High-Flux Synchrotron Alternatives Driven by Powerful Long-Wavelength Fiber Lasers

Project Information

SALT

Grant agreement ID: 835306

DOI

10.3030/835306

Project closed

EC signature date 18 June 2019

Start date 1 July 2019

End date 30 June 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 490 912,50

EU contribution

€ 2 490 912,50

Coordinated by

FRIEDRICH-SCHILLER-UNIVERSITÄT JENA
Germany

Periodic Reporting for period 4 - SALT (High-Flux Synchrotron Alternatives Driven by Powerful Long-Wavelength Fiber Lasers)

Reporting period: 2024-01-01 to 2024-06-30

Summary of the context and overall objectives of the project

The SALT (“High-Flux Synchrotron Alternatives Driven by Powerful Long-Wavelength Fiber Lasers”) project addressed the challenge of making synchrotron-like radiation more accessible through the development of high-flux, table-top light sources. These sources were designed to operate in three critical spectral regions: Terahertz (THz), mid-infrared (mid-IR), and soft X-rays. The motivation behind this endeavor lies in the immense application potential of synchrotron radiation, which is currently limited to large, expensive facilities. By miniaturizing the technology, the SALT project aimed to democratize access to advanced radiation sources, thus accelerating research and enabling new discoveries across multiple scientific disciplines.

Importance for Society
Synchrotron radiation has been instrumental in advancing fields such as materials science, biology, and chemistry by providing unique insights at the atomic and molecular levels. However, the high cost and limited availability of synchrotron facilities have restricted their use. The SALT project’s innovation in creating compact, high-performance light sources has the potential to revolutionize these fields by making this powerful tool more widely available. This democratization will not only speed up existing research but also open up new avenues of investigation, potentially leading to breakthroughs in healthcare, environmental science, and advanced manufacturing.

Achievements of the Project
The SALT project successfully achieved its ambitious goals by focusing on two main objectives:
1. Revolutionizing Ultrafast Laser Performance: The project unlocked the potential of Thulium-doped fiber lasers, significantly surpassing the performance of existing 2µm sources. These advancements included achieving record-high average power outputs and the development of a 4-channel, coherently combined Thulium laser, which set new benchmarks in pulse energy and duration.
2. Demonstrating New Realms of Radiation Flux: Through the innovative use of frequency conversion, the project demonstrated unprecedented levels of flux in the targeted spectral regions. This included the development of high-power THz sources, efficient mid-IR generation, and the advancement of soft X-ray sources capable of generating application-relevant photon flux.

These achievements not only established new state-of-the-art capabilities in laser and radiation source technology but also laid the groundwork for future applications that could drive significant societal and scientific advancements. The project’s outcomes are expected to attract further research investment and foster international collaboration, positioning the developed technologies at the forefront of their respective fields.

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

Within the project SALT several technological accomplishments have been achieved. A new generation of Tm-doped fiber lasers was developed. With the source realized in this project period, the performance level of state-of-the-art Tm-doped fiber laser technology and OPCPA technology could be surpassed. The laser delivers a pulse energy of 1.7 mJ and 170 W of average output power at 100 kHz repetition rate. Additionally, the output pulse duration was below 100 fs and the beam quality was close to the diffraction limit.
This renders this source the highest average power mJ-level SWIR source to date. These parameters are generated by a table-top source which is the ideal platform to scale frequency conversion processes into the soft X-ray, THz and mid infrared spectral region.
Regarding frequency conversion into the soft X-ray spectral region, first high-order harmonic generation experiments, driven by the established laser technology, have been conducted [1].
Here, a novel concept of simultaneous pulse self-compression and high-harmonic generation in a single hollow-core fiber has been demonstrated. Due to the tight confinement of light in the fiber, this design allows using moderate peak power driving laser pulses, which can potentially enhance both integration and the availability of these sources. Numerical modelling of the experiment additionally revealed important findings regarding the interaction length of the driving field with the generated high harmonic radiation in the waveguide. These findings are an important cornerstone for the next experimental steps.
For the preparation of driving a THz generation with the developed Tm-laser system an experiment using a 1 µm wavelength driving laser [2] has been carried out. By using an unprecedented average power of 640 W to drive the gas-plasma based THz generation an average power of 640 mW of broadband, single-cycle THz pulses has been generated [3]. This result is the current state-of-the-art of laser-driven THz sources in terms of average power as well as conversion efficiency for a 1 µm wavelength driving laser.
In alignment with the project goals, the same THz generation technique was investigated using 2 µm wavelength lasers A significant conversion efficiency enhancement with the doubled drive wavelength in a gas-plasma based THz generation could be demonstrated. The outcome was an average power of 380 mW of broadband, single-cycle THz pulses with an efficiency as high as 0.32% which is roughly a factor 4 higher than previously reported for a 1 µm drive wavelength [4]. This result will open new possibilities for THz applications which rely on highest flux levels.

[1] M. Gebhardt et al., Light Sci Appl 10,36(2021)
[2] H. Stark et al., Opt. Lett. 46, 969-972 (2021).
[3] J. Buldt, et al., Opt.Lett.46(20) 5256-5259 (2021)
[4] J. Buldt, et.al. Opt. Lett. 48, 3403-3406 (2023)

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)

The developed Thulium fiber CPA system defines the state of the art and has pushed the available performance by more than an order of magnitude in the 2 µm wavelength region. Worth to be mentioned, the architecture is highly scalable, research will continue and another performance step is feasible. In addition, we have developed novel pulse post-compression scheme which allow for highly efficient pulse shortening at 2µm wavelength.
Also noteworthy, the demonstrated THz source driven by a 1 µm ultrafast fiber laser is by far the most powerful in the world and our 2µm laser driven THz source have experimentally revealed a conversion efficiency scaling. In addition, the developed high-flux soft-Xray source employs nonlinear pulse compression to drive phase-matched HHG into the water window for the first time with record flux levels.
Overall, the developed coherent sources in application relevant spectral regions will open up many possibilities e.g. in imaging and spectroscopy.