Soft X-ray Laser beams

Overall, this scientific activity has been productive and has gone a long way on the path that began 15 years ago. At that time, a great deal of confusion was lying over the performances and possibilities offered by laser-driven SXRL: although it was a laser, the SXRL lasers at that time were not spatially coherent and therefore not efficiently focusable and usable in imaging, one of the more important application. The injection of high order harmonics, a fully coherent source, and its amplification made it possible to overcome this limit. It nevertheless took many years at the international level to be accepted as a must to operate plasma based XUV lasers. The work carried out in the UPX team allowed to show the unique capabilities of this mechanism: production of pulses of 100 fs (against a few ps before), full coherence having an excellent wavefront, polarization tuning (linear and circular).  A final step would have been to transpose the so-called CPA technique into XUV rays as we proposed in 2012 but already excellent performances were predicted using only two amplifying plasmas. It should be noted that the technique of XUV-CPA was successfully used in a free electron laser experiment (FERMI @ ELETTRA) to which we collaborated.

Here are highlights of our latest developments.

– Efficient guiding of ultra-intense laser pulses into high density plasmas

We have successfully implemented a waveguide by focusing with an axicon lens a sequence of “ignitor” (130 mJ, 30 fs) and ”heater” pulses (690 mJ, 600 ps) delayed by 600 ps. The electron density has been measured retrieving the phase change imparted by the plasma with a Mach-Zehnder interferometer. At the highest reported density, appropriate conditions for guiding in terms of size and transverse density gradient were granted for the pump pulse focused about 1.55 ns after the arrival of the ”ignitor”. When injecting a 5× laser pulse into the waveguide, the electronic density growth up to more than 1020cm-3. The transmitted beam is multimode and contains about 50% of the initial energy after 5 mm of propagation and decrease down to 20% after 20 mm when krypton is used. In parallel to the experimental measurements, intensive numerical calculations using hydrodynamic and particle-in-cell codes has been performed to understand and predict the creation and posterior evolution of the waveguide.

– Toward ultrashort laser driven soft x-ray lasing by collisional gain gating

We investigated the influence of electron density on the temporal properties of the 32.8 nm lasing emission. By seeding the SXRL amplifier using the 25th harmonic of the infrared driving laser we measured the temporal gain dynamic. As expected by our numerical simulations, the gain duration monotonically decreased from 7 ps to an unprecedented8 shortness of 450 fs FWHM as the amplification peak rose from 150 to 1,200 with an increase of the plasma density from 3 × 1018cm−3up to 1.2 × 1020cm−3. From our Maxwell–Bloch modelling, the inferred SXRL pulse duration varies from about 6.4 ± 0.3 ps for ne=3× 1018cm−3down to about 120 fs for ne= 1.2 × 1020 cm−3, which thus breaks the decade-long picosecond barrier of plasma-based SXRL.

– Circularly Polarized Plasma-Based Soft-X-Ray Laser

In the soft x-ray range, the availability of coherent circularly polarized light has been limited so far to few large-scale facilities and more recently to high-order harmonic generation. For generating circularly polarized SXRL beams, we have converted the HHG seed polarization from linear to circular using a grazing incidence four-reflector phase shifter and a λ/2 wave plate. The polarized seed is then injected and amplified into the plasma amplifier.  Using this technique, we were able to demonstrate that we are able to control the polarization of o the seeded SXRL (from linear to circular) and compensate the losses introduced by the phase shifter (1% transmission) after amplification.

– Modeling of the amplification of a high-order harmonic seed by a soft x-ray laser plasma

In collaboration with G. Maynard (LPGP-Orsay) and E. Oliva from University Politechnica de Madrid, we have performed intensive modeling of amplification of a high-order harmonic seed by a soft x-ray laser plasma. The time-dependent evolution of the x-ray signal is determined from a fully dynamic Maxwell-Bloch calculation. At high seed intensities, a simplified one-dimensional calculation leads to strong Rabi temporal oscillations of the output signal. This set of code have being recently modified to performed full 3D calculations of the amplification process. It has been used to demonstrate the concept of X-ray chirped Pulse Amplification, (that was successfully tested on FERMI free-electron laser)