BAXHHG · Bright attosecond x-rays in the water-window using phase-matched high-harmonic generation

Bright attosecond x-rays in the water-window using phase-matched high-harmonic generation

The aims of this project were to advance coherent x-ray generation driven by ultrafast lasers. Such technology offers a route to table-top coherent x-ray microscopy as well as the observation of electronic motion on the atomic scale, and is therefore of great technological and scientific interest. Our chosen route was to develop a source of few cycle laser pulses at infrared wavelengths. We then used these laser pulses to drive high-order harmonic generation (HHG). In HHG, an electron is pulled away from its parent atom before being accelerated back towards it. In the ensuring collision, extreme ultraviolet and x-ray radiation is produced with the precise timing-or coherence -of a laser. Whilst HHG has been studied since the 1980s, it remains difficult to produce intense radiation above photon energies of 100eV. One aim of the project was to produce bright coherent x-ray radiation in spectral region in which water is transparent. Another important property of HHG is that the radiation bursts it produces are of a duration comparable to the motion of electrons around an atom. In the last decade, these pulses have been used at photon energies up to 100eV. Another aim of our project was to measure these pulses at photon energies in the water window. One major part of the project was developing the requisite laser technology. We developed two sources of amplified ( 1 mJ) pulses at 2 µm. The first is homemade system that delivers 4 cycle pulses. The second is a commercial system. Both are pumped by a titanium-doped sapphire amplified laser system (Ti:Sapph). During the project, we realised that the quality of the output of both 2 μm systems was crucially dependent on the spatial quality of the Ti:Sapph system, which was insufficient for the task in its original 'out-of-the-box' commercial configuration. Therefore, we began a major upgrade of this system. Our aim of achieving an unprecedentedly high repetition rate (4 kHz) means that this is a challenging goal. As of March 2013, the completion date of the project, this upgrade is nearing completion. We also made some related developments to the manipulation and characterization of amplified laser pulses. We demonstrated a novel means of compressing these pulses in the infrared [1]. We also developed a new method for their full spatio-temporal characterization [2,3]. In parallel with these developments, we demonstrated progress in HHG driven by 2 μmsources. In particular, using a simple gas jet target of neon and argon, we observed generation up to 200eV. Although it has many advantageous characteristics, HHG is a quite inefficient process, and an understanding of the behaviour of the yield is important. In particular it is known that on a microscopic level, the efficiency drops as the laser wavelength is increased. We conducted a detailed theoretical study of this behaviour [4]. We also developed computer codes for simulating macroscopic HHG. These are crucial for understanding and designing real-world sources. As a means of overcoming the low microscopic yield, we investigated the use of hollow cylindrical waveguides to extend the interaction length. High gas pressures (up to 50 atmospheres) are required for efficient generation, the x-rays must be delivered and measured in vacuum. Therefore, we developed and interferometrically tested apparatus for containing these high pressures within a vacuum chamber. A possible future development is HHG driven by infrared pulses at high repetition rate (greater than 100 kHz) fibre lasers, which typically have lower pulse energies that lower repetition rate system. To this end the development of means for spectral broadening and compression of such pulses is necessary. We demonstrated unprecedented spectral broadening of pulses from a home-built optical parametric chirped pulse amplification system (OPCPA) [5]. For further information, contact Prof. Dr. Jens Biegert jens.biegert@icfo.es (group leader) or Dane Austin dane_austin@fastmail.com.au (IEF Fellow). References [1] Ricci, A., Silva, F., Jullien, A., Cousin, S. L., Austin, D. R., Biegert, J., and Lopez-Martens, R. Generation of high fidelity few-cycle pulses at 2.1 μmvia cross-polarized wave generation. submitted to Opt. Express (2013). [2] Cousin, S. L., Bueno, J. M., Forget, N., Austin, D. R., and Biegert, J.Three-dimensional spatiotemporal pulse characterization with an acousto-optic pulse shaper and a Hartmann-Shack wave front sensor. Opt. Lett. 37, 3291 (2012). 10.1364/OL.37.003291. [3] Cousin, S. L., Forget, N., Grun, A., Bates, P. K., Austin, D. R., and Biegert, J. Few-cycle pulse characterization with an acousto-optic pulse shaper. Opt. Lett. 36, 2803 (2011). 10.1364/OL.36.002803. [4] Austin, D. R. and Biegert, J. Strong-field approximation for the wavelength scaling of high-harmonic generation. Phys. Rev. A 86, 023813 (2012). 10.1103/PhysRevA.86.023813. [5] Silva, F., Austin, D., Thai, A., Baudisch, M., Hemmer, M., Faccio, D., Couairon, A., and Biegert, J. Multi-octave supercontinuum generation from mid-infrared filamentation in a bulk crystal. Nat. Commun. 3, 807 (2012). 10.1038/ncomms1816.

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