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High-order Harmonic Generation

This project is a part of the Australian Research Council (ARC) Centre of Excellence for Coherent X-Ray Science (CXS), which aims to be the world leader in the development of non-crystallographic techniques for the determination of membrane protein structures. The aims of our work involve generating a high flux of coherent photons in the water window (a spectral region from 4.2 to 2 nm where carbon is strongly absorbing and water is not) for imaging of biological molecules in-vivo. The process by which we intend to do this is known as high-order harmonic generation (HHG).

HHG uses the high peak energy of ultrashort laser pulses to generate even shorter pulses in the extreme ultravoilet (XUV) to soft X-ray region of the spectrum by focussing the beam into a gas. Odd harmonics of the exciting laser frequency are produced in a directed, narrow divergence, highly coherent beam. The simple classical picture describes this process in the following three steps [Corkum, Phys. Rev. Lett. 71, 1994 (1993)]:

We have recently (March 2007) installed a new laser and amplifier system capable of producing pulses less than 30 fs in duration with pulse energy >6 mJ at a repetition rate of 1kHz. The short duration and high peak intensity when focussing these pulses are necessary to achieve the sub-4nm radiation with high flux desired.

We are currently investigating several pathways to obtain harmonics with wavelength down to 4 nm:


HHG in a waveguide 

Generating fully coherent x-ray beams requires that the conversion process be phase matched. To build up coherently over an extended propagation distance the XUV and the laser light must travel with the same phase velocity; i.e. the process must be phase matched. When this is the case, the nonlinear response from the medium continues to add constructively to the signal beam. However, due to the large difference in wavelength between the harmonic beam and the laser beam, and depending on their refractive indices the noble gas and the created plasma, they can have greatly different phase velocities. By propagating the beam in a hollow waveguide the gas pressure in the interaction region can be precisely controlled to match the phase velocity of the laser light and the XUV harmonics.

For further information see e.g.: R.A. Bartels et al, Science 297, 376 (2002)

multiple becs

HHG in a modulated waveguide

It has been shown recently that where phase-matching the XUV harmonic beam and the laser beam becomes difficult, quasi-phase matching can be used instead (A. Paul et al, Nature 421, 51 (2003)). In this process, the nonlinear interaction responsible for generating the harmonics is supressed when the XUV and laser beams are out of phase and enhanced when they are in phase. In this way, harmonics are only generated in phase and add constructively.

In a modulated hollow core waveguide, the intensity of the laser is modulated with the waveguide diameter, and so the HHG process is switched on and off at the modulation period. We are developing a novel means of generating modulated waveguides that will allow much greater flexibility and control of the depth, shape, and period of the modulations. This is expected to have significant advantages over previously reported modulated waveguides.

atom chip

HHG in an ionic medium

The energy and flux attainable from HHG depends strongly on the medium used to generate the XUV beam. The energy depends strongly on the ionization potential of the atoms, and the flux varies with the cross-section of the atoms. Traditionally neutral noble gas atoms are used as the nonlinear medium, however the ionization potential decreases while the cross section increases as you move down the periodic table. One solution is to ionize the atoms prior to HHG, thereby significantly increasing the ionization potential of the nonlinear medium without changing the cross-section. We are exploring several ways to achieve this, including: sending through a pre-pulse to ionise the atoms, and using an electric discharge to ionise the atoms.