Optical components made of air: Gas-based sono-photonics as a new platform technology

“The question we asked ourselves in the DESY Ultrafast Photonics research group was this: Does this necessarily have to involve the interaction with a solid?” says Christoph Heyl, who heads the group. “And the answer was: No, it doesn’t.”

A well-known property of light is that it can also be deflected by differences in density within gases and liquids. We see this, for example, when the sky is reflected by the warm air above the tarmac on a hot summer’s day. Heyl’s team came up with the following idea: Optical effects like these could be deliberately induced in air or in a gas in order to control light. By means of sound. Because sound waves are simply fluctuations in the pressure and density of air. So if one could impose certain density patterns on the air using intense sound waves and do this in such a way as to modify the light in the desired way, one could dispense with lenses, mirrors or other solid optical components altogether.

Versatile applications

“This would have advantages for many different applications,” says Heyl. For example, when processing materials in the field of semiconductor technology, energy technology, accelerator technology or medical technology. Especially when using high-power lasers. Because these lasers can carry so much energy that conventional optical components would soon be destroyed by the incessant bombardment. “In some areas, existing optical systems are reaching their limits,” says Heyl. “Laser-based particle accelerators, for example, require enormous levels of radiant power. And laser-based nuclear fusion, which is being studied by many groups around the world because of its tremendous potential as a future power source, also calls for a new solution.”

Especially because it is not only the optical components that suffer. The quality of the laser beam itself also deteriorates when high-power laser pulses are sent through glass or other solid materials. Gas, on the other hand, is virtually indestructible and does not reduce the brilliance of the laser beam. In addition, this contactless method of modulating light would be adaptive rather than static. This means that it is not necessary to switch the optical components in order modify the deflection of the photons; instead, one simply readjusts the gaseous optical grating during operation. “Conceivably, this could even be programmed,” says Heyl.

New acoustic transducers thanks to strong partners

The researchers soon realised, however, that in order to imprint such sound patterns on the air they would need ultrasonic transducers capable of generating unprecedented sound pressure levels. “We called various companies that manufacture acoustic transducers and asked if they could supply a device capable of producing around 160 decibels,” explains Heyl. “They thought we were pulling their leg and hung up on us.”

So instead, Heyl’s team turned to specialists in ultrasound research and asked them to develop a transducer with the necessary power. Together with a team led by Mario Kupnik from TU Darmstadt and the ultrasound equipment manufacturer Inoson GmbH in the Saarland, they succeeded in developing a transducer capable of generating more than 140 decibels at a frequency of 500 kilohertz. “That is equivalent to the sound pressure of a roaring jet engine at a distance of a few metres,” says Kupnik. It is worth noting, though, that sound at such a high frequency is inaudible to humans and animals and therefore harmless. In any case, this sound pressure level was sufficient in order to conduct proof-of-principle experiments – demonstrating the viability of the idea.

Using the resulting optical grating, consisting of air layers of different densities, the partners were able to deflect the infrared laser pulse of a 20-gigawatt laser with an efficiency of more than 50 percent. The properties of the grating, and hence the deflection, could be controlled via the frequency and intensity of the sound.

The team’s work is being funded by the Carl-Zeiss-Stiftung, among others, as part of the SOPHIMA project with 750.000 €: “The project uses a highly innovative approach to control light and has the potential to facilitate the application of photonics in life science technologies and many other fields,” says Felix Streiter, managing director of the foundation. Aalen University, the University of Hamburg and the Helmholtz Institute Jena are also involved in the project.

“Of course, we want to optimise the method further and achieve efficiencies of over 90 percent in modulating the light, not least by using even more powerful transducers,” says Christoph Heyl. But even when the proof-of-principle study was published, numerous parties already expressed an interest, including members of industry. So the demand for such a technology is very high.

Game changer for research and industry

The researchers involved realise that gas-based sono-photonics could become a game changer for research and industry if it continues to live up to its promises. It represents a completely new approach to optics, a possible platform technology with applications in a wide range of future markets, from laser technology to new materials and medicine – the potential seems virtually unlimited. “This method of controlling laser beams operating at maximum optical power offers excellent prospects for many of our fibre-based products, for example in the field of power generation through laser fusion,” says Thomas Theeg, CEO of FiberBridge Photonics, a company that manufactures fibre-optical components. And Björn Betz of the Fraunhofer Institute for Photonic Microsystems (IPMS) anticipates “exciting new possibilities for a truly interdisciplinary field that combines optics with acoustic technologies. This offers excellent opportunities for opening up new markets.”

Which makes it all the more important to get industry partners involved at this early stage. Especially those who not only want to develop the new technology, but also apply it. “We primarily do fundamental research,” says Heyl. “Although the research and development we do for many projects is indeed application-oriented, industrial partners can bring a completely different perspective to the table. Especially in terms of the cost efficiency and user-friendliness of the technology being developed.” When it comes to the benefits to society and favourable market opportunities, these aspects are just as important as technical functionality. Conversely, it is of course very interesting for companies to be involved in the development of a new technology right from the start, so as to be among the first movers and gain a competitive advantage.

Hi-Acts brings together the right researchers and industrial partners

As a means of establishing such productive collaborations for the new project and thus developing gas-based sono-photonics to market maturity, Christoph Heyl’s group found just what they were looking for in Hi-Acts. Because this is precisely what the innovation platform is all about: bringing together researchers and suitable industrial partners at an early stage and organising the interdisciplinary exchange between them. “In June 2024, a colleague drew my attention to Hi-Acts and said I should have a chat with their innovation manager Johannes Blum,” Heyl recalls.

The two sat down together and discussed the benefits of working together on the new project: Hi-Acts offers scientists many opportunities to present their topics and innovative developments to the world at large, and to win over new partners for co-operation – especially from an application-oriented and industrial setting. These include network meetings, in-house webinars and other events held in collaboration with industry networks or clusters. The Hi-Acts team also knows its way around public funding programmes and even offers funding itself, for example through its Use Case Initiatives. For the upcoming work on gas-based sono-photonics, for example, the platform is helping to fund a post-doctoral position. “Financial resources – whether from companies or public authorities – are always very helpful, of course, in accelerating the development of a new technology,” says Heyl.

However, even just exchanging ideas with industry partners can be extremely fruitful and is therefore very important. On the one hand, it is easier to raise the necessary funds when working together. On the other hand, the time to market can be reduced: “The earlier we bring such partners on board and the better we understand possible applications and related aspects, the better we can adapt individual developments,” says Heyl.

Ideally, it will only be a few years before the first systems to use gas-based sono-photonic techniques are put into actual operation. Perhaps even at DESY itself: One initial field of application could be laser-plasma accelerators, which are currently being developed at DESY closely assisted by Hi-Acts. These ultra-compact yet very high-energy electron accelerators would be extremely useful in both research and medicine. The problem so far has been the enormously high electron flux required for such accelerators, which cannot yet be provided by current laser technologies,” reports Heyl. This problem could perhaps be solved by stacking the laser pulses on top of each other – so to speak. However, this calls for the already very powerful pulses to be guided with great precision. And that is exactly what gas-based sono-photonics may soon be capable of doing.