Laser based x-ray generation and microscopy

 

The aim of this project is to generate ultra-short pulses of x-rays, ranging in wavelength from 1 nm to tens of nm, to be able to ultimately study structures at the nanometer level in biological cells. It is part of an NWO funded infrastructure for imaging of biological samples using different optical techniques.

A significant part of the project is the development of the required laser system to generate x-rays. High peak power laser pulses are required so that focusing in a noble gas leads to high-harmonic generation. In this process the electric field of the focused optical wave deforms the Coulomb potential of the noble gas atoms so that an electron wave packet can escape via tunnelling. After about 3/4 of an optical cycle the wave packet returns at the atom it came from, and in this re-collision event high energy photons can be emitted.  The process is coherent and results in high-quality x-ray beams with (sub)mrad-level divergence. In order to obtain enough flux of x-ray photons several advanced techniques will be used. Amplification of few-cycle pulses to terawatt level peak power was pioneered in our group using parametric interaction in non-linear crystals. With this system radiation is routinely generated down to tens of nm through high-harmonic generation. A new facility is under construction utilizing the same technique at a higher repetition rate (kHz level vs. 30 Hz) to increase the number of x-ray photons, while new phase-matching techniques extend range of generated wavelengths down to the 'water window' (2-4 nm).

With this source x-ray microscopy will be performed based on lens-less imaging (by measuring diffraction patterns on a CCD camera, see figure), as well as by tomographic techniques.

Contact: Kjeld Eikema, email: kse.eikema@few.vu.nl, and Stefan Witte, email: s.witte@few.vu.nl
Physics of light and matter

kamlaser

Fig.: Principle of lens-less imaging, based on the recording on a CCD of an interference pattern resulting from scattered x-rays by the object under study and a reference beam or pattern.