s-NEBULA by Blazon Publishing and Media Ltd

Take Terahertz for a spin Despite important developments in photo-conductive switches and quantum cascade lasers for THz generation, THz technologies are used in only a few relatively niche applications. We spoke to Dr. Romain Lebrun, the project coordinator from Thales Research Center, about the work of the s-Nebula project in developing a new approach based on spintronics and exploring the potential applications of this emerging technology. The terahertz frequency band lies between the microwave and the far infrared range, yet it remains difficult to address this frequency band, an issue that is referred to as the terahertz (THz) gap. As the coordinator of the EU-funded s-Nebula project, Dr Romain Lebrun is working to address this issue, building on earlier research in the field of spintronics. “Several years ago people found that spintronics devices could be used to generate a THz (1 THz=1012 Hz) signal. So if you send a femtosecond (10 -15 of a second) laser pulse onto a nanometer thick magnetic heterostructure, this generates a THz signal,” he outlines. This approach has proved to be effective in comparison to existing methods of generating broadband THz signals from 1-30 THz. “The amplitude of the emission from a spintronics device is not far from what can be achieved with standard photoconductive switches, which are quite efficient below 4 THz, but don’t really operate above that. Their extremely large bandwidth has no equivalent in other systems, and is thus quite promising, especially for applications in spectroscopy,” says Dr Lebrun. Terahertz radiation This work provides solid foundations for researchers in the project as they aim to develop new solutions for the generation and detection of THz radiation using spintronics physics. Part of this work is exploratory and involves developing different spintronic stacks to provide new building blocks of spin-based THz technologies such as continuous THz emission or THz detection. Another part consists in boosting the emission efficiency of THz spintronic emitters. “We have been working on the integration of THz spintronic emitters. For example, we put an infra-red cavity on top of the ferromagnet/Heavy metal bi-layers. This helps to absorb the femtosecond laser pulse into the ferromagnet,” explains Dr Lebrun. “The project’s agenda also includes engineering work to integrate these emitters on chip within a wider platform. We also try to match these properties up with some applications.” A number of potential applications of this technology were identified at the

layers,” he outlines. The more fundamental part of the project also requires the development of new materials, including investigations into anti-ferromagnetic materials. “We have been trying to understand if we can get new THz functionalities with anti-ferromagnetic materials instead of ferromagnetic materials. This is really basic science, as it’s not clear if we can apply the same physics with this new type of magnet,” acknowledges Dr Lebrun. “Whilst the efficiency remains to be largely improved, we have been able to demonstrate that we can generate narrow band THz emission with them.”

Innovative ideas

Voltage controlled THz spintronic emitters. Reproduced from G. Lezier et al., Appl. Phys. Lett. 120 152404 (2022), with the permission of AIP Publishing.

beginning of the project, for example in nondestructive testing. Technologies capable of procuding THz waves could thus be used in the automotive industries to detect the thickness of metallic layers in a vehicle. “If you send a THz signal through a metal it will be absorbed and cannot be transmitted further. This can then provide you with key information about the properties of ultrathin metallic layers,” says Dr Lebrun. The thickness of the layers can be detected with great precision, down to a resolution of less than 10 micrometers, and Dr Lebrun says this approach could brings some significant benefits over more conventional alternatives.

“There is also interest in using these types of emitters for THz imaging biosensing, as well as for ellipsommetry.” These various application areas will have different requirements in terms of THz generation and detection, and so the ability to predict the nature of the emission based on the composition of the stack would be highly valuable. One work package in the project is also dedicated to modelling, and Dr Lebrun says significant progress is being made. “We can now make quite accurate predictions. So we can see the impact of combining different layers, or integrating in standard photonic stacks including cavities or anti-refractive coating

The s-Nebula project itself is a FET Open initiative, where the primary focus is on exploring innovative idea, and to develop proof-of-concept of device functionality rather than translating research into commercial development. However, a spin-off company has already been established, offering products developed on the basis of the project’s work. “Some of our German colleagues have started a spin-off, selling a THz emitter system that could be useful for applications which needs high amplitude THz pulses,” says Dr Lebrun. There are several of these kinds of relatively niche applications of THz technologies, now researchers aim to broaden the range of applications, with a 3-day workshop planned

You can generate intense THz signals from nanometer

thick magnetic films

for later this year to heighten awareness. “We plan to gather experts from materials science, spintronics and optics fields to identify new perspectives of research, as well as engineers in order to explore the full application potential of this emerging technology,” continues Dr Lebrun. The maturity of the technology varies according to the application. The emitter intended for use in research laboratories or synchrotron based facilities is already available for example, while others are further away from practical applications. “For the fibreintegrated tip for non-destructive testing and THz imaging that is being developed in s-Nebula, I would expect that we’ll be close to Technology Readiness Level (TRL) 3 by the end of the project, which is a proof-of-concept, while with the continuous wave emitter for potential communication applications, we are still at the first observations at TRL 2,” says Dr Lebrun. Further research is planned to build on the progress that has been made in s-Nebula, with Dr Lebrun planning to focus more precisely on certain parts of the THz frequency band in future. “Currently there is no technology to efficiently emit a THz signal between 5-10 THz,” he explains. “Below 5 THz there are photoconductive switches, but between 5-10 THz there is really nothing efficient at the moment, so we will try to look in that direction in the future.”

s-NEBULA Novel Spin-Based Building Blocks for Advanced TeraHertz Applications

Project Objectives

s-Nebula is a FET OPEN project that explores and develops a revolutionary approach to TeraHertz (THz) technology, both for generation and detection of THz radiation, initiating the new field of spinbased TeraHertz (s-THz) technology, a game changer for the future of THz field. The ambition of s-NEBULA is to provide a platform of room-temperature innovative spinbased THz building blocks, arising from novel combinations of magnetism and optics.

Project Funding

The s-Nebula project has received funding from the Horizon 2020 Framework Programme of the European Commission under FET-Open Grant No. 863155. The content of this article reflects the views only of the s-Nebula Consortium, and the European Union cannot be held responsible for any use which may be made of the information contained therein.

Project Partners

Our consortium is composed of 7 european research institutions from 4 different countries (France, Germany, Sweden and Czech Republic). https://s-nebula.eu/theconsortium/

Contact Details

Project Coordinator, Romain Lebrun, PhD Msc Physics Researcher at CNRS-Thales, UMR-137 Thales Research & Technology – France (TRT-fr) 1 Av. Augustin Fresnel, 91767 Palaiseau cedex - France T: +33 1 69 41 60 78 E: romain.lebrun@thalesgroup.com E: THALES GROUP INTERNAL@sNEBULA_2020 W: https://s-nebula.eu

used in memory devices by the microelectronic industry, which brings promising prospects for terahertz technologies. Romain Lebrun, PhD

Fiber on tip spintronic terahertz emitters. Adapted with permission from F. Paries et al., Opt. Express 31, 30884-30893 (2023) © The Optical Society

EU Research

www.euresearcher.com

Romain Lebrun is a research scientist at the Thales research centre in Palaiseau, France, having previously held positions at several different academic institutions across Europe. He is a condensed matter experimentalist, and has published papers in many prominent journals.