Periodic Reporting for period 2 - s-NEBULA (Novel Spin-Based Building Blocks for Advanced TeraHertz Applications)
Reporting period: 2021-01-01 to 2022-06-30
s-Nebula develops terahertz technologies based on magnetic materials and spintronic phenomena that can have an impact on high frequency communications, non-destructive testing and ellipsometry. The project gathers expertise from France, Germany, Sweden and Czech Republic to face challenges in materials growth, terahertz spectroscopy, device modelling and evaluation as well as technological integration for industrialisation.
The TeraHertz (THz) frequency band represents a spectral window that offers rich opportunities for advanced applications in many fields, e.g. industrial quality control, spectroscopy, imaging, medical diagnostics, security, telecommunication and high-speed electronics. However, the development of THz technology is currently hampered by the limitations of available technological paradigms and there is an urgent need for a radically new THz technological framework. Recently, starting with pioneering work of consortium members, optically-driven spin based THz (s-THz) emitters were demonstrated, based on optically triggered spin currents and spin-to-charge conversion. A typical s-THz emitter is a nanometer-thick metallic heterostructure consisting of ferromagnetic (FM) and strongly spin-orbit-coupled nonmagnetic (NM) materials (see Fig 1). Pumping the s-THz emitter by a fs laser pulse leads to ultrafast demagnetization, followed by the emission of spin-current pulse converted by the inverse spin-Hall effect into an electric-dipole type emission of a broadband THz pulse (> 20 THz) into the optical far field. This s-THz emission possesses an efficiency matching state-of-the-art THz standards. In s-Nebula, we propose to combine recent development in the fields of spintronic with the expertise from THz technologist to address the full THz chain. The s-Nebula approach thus relies in both exploring new materials and new device geometries to enhance s-THz emission efficiency, together with developing new s-THz functionalities. The s-NEBULA project will provide cutting-edge solutions to solve bottleneck scientific issues in the THz field motivated by clear needs in target applications, such as variable-baseline broadband pulsed emitters and voltage-controlled compact detectors for non-destructive testing (NDT), intrinsically-modulated CW emitters for communication and programmable-polarization emitter for ellipsometry measurements.
The TeraHertz (THz) frequency band represents a spectral window that offers rich opportunities for advanced applications in many fields, e.g. industrial quality control, spectroscopy, imaging, medical diagnostics, security, telecommunication and high-speed electronics. However, the development of THz technology is currently hampered by the limitations of available technological paradigms and there is an urgent need for a radically new THz technological framework. Recently, starting with pioneering work of consortium members, optically-driven spin based THz (s-THz) emitters were demonstrated, based on optically triggered spin currents and spin-to-charge conversion. A typical s-THz emitter is a nanometer-thick metallic heterostructure consisting of ferromagnetic (FM) and strongly spin-orbit-coupled nonmagnetic (NM) materials (see Fig 1). Pumping the s-THz emitter by a fs laser pulse leads to ultrafast demagnetization, followed by the emission of spin-current pulse converted by the inverse spin-Hall effect into an electric-dipole type emission of a broadband THz pulse (> 20 THz) into the optical far field. This s-THz emission possesses an efficiency matching state-of-the-art THz standards. In s-Nebula, we propose to combine recent development in the fields of spintronic with the expertise from THz technologist to address the full THz chain. The s-Nebula approach thus relies in both exploring new materials and new device geometries to enhance s-THz emission efficiency, together with developing new s-THz functionalities. The s-NEBULA project will provide cutting-edge solutions to solve bottleneck scientific issues in the THz field motivated by clear needs in target applications, such as variable-baseline broadband pulsed emitters and voltage-controlled compact detectors for non-destructive testing (NDT), intrinsically-modulated CW emitters for communication and programmable-polarization emitter for ellipsometry measurements.
During the first year, s-Nebula consortium harnessed the necessary synergies among its members and developed suitable protocols for validating s-THz emission efficiencies. The consortium screened and benchmarked a large number of heavy metal (HM)/ferromagnet (FM) heterostructures with a state-of-the art s-THz emitter from the JGU partner. Combining theory and experiments, the CNRS and UU partners established important figure of merits for the efficiency of the THz emission in (HM)/ferromagnet(FM) heterostructures. In parallel, UU and VSB modelled the mechanisms being THz emission in different types of heterostructures and identified some topological insulators as promising systems for THz spin-to-charge conversion. In the second reporting period, the consortium has thus developped various approaches to enhance the output power of standard STEs, and obtained a first increase by a factor 5. Regarding the development of new functionalities for s-THz emitters, the objective of continuous THz emission was demonstrated first at 300 GHz, and then up to 1 THz. Furthermore, static 360° polarization control was achieved and a modulation at rate of 100 khz was achieved.
On the detection side, a key accomplishment of the consortium has been to demonstrate at the FUB partner that s-THz detection can be achieved within a single nanometer thick metallic ferromagnet, increasing the detection efficiency by 2 orders of magnitudes compared to magnetic garnets and showing a gapless detection bandwidth from 1-20 THz. In parallel, the JGU partner achieved to develop high-quality antiferromagnetic thin films with large antiferromagnetic domains that could be used to develop the concept of pulsed narrow band THz emitters. Identifying potential manipulation of the antiferromagnetic order by spin-orbit torques could then make them suitable for continuous THz emission and device integration at a later stage. Preliminary investigations between the Thales, CNRS, JGU and FUB highlighted that their resonant THz response can be detected optically and electrically and could thus be used to develop narrow band THz detectors.
