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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:
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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) |
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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. |
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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.
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