A fully coherent soft X-ray free-electron laser
An international team of researchers implements a new method for generating intense and nearly fully coherent soft-X-ray pulses at a free-electron laser with high potential for use in novel spectroscopic techniques.
An international research group led by the FERMI free-electron laser (FEL) physics team from the Elettra laboratory in Trieste demonstrated the first lasing of a soft X-ray FEL based on the recently proposed electron beam echo effect. The scheme has the potential to generate fully coherent pulses at wavelengths as short as 1 nm, opening up opportunities for new experiments in the emerging field of nonlinear X-ray science. The results were recently published online in the journal Nature Photonics under the title “Coherent soft x-ray pulses from an echo-enabled harmonic generation free-electron laser”.
As opposed to conventional lasers, FELs use free electrons as the lasing medium. An electron beam is first accelerated in a linear accelerator to a relativistic speed and then injected into a periodic magnetic device called an undulator where it emits light. The coupling between the wiggling motion of the electrons in the undulator and the electric field of the emitted light results in amplification of the radiation.
Most X-ray FELs amplify spontaneous undulator emission due to electron shot-noise, originating from the discrete nature of the electric charge in the electron beam. This process leads to a highly fluctuating output, characterised by a broad and noisy spectrum and poor temporal coherence. Coherence of a FEL can be improved by letting the electron beam interact with an external conventional laser – the “seed” – before injection into the undulator. Such a scheme is used at the FERMI FEL in Trieste – the world’s first externally seeded soft X -ray FEL. However, the method is not immune to possible instabilities in the electron beam that may build up during the acceleration phase. As a result, the coherence properties of the conventional laser are not fully preserved during the interaction with the electron beam, leading to a degradation of the FEL performance, especially at short wavelengths.
The research team lead by Elettra Sincrotrone Trieste implemented a new method, originally proposed by Gennady Stupakov from SLAC, at the FERMI FEL. Called Echo-Enabled Harmonic Generation (EEHG), the method uses two external conventional lasers to precisely tailor the electron beam prior to emission of X-rays. This allowed them to generate intense and nearly fully coherent pulses at wavelengths as short as 5.9 nm. Coherent narrow band (but not amplified) emission was observed even at 2.6 nm. Because the technique is virtually insensitive to the possible electron beam instabilities, it may allow the generation of pulses with laser-like properties down to 1 nm, opening the door to entirely new experiments with coherent light.
“FERMI is currently the only FEL in the world that can operate in the EEHG configuration at such short wavelengths”, says Enrico Allaria who coordinated the experiment. “In the future, we aim to further exploit the capabilities offered by the technique in order to offer to the broad user community a fully tunable FEL capable of delivering multi-color laser-like pulses with wavelengths extending into the water window (2-4 nm)”.
The full article is available at https://rdcu.be/bAxPt
