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Similarly to Fourier Transform Spectroscopy, Dual Comb Spectroscopy enables the measurement of ultra-broadband absorption spectra with high spectral resolution but the acquisition times can be up to one million times shorter . The new dual comb spectroscopy project at the IEP is dedicated to the further development of this young spectroscopy method towards novel applications in atomic, molecular and solid state physics via Raman and ultraviolet absorption spectroscopy.
Figure 1: Extreme Ultraviolet (XUV) Dual Comb Spectroscopy yielding ultra-broadband (>150 THz) absorption spectra of atoms and molecules with an unparalleled spectral resolution on the order of µeV. This resolution is hitherto unprecedented for XUV table top sources but mandatory for example for the resolution of high principal quantum numbers in Rydberg series (the example shows the Rydberg series in argon converging against the ionization limit at 15.76 eV).
With this high level of spectroscopic sensitivity, the effect of the nuclear spin in the process of photo ionization can for example be studied with different isotopes.
Following up on the first nonlinear realizations of dual comb Raman spectroscopy at the Max- Planck-Institute of Quantum Optics [2,3], one of our goals is the further development of dual comb Raman spectroscopy for directly accessing strong fundamental molecular fingerprint transitions in liquids and solids and, in combination with imaging modalities, the study of chemical reaction processes, also on surfaces by the use of ultrashort laser pulses in the near-infrared and visible spectral region .
So far our knowledge about (extreme) UV characteristics of atomic and molecular gasses and condensed matter is scares due to the lack of readily available laser sources. Especially high resolution spectroscopic studies with an advanced relative spectral resolution on the order of 10-5 and 10-6 still have to be performed at synchrotron sources that are only restrictedly accessible. In the recent years, the high power laser technology has prospered such that table top high photon flux (X)UV sources with high repetition rates become possible not only with enhancement cavities  but also in single pass geometry. However, laboratory XUV spectroscopic studies are still limited by conventional grating spectrometers that typically provide a spectral resolution on the order of 10-3 [5,6]. At the IEP, we are currently developing the world`s first UV dual comb spectrometer that will enable an unparalleled relative spectral resolution of up to 10-9 outperforming synchrotron based spectrometers by up to three orders of magnitude (see figure 1) and grating based XUV spectrometers by even up to six orders of magnitude. This new level of sensitivity will enable for example to study the role of the nuclear spin in the process of photo ionization to an unprecedented level of detail.
 B. Bernhardt, A. Ozawa, P. Jacquet, M. Jacquey, Y., T. Udem, R. Holzwarth, G. Guelachvili, T. W. Hänsch and N. Picqué, Cavity-enhanced dual-comb spectroscopy, Nature Photonics 4, 55-57 (2009)
 T. Ideguchi, S. Holzner, B. Bernhardt, G. Guelachvili, N. Picqué and T. W. Hänsch, Coherent Raman spectro-imaging with laser frequency combs, Nature 502, 355 (2013)
 T. Ideguchi, B. Bernhardt, G. Guelachvili, T.W. Hänsch and N. Picqué, Raman-induced Kerr-effect dual-comb spectroscopy, Optics Letters, Vol. 37, 4498 (2012)
 B. Bernhardt, A. Ozawa, A. Vernaleken, I. Pupeza, J. Kaster, Y. Kobayashi, R. Holzwarth E. Fill, F. Krausz, T. W. Hänsch, and Th. Udem, Vacuum ultraviolet frequency combs generated by a femtosecond enhancement cavity in the visible, Optics Letters 4, 503 (2012)
 B. Bernhardt, A. R. Beck, X. Li, E. R. Warrick, M. J. Bell, D. J. Haxton, C. W. McCurdy, D. M. Neumark and S. R. Leone, High-spectral-resolution attosecond absorption spectroscopy of autoionization in xenon, Physical Review A 89, 023408 (2014)
 K. Hütten, M. Mittermair, S. Stock, R. Beerwerth, V. Shirvanyan, J. Riemensberger, A. Duensing, R. Heider, M. Wagner, A. Guggenmos, S. Fritzsche, N. M. Kabachnik, R. Kienberger and B. Bernhardt, Ultrafast Quantum Control of Ionization Dynamics, Nature Communications 9, 719 (2018)
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Institut of Experimental Physics
Graz University of Technology
Dipl.-Phys. (Univ.) Dr.rer.nat.
Tel.: +43 (0) 316 / 873 - 8663
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Dr. Birgitta BERNHARDT
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