With the progresses achieved on the development of high quality, highly coherent soft X-ray (or EUV) sources from synchrotrons, free-electrons laser, high harmonics and plasma based soft X-ray laser, as well as for the need of always better optics for imaging (EUV lithography, EUV spatial telescopes, EUV microscopes), the demand on metrology is increasing very fast. Nowadays, many synchrotrons have developed metrology beamlines but with the limit of being too expensive and too large for transposing them to university-scale laboratories or optical firms. At Laboratoire d'Optique Appliquée, we have developed a compact and versatile metrology beamline to test at-wavelength different EUV optics, from single component to full assembly and adaptive optics.
The beamline is based on the use of high harmonics generated by the interaction of a 35 fs, 4 kHz, 3 mJ laser with neutral gases. The high harmonics span from 10 to 50 nm and are fully coherent, collimated and exhibit a good wavefront of about lambda/5 rms.
The beamline covers a footprint of about 5*1.5 m2 while the driving laser occupies about 4 m2. Itis composed of an interaction chamber where high harmonics are generated, a spectrometer and the metrology chamber (1.5m*0.7m) .
We have tested many optical components from flat or curved mirrors to toroidal mirror or Schwarzschild microscope. We will present in detail the beamline as well as results from optic metrology. The beamline is also used for calibration of wavefront sensors.
This beamline is well suited for testing EUV adaptive optic in any configurations.
We present an optical system based on two toroidal mirrors in a Wolter configuration to focus broadband XUV high harmonic radiation generated by the non-linear interaction of a fs, 10 Hz laser with neutral gas. The experiment was carried out at Lund University in collaboration with Laboratoire d’Optique Appliquée, ELI-ALPS and Imagine Optic. Optimization of the focusing optics alignment is carried out with the aid of an XUV Hartmann wavefront sensor commercialized by Imagine Optic.
Back-propagation of the optimized wavefront to the focus yields a focal spot of 3.6 * 4.0 µm2 full width at half maximum, which is consistent with ray-tracing simulations that predict a minimum size of 3.0 *3.2 µm2.
We will show also how the optimization of the high harmonic beam by the use of an infrared adaptive optic may help for compensating the residual aberrations of the Wolter, leading to a clear improvement of the focal spot.
X-ray free-electron lasers (FELs) are powerful tools for probing matter properties down to sub-nanometer scales with femtosecond time resolution, allowing a growing number of physical, chemical, biological and medical investigations to be carried out. FELs operating in seeding mode intrinsically present enhanced temporal coherence properties with respect to those relying on the self-amplified spontaneous emission (SASE) process. They are however limited, for the moment, to extreme ultraviolet (XUV) wavelengths, or in some cases to soft X-rays, and durations of tens of femtoseconds. We studied how these limits can be overcome by means of X-ray chirped pulse amplification, inspired by infrared lasers.
As a matter of fact, the use of a seed enables a fine control of the chirp and a spectro-temporal shaping of the FEL emission. Moreover, ultrashort wavelengths can be envisaged through schemes of high-gain harmonic generation and echo-enabled harmonic generation. We will present FEL simulations coupled with the study of a compressor in conical diffraction geometry.