EGAS41 - Swansea University

41 st EGAS CP 114 Gdańsk 2009
XUV frequency comb spectroscopy
D.Z. Kandula, C. Gohle, T.J. Pinkert, A. Renault,
W. Ubachs, K.S.E. Eikema ∗
LCVU, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081HV Amsterdam,
The Netherlands
∗ Corresponding author: kjeld@few.vu.nl,
Precision spectroscopy in the extreme ultraviolet on helium atoms and ions is very interesting
as it could provide more stringent tests of quantum electrodynamics than currently
possible with hydrogen. One problem is that spectroscopy in this region typically requires
amplification and harmonic upconversion of visible light sources, which can introduce difficult
to control systematic errors as a result of so-called frequency-chirping effects [1,2].
This issue can be overcome using a combination of amplification and harmonic upconversion
of a frequency comb laser. With this technique a pair of phase-locked extreme
ultraviolet pulses is generated and used to excite the 1s 2 1 S 0 −1s5p 1 P 1 transition in helium.
Viewed in the frequency domain, the spectrum of the pulses in the XUV resembles
again a frequency comb, but now in the form of a cosine-modulated spectrum. Viewed in
the time-domain, excitation with phase-locked pulses is in fact a form of Ramsey excitation
(see e.g. [3,4]).
The setup consists of a Ti:Sapphire frequency comb as a source of phase controlled
pulses at 773 nm. A dedicated parametric amplifier is used to amplify two subsequent
pulses from the comb laser to a few mJ per pulse. The 15th harmonic is used to excite
the 1s 2 1s5p transition in atomic helium at 51.56 nm. The excited atoms are ionized with
an infrared laser pulse at 1064 nm and detected in a time of flight spectrometer. By
changing the delay between the pulses (i.e. the repetition rate of the frequency comb
laser) in one-attosecond steps, we scan the cosine-shaped comb over the transition.
To achieve an accuracy in the MHz range, a careful analysis is required of phase shifts
and wave front deformation in the amplification and the harmonic upconversion process,
as it shifts the positions of the modes in the XUV. This is achieved with an interferometric
measuring technique which has a single-laser shot accuracy of 10 mrad [5]. Many other
systematic effects have been investigated in detail, which should allow an initial target
accuracy of 10 MHz for the ground state of helium (which is 5 times the current best
value [1,2])
References
[1] K.S.E. Eikema et al., Phys. Rev. A55, 1866-1883 (1997)
[2] S.D. Bergeson et al., Phys. Rev. Lett. 80, 3475 (1998)
[3] S. Witte et al., Science 307, 400-403 (2005)
[4] S. Cavalieri et al., Phys. Rev. Lett. 89, 133002 (2002)
[5] D.Z. Kandula et al., Optics Express 16, 7071-7082 (2008)
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