The junior research group for ultraviolet dual comb spectroscopy, led by Jun. Prof. Birgitta Bernhardt and funded by the Carl Zeiss Foundation, has been established at the Institute for Applied Physics in 2017 and is devoted to absorption spectroscopy with ultrahigh spectral resolution, especially in the (extreme) ultraviolet (XUV) region.
Dual comb spectroscopy is an innovative form of traditional Fourier transform spectroscopy (FTS). The combination of two short pulse lasers with slightly different repetition rates abandons a moving mirror, so far being the characteristic but also limiting component of FTS as the most abundant technique for broadband high-resolution spectroscopy. In this way, one million times shorter measurement times with simultaneously unprecedented spectral resolution become possible. Due to the versatility of the method, a variety of dual comb spectrometers have been realized recently in the THz, visible and infrared spectral region (see for example [1,2]). Dual Comb Spectroscopy has lately been introduced also to nonlinear spectroscopic techniques (CARS, [3]).
The group is currently working on the world's first dual comb spectrometer in the (X)UV between 4 and 60 eV via high harmonic generation [4]. The innovative spectrometer with table-top size that will outperform any synchrotron source in terms of spectral resolution by at least one order of magnitude will enable the clarification of diverse questions about physical properties of atoms, molecules and solids.
Further projects of the group involve absorption spectroscopy of gasses and solids with temporal resolution in the femtosecond and attosecond time domain (following the studies of [5-7]). Most recently, the combination of transient absorption spectroscopy and ionization spectroscopy enabled the ultrafast quantum control of the ionization dynamics in krypton [8].

Simplified scheme of ultraviolet dual comb spectroscopy: Two near infrared high power fiber frequency
combs with slightly detuned repetition frequencies are frequency-converted via high harmonic generation
into the extreme ultraviolet region. The combs are spatially overlapped, interacting with a sample and
generate a time-dependent beating signal on a photo detector. After Fourier transformation of this inter-
ferogram, broadband XUV absorption spectra of the sample with extreme spectral resolution (ΔE/E ~ 10-6)
can be retrieved.

[1] 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, Vol. 4, 55-57 (2010)
[2] B. Bernhardt, E. Sorokin, P. Jacquet, R. Thon, T. Becker, I. T. Sorokina,
     N. Picqué and T. W. Hänsch, Mid-infrared dual-comb spectroscopy with
     2.4 µm Cr2+:ZnSe femtosecond lasers, Applied Physics B - Lasers and
     Optics, Vol. 100, 3 - 8 (2010)
[3] 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)
[4] 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 Vol. 4, 503 (2012)
[5] 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)
[6] A. R. Beck, B. Bernhardt, E. R. Warrick, M. Wu, S. Chen, M. B. Gaarde, K. Schafer,
     D. M. Neumark and S. R. Leone, Attosecond transient absorption probing of
     electronic superpositions of bound states in neon: detection of quantum beats,
     New J. Phys. 16, 113016 (2014)
[7] X. Li, B. Bernhardt, A. R. Beck, E. R. Warrick, M. J. Bell, D. J. Haxton,
     C. W. McCurdy, D. M. Neumark and S. R. Leone, Investigation of coupling
     mechanisms in attosecond transient absorption of auto-ionizing states:
     comparison of theory and experiment in xenon, J. Phys. B: At. Mol. Opt. Phys. 48,
    125601 (2015)
[8] K. Hütten, M. Mittermair, S. Stock, R. Beerwerth, V. Shirvanyan, J. Riemensberger,
     A. Duensing, R. Heider, M. S. Wagner, A. Guggenmos, S. Fritzsche, N. M. 
     Kabachnik, R. Kienberger and B. Bernhardt, Ultrafast Quantum Control of 
     Ionization Dynamics in Krypton, Nature Communications, Vol. 9, 719 (2018)

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