, together with colleagues from EMPA in Dübendorf and ICFO in Barcelona, has now discovered a new method that can create small and effective light in ‘the future. The results of their research were recently published outside the pages of the scientific journal Nature Materialscall_made.
Try the unexpected
“To achieve this, we must first try the unexpected,” explains Novotny. For several years, he and his colleagues have been working on small-scale lighting based on the tunnel effect. Between two electrons (gold and graphene in this case) separated by a barrier, the electron can tunnel according to the laws of quantum mechanics. Under special conditions, that is, if the process of rotation does not change ie the electrical energy is not conserved, it is possible to generate electricity.
Sotirios Papadopoulos postdoc explains: “Unfortunately, the efficiency of these light sources is not good because the radiant atmosphere is not very efficient.” This air problem is well known in other technical fields. In a cell phone, for example, the chips that generate the microwaves needed for transmission are only a few millimeters in size. In contrast, microwaves themselves are about 20 centimeters long, making them a hundred times larger than a chip. To overcome this difference in size, an antenna is needed (which in modern phones is no longer visible outside). Similarly, in the research of the Zurich researchers, the wavelength of the light is greater than the source.
Semiconductor outside the tunnel junction
“You would think that we are carefully looking for an antenna solution – but in reality, we are not,” says Papadopoulos. Like other groups before him, the researchers were studying a variety of semiconductors such as one-atom-thick tungsten disulfide sandwiched between open space electrodes to produce light in this way. In principle, one would think that the best position should be somewhere between the two electrodes, maybe a little closer to one than the other. Instead, the researchers tried something different by placing a semiconductor on top of the graphene electrode, completely outside the tunnel cavity.
Amazing air performance
Surprisingly, this seemingly mindless situation actually works. Researchers discovered the reason for this by changing the voltage applied to the junction of the tunnel and measuring the current passing through it. This measurement shows a clear resonance, which corresponds to the so-called exciton resonance of semiconductor materials. Excitons have a positively charged hole, which corresponds to a missing electron, and an electron that is attached to the hole. They can be excited, for example, by light irradiation. The exciton resonance is a clear sign that the semiconductor is not directly excited by the charge carriers – after all, no electrons pass through it – but that it absorbs the energy created in the tunnel and releases it again. In other words, it acted like an antenna.
Applications in nanoscale light sources
“At the moment the antenna is not very good because in the semiconductor the so-called dark excitons are produced, which means that little light comes out”, admitted Novotny: “To improve this will be a task us in the near future.” If researchers are successful in making the light source of the semiconductor work well, it will be possible to create a light source that measures only a few nanometers and is therefore a thousand times larger slightly more than the wavelength of light they have. Since there are no electrons passing through the semiconductor antenna, there is no negative effect that usually occurs in the range that can reduce efficiency. “In any case, we opened the door to new applications,” says Novotny. Trying the unexpected is a lost cause.
Source: ETH Zurich