On the detection side, a key accomplishment of the consortium has been to demonstrate at the FUB partner that s-THz detection can be achieved within a single nanometer thick metallic ferromagnet, increasing the detection efficiency by 2 orders of magnitudes compared to magnetic garnets and showing a gapless detection bandwidth from 1-20 THz. In parallel, the JGU partner achieved to develop high-quality antiferromagnetic thin films with large antiferromagnetic domains that could be used to develop the concept of pulsed narrow band THz emitters. Identifying potential manipulation of the antiferromagnetic order by spin-orbit torques could then make them suitable for continuous THz emission and device integration at a later stage. Preliminary investigations between the Thales, CNRS, JGU and FUB highlighted that their resonant THz response can be detected optically and electrically and could thus be used to develop narrow band THz detectors.
s-Nebula is a high risk/high reward fundamental science project, fully in line with the objectives of the FET program. Our main goal as far as the impact on future technology is concerned, is prepare to prepare the ground for the development of a full spin-based THz chain, from THz excitation to THz detection, but also the control of radiation properties, such as polarization (key point for ellipsometry application) and signal modulation, required for information encoding and decoding.
After the first two reporting periods, the fact that we have succeeded in developing a joint strategy to benchmark s-THz emitters and to enhance their efficiency and functionalities represents an important step towards the potential integration in THz devices. These results are clearly at the state-of-the-art of standard passive THz emitters and the FUB partner thus launched a start-up to commercialize this type of broadband THz emitters. In parallel, we demonstrated proof of concept of s-THz detection with a record efficiency and identified new materials to bridge the gap with conventional ZnTe detectors. There is thus a potential to develop THz detectors to precisely benchmark THz emitters. In paralle, due to their THz resonant modes, antiferromagnetic materials present key advantages to develop narrow band and tunable s-THz devices. The consortium succeeded in developing high quality antiferromagnetic thin films with large antiferromagnetic domains which is extremely important to achieve electrical detection of sub-THz (in the consortium) and THz (out of the consortium) signals not only with antiferromagnetic single crystals. At this stage, we achieved an electrical controlling of the static magnetic properties of thin films over 100 μm² and evidenced the competing roles of spin-orbit and magneto-elastic torques. It makes us confident that our control of antiferromagnetic material will keep improving.
Finally, the work done in s-Nebula is still at a pre-competitive stage for some of the targeted THz technologies. The potential of s-THz emitters is studied for non-destructive testing and ellipsommetry is under way and will be further developped in the next period. However, spin-based THz technologies are already on the market with the recent development of a start-up from the FUB partner, commercializing standard s-THz emitters.
In parallel, s-Nebula consortium largely dissiminates its activities by organizing a symposium on spin-based THz devices promote the strategy proposed in s-Nebula and reinforce the joint efforts in this emerging topic with promising new technological developments for future THz devices. The consortium members also gave multiple conferences presentations (including invited talks) and published more than 35 papers in peer-reviewed high impact journals. A complete list of publications and communications can be found on the s-Nebula page of OpenAIRE, together with an access to datas.
After the first two reporting periods, the fact that we have succeeded in developing a joint strategy to benchmark s-THz emitters and to enhance their efficiency and functionalities represents an important step towards the potential integration in THz devices. These results are clearly at the state-of-the-art of standard passive THz emitters and the FUB partner thus launched a start-up to commercialize this type of broadband THz emitters. In parallel, we demonstrated proof of concept of s-THz detection with a record efficiency and identified new materials to bridge the gap with conventional ZnTe detectors. There is thus a potential to develop THz detectors to precisely benchmark THz emitters. In paralle, due to their THz resonant modes, antiferromagnetic materials present key advantages to develop narrow band and tunable s-THz devices. The consortium succeeded in developing high quality antiferromagnetic thin films with large antiferromagnetic domains which is extremely important to achieve electrical detection of sub-THz (in the consortium) and THz (out of the consortium) signals not only with antiferromagnetic single crystals. At this stage, we achieved an electrical controlling of the static magnetic properties of thin films over 100 μm² and evidenced the competing roles of spin-orbit and magneto-elastic torques. It makes us confident that our control of antiferromagnetic material will keep improving.
Finally, the work done in s-Nebula is still at a pre-competitive stage for some of the targeted THz technologies. The potential of s-THz emitters is studied for non-destructive testing and ellipsommetry is under way and will be further developped in the next period. However, spin-based THz technologies are already on the market with the recent development of a start-up from the FUB partner, commercializing standard s-THz emitters.
In parallel, s-Nebula consortium largely dissiminates its activities by organizing a symposium on spin-based THz devices promote the strategy proposed in s-Nebula and reinforce the joint efforts in this emerging topic with promising new technological developments for future THz devices. The consortium members also gave multiple conferences presentations (including invited talks) and published more than 35 papers in peer-reviewed high impact journals. A complete list of publications and communications can be found on the s-Nebula page of OpenAIRE, together with an access to datas.