131 130 Principal Scientist Profiles Jan Rothhardt Jan Rothhardt Principal Scientist Profiles GROUP LEADER AT THE HELMHOLTZ INSTITUTE JENA AND AT THE INSTITUTE OF APPLIED PHYSICS Dr. Rothhardt is head of the Soft X-ray Spectroscopy and Microscopy Group at the Helmholtz-Institute Jena and a member of the extended board of directors of the Helmholtz Institute Jena. JAN ROTHHARDT RESEARCH AREAS Dr. Rothhardt investigates matter on smallest spatial and temporal scales by using modern laser-based XUV and soft x-ray sources. His research interests include: • Laser-based short wavelength sources • Nanometer scale imaging techniques • Ultrafast XUV spectroscopy of molecules and highlycharged ions TEACHING FIELDS Dr. Rothhardt’s teaching activities cover Nonlinear Optics, Laser Physics and Ultrafast Laser Spectroscopy. RESEARCH METHODS Dr. Rothhardt’s group utilizes a variety of modern imaging and spectroscopy techniques including: • Coherent diffraction imaging and holographic techniques • XUV Laser spectroscopy & XUV Fourier-Transform spectroscopy • Ultrafast pump-probe spectroscopy The group utilizes modern experimental equipment including: • High average power femtosecond lasers • High photon flux table-top XUV and soft x-ray sources • XUV and soft x-ray spectrometers and detectors • High performance computers and clusters for image processing RECENT RESEARCH RESULTS Recent research of Dr. Rothhardt has been focused on the development and applications of high photon flux XUV and soft x-ray sources. This included the demonstration of phase matching and efficient high harmonic generation at high repetition rates in the tight focusing regime [1], resonant enhancement of the macroscopic yield of high harmonic generation by Fano-resonances [2], the demonstration of a high photon flux XUV sources delivering up to 1 mW of average power per harmonic in the XUV [3, 4]. These unique sources have recently been employed for lensless imaging at the nanoscale. WAVELENTH-SCALE LENSLESS XUV IMAGING Short wavelength radiation in the extreme ultraviolet (XUV) and soft X-ray spectral region enables high contrast and high-resolution imaging. In our group, we utilize the highly coherent radiation of table-top high harmonic sources to perform lensless imaging with wavelength-scale resolution down to only a few nanometers. This novel method offers insights into e.g biological samples, which is so far not possible with other techniques. The employed lensless imaging methods are based on computational image formation by “digital optics”. Hence, the image forming lens is replaced by an iterative algorithm or neural network, which completely eliminates aberrations and losses. This approach enables high resolution, high contrast images with a minimized dose on the sample. In our latest experiments we used a high photon flux, 18 nm wavelength, laser-like high-order harmonic source to achieve record imaging performance by different lensless methods. A record resolution of 13 nm has been achieved using coherent diffractive imaging on isolated samples [5]. Waveguiding effects in nanoscale structures have been observed with wavelength-scale resolution via Fourier-transform holography [6]. Recently, we achieved 45 nm resolution on an extended Siemens-Star test object by employing Ptchography, a scanning lensless imaging technique [7], which enables real-world applications e.g. in EUV metrology and biology. Our current research is focused on pushing resolution limits to the few-nm range [8], exploring broadband (potentially materialselective and ultrafast) lensless imaging, and enabling imaging in the so-called water window for biological and solid-state applications. Figure 1: Large field of view XUV image of a Siemens-Star test pattern recorded with Ptychography. The obtained resolution is as small as 45 nm [7]. Figure 2: a) Simulation of the transmitted XUV light field behind a test sample. b) XUV image of the letter P obtained via FourierTransform-Holography (FTH). c) Helium-ion microscope image of the sample structure [6]. [1] Rothhardt et al., New J. Phys., 16, 033022 (2014). [2] Rothhardt et al., Phys. Rev. Lett. 112, 233002 (2014). [3] Hädrich et al., Nat. Photonics 8, 779 (2014). [4] Klas et al., Optica 3, 1167 (2016). [5] Tadesse et Al., Opt. Lett. 41, 5170 (2016). [6] Tadesse et al., Sci. Rep. 8, 8677 (2018). [7] Tadesse et al., Sci. Rep. 9, 1 (2019). [8] Rothhardt et al., J. Opt. 20, 113001 (2018). Contact: Phone: + 49 3641 9-47818 Email: jan.rothhardt@uni-jena.de
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