Scientists from Stanford University have managed to made a significant step in the field of nanophotonics. They found a way to increase the intensity of terahertz electromagnetic waves, while also showing another way in which these waves could be controlled. They published their discovery in the prestigious journal Science. Among the authors are two scientists from the Faculty of Mechanical Engineering of the Brno University of Technology, who supplied the theoretical model and calculations necessary for the research. Terahertz waves may be of interest in the future, for example, for the development of communication technologies.

Nanophotonics is a field dedicated to investigating how light interacts with nanostructures whose dimensions range from tens to hundreds of nanometers. "The theory of the electromagnetic field was completed in the second half of the 19th century, when the idea of an arbitrarily small dose of energy that a substance can absorb or emit was born. The area of investigation of the electromagnetic field in the visible region of the spectrum is called optics. Since the beginning of the 20th century, we have known that field energy can only be transferred in multiples of a very small "dose" called a photon. Let's think of them as packets of energy that we can transmit or receive from matter. Of course, other quantum phenomena also play a role at this level. In the second half of the 20th century, research gradually focused on gradually smaller areas, which is why we now call this science nanophotonics," explains Radek Kalousek, a physical engineer from BUT.

One of the interesting topics of nanophotonics is to use an electromagnetic field to disrupt the "calm" electrons in metals, in this case gold nanostructures surrounded by titanium oxide. The experimental part of the published research is the work of scientists of Stanford University. Two years ago, Radek Kalousek and his colleague Martin Hrtoň were research fellows at this prestigious American university in the team of Professor Mark L. Brongersma . "It was an amazing experience. Our colleagues paid a lot of attention to us, there is a very open atmosphere," Kalousek praises.

It soon became clear that the focus of the fellow researches from BUT was very useful for them: while the Stanford team is at the forefront of experiments, colleagues from Brno could contribute their know-how as theoretical physicists. "Our contribution was in the field of theory and modelling. We thought about how electrons behave, how they absorb energy from light and what happens to them next," Kalousek describes. It is thanks to this stay that Brno physicists are now part of the success published in the prestigious journal Science.

(In)Visible Wave

The field of nanophotonics is very demanding for experimental research, as the problem it faces is the weak signal. In layman's terms: researchers who are trying to detect only a few photons receive a variety of parasitic noises from the environment interfering with their measurements. On such small scales, it is easy for the signal they are looking for to disappear in such noise.

One of the tasks was to observe the electric field in an electromagnetic wave itself, which is quite difficult. "Imagine tying a rope to a tree and holding the other end in your hand. Then you wave your hand and see a wave running down the rope and coming back to you. This is because you are observing the flickering with your eyes in terms of light in a very slow perspective. On the other hand, the light itself, or the electromagnetic field, oscillates so fast that we do not have instruments to monitor its deflections in the visible part of the spectrum. We can only observe its consequences – as in the case of the rope – where if you were blindfolded, you would only know that the pulse has returned by the rope twitching your hand," Kalousek explains.

An article published in the journal Science describes a way to deal with these problems. "Our colleagues at Stanford have managed to figure out how to increase the intensity of terahertz waves. At the same time, they can, to some extent, control the electric current that generates the terahertz wave, at a level well below the diffraction limit. In this way, it is possible to achieve an effect that existing methods do not allow: not only are we able to map the waveform, but also to shape it spatially. And it is the control over the resulting wave that represents a fact that also has application potential," says physicist Martin Hrtoň.

The article in the journal Science can be accessed on the web here.

Kalousek calls the discovered method of generating terahertz waves very important, but at the same time he adds that all this enthusiasm is still reserved "only for physicists". "The path between basic research and application is terribly long. In the future, terahertz waves, which have a thousand times higher frequency than the gigahertz used by today's technologies in computers, for example, could be interesting. Waves could be used to investigate the properties of materials, or to transmit information. However, we are still working in the field of basic research," adds Hrtoň.

In addition to their achievement, the pair of Brno scientists also brought a new perspective from overseas. "The strategy of the scientists there is that they try to publish their discoveries in the best journals. Although the risk of failure is greater, the effort put in is de facto the same as for average titles. And sometimes it just works out. I think this should be an inspiration for us: let's not be afraid to try the most prestigious scientific forums, let's be more ambitious. I believe that we have all the prerequisites for this," concludes Kalousek.