The flying focus effect optimizes soft X-ray lasers

Negative plasma dispersion is a limiting effect in numerous laser-plasma interaction experiments performed at high density. In longitudinally pumped seeded soft X-ray lasers (SXRLs), the plasma dispersion results in a desynchronization between the infrared pump beam, propagating with a velocity <c, and the soft X-ray seed beam, propagating at ≈c, and therefore severely restricts the effective amplification length.

In collaboration with researchers from Madrid and Prague, scientists from LOA’s FLUX group have demonstrated experimentally for the first time the ability to control the group velocity of a high intensity (> 1018 W/cm²) laser pulse in a plasma thanks to spatio-temporal couplings which generate the so-called ‘flying focus effect’.

Measurements show that this allows speeding up the pump pulse to keep it synchronized with the seed, so as to boost the energy contained in the amplified soft X-ray beam up to an optimal energy extraction of 60%.  Furthermore, as the x-ray pulse and infrared pump no longer drift apart, the soft x-ray pulse duration does not increases with the amplification length but remains constant. Measurements of the duration of 32.8 nm-soft-x-ray pulses give, in good agreement with 3D Maxwell-Bloch simulations, a record-low value of 350 fs RMS, constant over 10 mm of propagation at an electron density of 8×1019 cm-3.

Dispersion compensation offers great potential for optimizing SXRL and fosters opportunities for many applications requiring ultrashort pulses or high on-target intensities with short wavelengths. This work also illustrates that fundamental limitations of laser-plasma interactions induced by plasma dispersion can be overcome thanks to spatio-temporal couplings, opening prospects in a wide scope of domains such as photon acceleration or laser wakefield acceleration.

This research has been published in :

Kabacinski et al., Spatio-temporal couplings for controlling group velocity in longitudinally pumped seeded soft X-ray lasers, Nature Photonics, March 2023

In blue, measurements of the RMS pulse duration of the SXRL with respect to the jet length. Measurements have been performed for different group velocities of the pump beam (larger, equal and smaller than c) at an electron density close to ne = 7x1019 cm-3. In red, linear regressions of these measurements. Small symbols are used for individual points, large symbols for averages over few shots.