Modern data transfer, in which data is conveyed through fiber optic cables in the form of modulated light beams, is made possible by the quick switching and modulation of light. Although light modulators can now be made smaller and integrated into chips, engineers still have difficulty working with light sources like lasers or light-emitting diodes (LEDs).
Researchers at ETH Zurich, under the direction of Prof. Lukas Novotny, along with associates at EMPA in Dübendorf and at ICFO in Barcelona, have recently discovered a new mechanism that could one day be applied to manufacture small yet effective light sources. Their study’s findings were only recently published in the academic journal Nature Materials.
Trying the Unexpected
To achieve this, we first had to try the unexpected.
Lukas Novotny, Professor, ETH Zurich
He has been working on small light sources based on the tunnel effect with his teammates for several years. According to the principles of quantum mechanics, electrons can tunnel between two electrodes (in this example formed of gold and graphene) separated by an insulating substance.
Specifically, light can be produced if the tunnel process is inelastic, which means that the electrons’ energy is not conserved.
Unfortunately, the yield of those light sources is rather poor because the radiative emission is very inefficient.
Sotirios Papadopoulos, Postdoctoral Researcher, ETH Zurich
In other technological fields, this emission issue is well-known. The chips used in mobile phones, for example, are only a few millimeters in size and produce the microwaves required for transmission. The microwaves themselves, in comparison, are a hundred times bigger than the semiconductor and have a wavelength of about 20 cm.
An antenna is required to compensate for this size disparity (although, in current phones, the antenna is no longer visible from the outside). The wavelength of the light is also substantially bigger than the light source in the experiments of the Zurich researchers.
Semiconductor Outside the Tunnel Junction
Papadopoulos added, “One might think, then, that we were consciously looking for an antenna solution—but in reality, we weren’t.”
The researchers, like previous groups, were researching layers of semiconductor materials such as tungsten disulfide with a single atom thickness sandwiched between the tunnel junction electrodes to create light in this manner.
In theory, the ideal position should be somewhere between the two electrodes, perhaps a little closer to one than the other. Instead, the researchers tried something altogether different by placing the semiconductor on top of the graphene electrode, outside of the tunnel junction.
Surprising Antenna Action
Surprisingly, this seemingly counterintuitive position performed exceptionally well. The cause for this was discovered by the researchers by adjusting the voltage applied to the tunnel junction and monitoring the current flowing through it. This measurement revealed a distinct frequency, which corresponded to the semiconductor material’s so-called exciton resonance.
Excitons are made up of a positively charged hole that represents a missing electron and an electron that is bound by the hole. Light irradiation, for example, can excite them.
The exciton resonance was a strong indication that the semiconductor was not activated directly by charge carriers—after all, there were no electrons flowing through it—but rather that it absorbed and re-emitted the energy released in the tunnel junction. In other words, it functioned similarly to an antenna.
Applications in Nanoscale Light Sources
Novotny further stated, “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. Improving this will be our homework for the near future.”
It should be able to develop light sources that measure only a few nanometers and are thus a thousand times smaller than the wavelength of the light they produce if the researchers are successful in increasing the efficiency of the light emission by the semiconductor.
The semiconductor antenna does not have any electrons flowing through it, so it also does not have any undesirable effects that frequently happen at boundaries and can lower efficiency.
Novotny concluded, “In any case, we have opened a door to new applications.”
Evidently, attempting the unusual has been successful.
Wang, L., et al. (2023) Exciton-assisted electron tunnelling in van der Waals heterostructures. Nature Materials. doi:10.1038/s41563-023-01556-7.