The art of giving light a different color

Red does not turn green and infrared light does not suddenly become visible when sent through a light guide. Light does not change its wavelength just like that. Unless, that is, you resort to a trick. An international research team has now been able to use this trick effectively in optical fibers for the first time. They are the first to have succeeded in functionalizing optical fibers in such a way that they transform invisible infrared light into red light. Their special fibers have the potential to be used as miniature light converters in the future. The invention is the result of collaboration between four research groups from the University of Jena's Collaborative Research Center NOA and partners at Fraunhofer IOF, Leibniz IPHT and the universities of Sydney and Adelaide (Australia). The scientists from the research group "Photonics in 2D Materials" led by Dr. Falk Eilenberger, Institute of Applied Physics at Friedrich Schiller University, have now been able to publish their research results in "Nature Photonics". Lead author is doctoral student Quyet Ngo.

Computers, cell phones, and supercomputing centers are becoming more and more powerful. Unimaginable amounts of data are processed and transported around the world in ever shorter times. The negative effect is that the energy consumption of the chips that perform the necessary computing tasks is also growing immeasurably.

In the pandemic year 2020, data centers in Germany alone consumed 16 billion kilowatt hours of energy, according to the Borderstep Institute for Innovation and Sustainability gGmbH. "The problem is that chips with common semiconductor materials consume almost 50 percent of the energy just for moving information by means of electrons. If we can find a more energy-efficient way of transporting data than using electrons, a cell phone battery could last longer before needing to be recharged," says Dr. Falk Eilenberger, head of the Photonics in 2D Materials research group at the Institute of Applied Physics at Friedrich Schiller University in Jena, describing an advantage that all cell phone users would probably like to take advantage of; not to mention the millions of kilowatt-hours that could be saved in data centers and servers through which cloud computing and video streaming operate.

New tasks for the tool light

This is one of the reasons why researchers led by Falk Eilenberger are looking for alternatives to the energy-guzzling electrons. They are relying on photons as a medium – also for data transport – and on light guides made of glass. However, simple optical fibers are not suitable for this purpose in all circumstances. They have to be specially designed and "upgraded" to take on new functions. The team in Eilenberger's research field, led by doctoral student Quyet Ngo, uses so-called 2D materials for this purpose – materials that consist of only one layer of atoms.

"Our idea was to use these new materials to change the properties of light," he explains – for example, its wavelength and thus its color. "Normally, light does not change its color," adds Falk Eilenberger. "Unless a lot of light interacts with special materials, such as certain crystals." But those are difficult to handle, he points out.

For the team from Jena, 2D materials are the better alternative. "In this particular case, we experimented with a very old material, molybdenum disulfide," Quyet Ngo shares. This has long been used as a lubricant in engine oils, he says. In Jena, the team found a way to give this material a new high-tech task: altering light.

Growing high-tech material

This meant that the researchers also had to modify the optical fibers. They resorted to specially shaped fibers developed by the team led by Prof. Dr. Markus Schmidt at IPHT in Jena and Prof. Dr. Heike Heidepriem-Ebendorff at the University of Adelaide. "These fibers are shaped like a hollow "C," which means that the light is guided more on the surface rather than in the center," Ngo explains. This facilitates the reaction of the light particles with the 2D material. Incidentally, the material is not generated separately in the experimental setup and applied to the glass fiber in a complicated process but grows directly in its cavity – like in a Petri dish.

The reactor in which this happens at around 700 degrees Celsius is located in the Institute of Physical Chemistry at Jena University. "We were able to draw on the research results of Prof. Dr. Andrey Turchanin, who developed the technology with which the novel 2D materials can be grown effectively and over large areas," says Falk Eilenberger. "Only the combination of the special fibers from IPHT with the 2D material from the Institute of Physical Chemistry and the technical solutions from the working group of colleague Eilenberger ultimately made the present result possible," adds Andrey Turchanin. Furthermore, the researchers were able to base their work on research results from the universities in Sydney and Adelaide. A total of 15 people from six institutions worked together on the topic.

"In the glass fibers, which carry an ultra-thin layer of molybdenum disulfide, we managed to convert infrared light into red light. We send the light through the fiber at a wavelength of 1240 nanometers, and it comes out at 620 nanometers at the end," Ngo explains. This makes the researchers from Jena the first in the world to succeed in functionalizing optical fibers in such a way that they can be used in the future as nonlinear light converters, for example.

New method to open up new opportunities for laser technology

Falk Eilenberger is convinced that being able to change light in this way opens up new possibilities for example in laser technology especially in Jena, where lasers are a big topic. "I think our technology will find many more applications here in the optical fiber toolbox." The advantages are obvious: the technology works at room temperature, the material is chemically robust, easy to process and offers interesting properties. It is conceivable, he adds, to grow it in multiple layers on the fibers or to modify it further to achieve more interactions with light. Quyet Ngo, who will describe the current research results in detail in his doctoral thesis, plans to explore the use of the new material in sensor technology in the future.

And as far as the problem of energy-guzzling chips mentioned at the beginning is concerned, Eilenberger and Ngo are optimistic: "What grows on glass also grows on silicon." The idea that photons instead of electrons could soon be used for digital data transfer is not a utopia for them.

(Author: Angelika Schimmel)

Original publication

Gia Quyet Ngo et al.: In-fibre second-harmonic generation with embedded two-dimensional materials, Nature Photonics 1 September 2022, DOI: 10.1038/s41566-022-01067-y.