Novel Spin-Based Building Blocks for Advanced TeraHertz Applications

Project Information

s-NEBULA

Grant agreement ID: 863155

DOI

10.3030/863155

Project closed

EC signature date 30 August 2019

Start date 1 January 2020

End date 31 December 2023

Funded under

• EXCELLENT SCIENCE - Future and Emerging Technologies (FET)

Total cost

€ 3 390 876,25

EU contribution

€ 3 390 876,25

Coordinated by

THALES
France

Periodic Reporting for period 3 - s-NEBULA (Novel Spin-Based Building Blocks for Advanced TeraHertz Applications)

Reporting period: 2022-07-01 to 2023-12-31

Summary of the context and overall objectives of the project

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 combined developments 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 achieved overall objectives are thus:
- The development of pulsed broadband (> 20 THz) with output power larger than conventional photoconductive switches, and the demonstration of tunable low power CW s-THz emitters with no-roll off above 1 THz.:
- The development of unified models describing spin transport, THz generation and propagation in complex structures,
- The demonstrate a fully operative spin-based THz detection mechanisms based on either Zeemand torque or spin-accumulation for pulsed regimes, and of proof of concepts of CW detectors using antiferromagnetic materials.
- The development of s-THz devices with modulation rate as large as 10 MHz and with a full polarization control (extinction ratior > 25 dB).
The s-NEBULA project also provided evidences that spin-based THz emitters can be integrated and validated in target applications, such as non-destructive testing (NDT) and for THz ellipsometry measurements.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

In the first part of the project, s-Nebula consortium first harnessed the necessary synergies among its members and developed suitable protocols for validating s-THz emission efficiencies. The consortium has 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 partners established an important figure of merit 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.
Along the project duration, the consortium developed many new functionalities of spin based THz devices, including among others : an enhanced THz emission of standard pulsed THz s-STE by a factor 10 using modified substrates and integration in IR and THz cavity, with a static 360° polarization control and > 25dB extinction ratio, the development of new types of spin-based THz emitters (narrow band pulsed emitters, and CW emitters using two detuned DFB lasers) and detectors (B field detection by Zeeman torques, and E field detection by Néel spin-orbit torque in AFM and by spin-accumulation in standard FMs). These important results led to the publication of around 70 articles in high impact journals (including 10 articles in Nature journals, 9 APL articles with 2 covers, 9 PRApplied articles and 6 Optics express articles among others) along the project duration. A complete list of publications and communications can be found on the s-Nebula page of OpenAIRE, together with an access to datas.
The consortium initially organized in 2021 a first workshop on THz spintronic at the virtual CLEO conference. Then, the consortium organized in October 2023 the first conference on THz spintronic, a key event to gather scientists from the THz optics, spintronic and material science research communities as well as THz engineers and structure this emerging community.
Furthermore, on the exploitation part, the ITWM partner successfully integrated a s-THz emitter in a conventional setup for layer thickness measurements and demonstrated that it can be used to measure micrometre thick dielectric layers. The VSB partner also demonstrated the potential of THz s-STE for THz ellipsometry, developing a dedicated THz ellipsometry equipment and demonstrating that their performances outperform their competitor photoconductive switches.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

s-Nebula was 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, was 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.
In the first part of the project, we successfully developped 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. The s-THz emitters also outperform the photoconductive switches at high power, whilst at the end of the project remaining at low power (< 300 mW) behind best photoconductive switches around 1 THz and better above 4 THz. In parallel, we demonstrated proof of concept of s-THz detection using different mechanisms with a record efficiency and identified new materials to bridge the gap with conventional ZnTe detectors. Due to their THz resonant modes, we demonstrated that antiferromagnetic materials present key advantages to develop narrow band and tunable s-THz devices but still requires material developments to exploit their full potential.
Finally, the work done in s-Nebula favorized a first dissemination towards applied technologies with the creation of a spin-off Teraspintec by the FUB partner, commercializing various integration of pulsed THz emitters. The integration of THz s-STE in THz NDT and THz ellipsometry devices will also have important impacts for the future integration of spin-based THz devices in applications beyond THz spectroscopy.

THz emission from a spintronic emitter based on a nm thick bilayer FM/HM