Spin waves as the key to more sustainable AI: Ondřej Wojewoda makes a breakthrough in measurement using Brillouin spectroscopy of light

The growing influence of artificial intelligence brings with it huge energy demands, which in turn bring large-scale climate challenges. Ondřej Wojewoda from CEITEC BUT brings a revolutionary solution in his PhD thesis: using Mie resonances in dielectric materials, he has developed a method that enables efficient measurement of short spin waves using common laboratory equipment. Ondřej’s discovery brings a fundamental shift in the availability of spin wave research, which has the potential to become a key platform for energy-efficient data transmission and processing with applications in AI, but also in medicine and mechano-biology.

Artificial Intelligence (AI) has experienced an extraordinary boom in recent years, and its potential to fundamentally change and streamline human activities is undeniable. A groundbreaking example of this development is the emergence of large-scale language models (LLMs) such as ChatGPT or Gemini. However, with their rise comes a major problem: the enormous energy consumption, which represents a significant obstacle to meeting global climate goals, especially in terms of water consumption – the problem being the heating, which necessitates the associated cooling of infrastructure, requiring up to a litre of water for just a few prompts.

One promising way to reduce the energy intensity of AI, especially LLM, is to move away from the classical von Neumann architecture to new technologies using wave systems. In this context, spin waves seem to be a promising candidate – they allow easy integration into existing technologies and provide natural nonlinearity, crucial for the proper functioning of AI (in current implementations, this function is provided by activation functions). However, the main challenge for implementing spin waves in functional chips is their miniaturization. Short spin waves with lengths below 400 nm can currently only be measured using synchrotrons, which is expensive, time consuming, and therefore slows down further development in this area.

CEITEC: Spin waves as the key to more sustainable AI: Ondřej Wojewoda makes a breakthrough in measurement using Brillouin spectroscopy of light

Under the guidance of Michal Urbánek, head of the CEITEC Nano research infrastructure, Ondřej Wojewoda has been working on this problem as part of his PhD together with his colleagues. In his PhD thesis, Ondřej developed a new methodology that addresses this problem by exploiting so-called Mie resonances in dielectric materials and allows these short spin waves to be measured using standard laboratory equipment called Brillouin Light Spectroscopy (BLS). This technique not only paves the way for the further development of spin waves as an energy-efficient technology for AI, but also finds applications in other fields. With the theoretical description and practical implementation of Mie resonances in Ondřej’s work, this technique has the potential to significantly contribute to advances in medical and mechano-biological applications, where BLS is currently experiencing a tremendous boom, for example, in the early diagnosis of eye defects.

“Spin waves (magnons) have the potential to be used as a new platform for data transmission and processing because they can reach wavelengths in the nanometer range and frequencies from hundreds of megahertz to units of terahertz. However, until now, it has only been possible to image spin waves with wavelengths below the diffraction limit of light using X-ray microscopy with large particle accelerators,” says Ondřej, introducing the context of his work. In Europe, for example, synchrotron accelerators able to measure spin waves are only in two places – Berlin and Zurich. This limitation makes research and development of spin wave devices much slower as well as more expensive. With this in mind, during his PhD studies, Ondřej looked for ways to overcome this problem.

“By exploiting Mie resonances in dielectric structures, we can measure spin waves with comparable wavelengths using a standard optical setup designed for Brillouin light scattering microscopy measurements,” says Ondřej. He studied the whole process on a silicon disk placed on a nickel-iron layer. The technique can be adapted to obtain resolution for wavelengths as short as 50 nm in the sample plane using an array of silicon strips with a subdiffraction period. In the last part of his thesis, Ondřej focused on the measurement of coherently excited spin waves, where he demonstrated phase resolution by measuring the dispersion relation of the spin waves.

CEITEC: Spin waves as the key to more sustainable AI: Ondřej Wojewoda makes a breakthrough in measurement using Brillouin spectroscopy of light

How did Ondřej’s research actually proceed? When choosing the topic for his PhD, Ondřej saw the use of plasmonic resonances in Brillouin scattering as natural because Brillouin scattering is similar to Raman scattering, and in Raman scattering, plasmonic resonances are often used to amplify the signal.

However, since Ondřej and his colleagues were new to Brillouin scattering, they were unaware that several teams had been made to try a similar approach before, to no avail. On their first attempt, however, the team obtained excellent results, which, unfortunately, they were unable to repeat. “Thanks to the advanced nanocharacterisation capabilities available at CEITEC Nano, we discovered that the samples we did not measure metal particles as we had originally planned, but instead, a silicon "mess" that has inadvertently fallen on our samples,” explains Ondřej.

This discovery led Ondřej to create new silicon resonators with which he and his colleagues were the first to not only amplify the signal but also to extend the detection capabilities of Brillouin scattering to very small wavelengths below 50 nm. The results presented have the potential to fundamentally change the research paradigm in solid-state physics and mechano-biology.

Ondřej’s method is replicable, and similar results can now be achieved by teams around the world – all they need is a nanofabrication lab and an optical setup to measure Brillouin light scattering. This is a major advance; as we mentioned above, previously, scientists were only able to image spin waves at the nanometer level in synchrotron accelerators, which are space and cost intensive. Using the BLS to measure spin waves thus makes magnonics a much more competent field.

At the end of last year, Ondřej received the Hlávka Foundation Award for his outstanding academic achievements and exceptional contribution to the field of laser spectroscopy. The Hlávka Foundation, the oldest Czech foundation founded on 25 January 1904 by architect Josef Hlávka, supports science, literature, art, and gifted students at Czech universities. It awards the Josef Hlávka Prize to the best students, graduates and young scientists. Six students of Brno University of Technology received the Hlávka Foundation 2024 Prize, together with a financial reward, at the Lužany castle.

Ondřej Wojewoda continues to work at CEITEC BUT. Thanks to the Postdoc Individual Fellowship grant from GAČR, he is now preparing for a two-year postdoctoral fellowship at the prestigious Massachusetts Institute of Technology (MIT), where he intends to research non-reciprocal magnetic materials for use in microwave technology.

CEITEC: Spin waves as the key to more sustainable AI: Ondřej Wojewoda makes a breakthrough in measurement using Brillouin spectroscopy of light