Coherent X-Ray sources

High harmonic generation

We have pioneered High Harmonic Generation in Portugal, making the first source of coherent soft X-rays at the L2I laboratory. High Harmonic Generation is a non-linear process occurring in the interaction of an intermediate intensity short-pulse laser with a gas or molecule, resulting in a frequency comb of odd multiples of the laser frequency, with a high degree of coherence and ultra-short pulse duration. These harmonics span down to XUV wavelengths, as here in IST where we produced HHG up to harmonic q=51 (20.6 nm radiation).

Argon High Harmonics Spectrum at L2I. The strongest harmonic is q=39 at 27 nm

With the goal to use harmonics as a source for X-ray coherent imaging, we have led a series of experiments in international facilities on HHG. For instance, we have looked at how harmonics are polarized when they are generated using two colored fields (the fundamental laser frequency and its double) instead of just one, at LOA-Ecole Polytechnique, using their kHz laser facility. These experiments abroad are possible thanks to a program (Laserlab-Europe) that grants access to international facilities to European researchers through a competition for beam-time.
Our most recent experiment (July 2012) was a 3 week run at the MEC station at the LCLS in Stanford, the word’s first X-ray laser, where we have produced High Harmonics and characterized them in terms of spectrum and wavefront. This line of research has been partially financed by PTDC/FIS/112392/2009, an FCT project starting Jan 1st 2011.

Coherent imaging with HHG

With coherent sources, new imaging techniques such as holography or diffraction imaging may be implemented. We are interested in exploring these techniques to image fast moving phenomena with unprecedented resolution. In imaging, one of the limits to the resolution is the wavelength of the light source used for imaging. With soft-X-rays, the diffraction limit is in the nm range, compared to visible light (around 500 nm). HHG are not only emitting at short wavelengths, but they are also ultra-short (lasting for less than the duration of the ultra-short laser, in the fs range or less).
We have collaborated with CEA-SPAM to produce single-shot coherent images in the XUV. We are currently implementing our own holography program in collaboration with LOA, thanks to Laserlab beamtime at LOA.

Experimental setup at LOA for dynamic coherent imaging

Novel diagnostics with HHG

Beyond imaging, a wealth of information can be obtained when probing matter in the XUV. For instance, the refractive index of ionized matter is poorly known at these wavelengths. As soon as a solid heats up, it undergoes a phase transition leading up to the plasma state, if sufficient energy is supplied to the system. A visible probe cannot penetrate in warm dense matter, therefore XUV wavelengths or even shorter (hard X-rays) must be used to diagnose the changes in the core of the heated matter.
We have been using HHG to probe dynamic changes in solids heated by lasers, and are developing novel diagnostics such as plasma wavefront sensing to directly measure the plasma refractive index in the XUV. We have had a series of campaigns in LOA for the development of this new diagnostic. We will benefit from a wide network through the COST-Action, a collaboration to develop novel XUV optics and metrology.

XUV Hartmann wavefront sensor

Plasma-based x-ray lasers

X-ray lasers were not always based on free electrons out of Linear Accelerators. For decades, high temperature plasmas were the medium available to amplify soft X-rays. The process was equivalent to lasing in an optical medium, but the population inversion was between two highly ionized states, leading to transitions in the X-ray range. These X-ray lasers are still the most energetic lasers in the soft X-rays (up to 10 mJ per shot), but they have poor coherence and longer pulse duration. After working on the demonstration that these plasma amplifiers can be seeded by HHG to keep the excellent optical properties of the seed, we kept working with Ph. Zeitoun’s group on the extension of this seeding technique to shorter, more energetic x-ray lasers.

An idealized soft x-ray laser chain