# Publications by Wilhelm Eschen

All publications of HI Jena

## 2022

W. Eschen, L. Loetgering, V. Schuster, R. Klas, A. Kirsche, L. Berthold, M. Steinert, T. Pertsch, H. Gross, M. Krause, J. Limpert, and J. Rothhardt
Material-specific high-resolution table-top extreme ultraviolet microscopy
Light: Science & Applications 11, 2678 (2022)

Abstract: Microscopy with extreme ultraviolet (EUV) radiation holds promise for high-resolution imaging with excellent material contrast, due to the short wavelength and numerous element-specific absorption edges available in this spectral range. At the same time, EUV radiation has significantly larger penetration depths than electrons. It thus enables a nano-scale view into complex three-dimensional structures that are important for material science, semiconductor metrology, and next-generation nano-devices. Here, we present high-resolution and material-specific microscopy at 13.5 nm wavelength. We combine a highly stable, high photon-flux, table-top EUV source with an interferometrically stabilized ptychography setup. By utilizing structured EUV illumination, we overcome the limitations of conventional EUV focusing optics and demonstrate high-resolution microscopy at a half-pitch lateral resolution of 16 nm. Moreover, we propose mixed-state orthogonal probe relaxation ptychography, enabling robust phase-contrast imaging over wide fields of view and long acquisition times. In this way, the complex transmission of an integrated circuit is precisely reconstructed, allowing for the classification of the material composition of mesoscopic semiconductor systems.

## 2021

W. Eschen, S. Wang, C. Liu, R. Klas, M. Steinert, S. Yulin, H. Meissner, M. Bussmann, T. Pertsch, J. Limpert, and J. Rothhardt
Towards attosecond imaging at the nanoscale using broadband holography-assisted coherent imaging in the extreme ultraviolet
Communications Physics 4, 154 (2021)

Abstract: The inherently broad bandwidth of attosecond pulses conflicts with the coherence requirements of lensless imaging. Here, broadband holography-assisted coherent imaging is demonstrated with a resolution of less than 35 nm. Nanoscale coherent imaging has emerged as an indispensable modality, allowing to surpass the resolution limit given by classical imaging optics. At the same time, attosecond science has experienced enormous progress and has revealed the ultrafast dynamics in complex materials. Combining attosecond temporal resolution of pump-probe experiments with nanometer spatial resolution would allow studying ultrafast dynamics on the smallest spatio-temporal scales but has not been demonstrated yet. To date, the large bandwidth of attosecond pulses poses a major challenge to high-resolution coherent imaging. Here, we present broadband holography-enhanced coherent imaging, which enables the combination of high-resolution coherent imaging with a large spectral bandwidth. By implementing our method at a high harmonic source, we demonstrate a spatial resolution of 34 nm in combination with a spectral bandwidth of 5.5 eV at a central photon energy of 92 eV. The method is single-shot capable and retrieves the spectrum from the measured diffraction pattern.

J. White, S. Wang, W. Eschen, and J. Rothhardt
Real-time phase-retrieval and wavefront sensing enabled by an artificial neural network
Optics Express 29, 1 (2021)

Abstract: In this manuscript we demonstrate a method to reconstruct the wavefront of focused beams from a measured diffraction pattern behind a diffracting mask in real-time. The phase problem is solved by means of a neural network, which is trained with simulated data and verified with experimental data. The neural network allows live reconstructions within a few milliseconds, which previously with iterative phase retrieval took several seconds, thus allowing the adjustment of complex systems and correction by adaptive optics in real time. The neural network additionally outperforms iterative phase retrieval with high noise diffraction patterns.

L. Loetgering, T. Aidukas, M. Du, D. B. Flaes, D. Penagos, M. Rose, A. Pelekanidis, A. D. Beurs, W. Eschen, J. Hess, T. Wilhein, R. Heintzmann, J. Rothhardt, and S. Witte
ptyLab: a cross-platform inverse modeling toolbox for conventional and Fourier ptychography
(2021)

Abstract: We present a cross-platform software, called ptyLab, enabling both conventional and Fourier ptychographic data analysis. The unified framework will accelerate cross- pollination between the two techniques. The code is available open-source in both MAT-LAB and Python.

W. Eschen, L. Loetgering, V. Schuster, R. Klas, J. Limpert, and J. Rothhardt
Table-top high-resolution ptychographic EUV imaging
(2021)

Abstract: Nanoscale imaging at 13.5 nm provides ideal opportunities for ‘at wavelength’ metrology. We present a setup and the latest results on lensless ptychographic imaging at 92 eV achieving sub 30 nm resolution.

A. Kirsche, R. Klas, M. Gebhardt, L. Eisenbach, W. Eschen, J. Buldt, H. Stark, J. Rothhardt, and J. Limpert
Continuously tunable high photon flux high harmonic source at 50-70 eV
(2021)

Abstract: Table-top extreme ultraviolet (XUV) light sources based on high harmonic generation (HHG) have emerged as a user-friendly, complementary technique to large scale facilities like synchrotrons and free electron lasers [1] . The ultrashort pulse duration (femtosecond to attosecond) and coherence of these laser-driven sources are highly advantageous for many applications, e.g. coherent diffractive imaging [2] or XUV spectroscopy of atoms, molecules and ions [3] . In addition to the precise control of temporal and spatial properties of the HHG radiation, investigations of narrow band resonances and detailed spectral features in the XUV require tuning of the discrete harmonic lines. Therefore, there is a strong application-driven demand for powerful, tunable XUV combs. To date, a large variety of tunable XUV sources have been demonstrated e.g. by shifting the driving wavelength using optical parametric amplifiers [4] or soliton-plasma dynamics [5] , among others [6] .

W. Eschen, V. Schuster, S. Wang, L. Loetgering, C. Liu, R. Klas, J. Limpert, and J. Rothhardt
Ultrafast nanoscale XUV table-top coherent diffractive imaging
(2021)

## 2020

W. Eschen, G. Tadesse, Y. Peng, M. Steinert, T. Pertsch, J. Limpert, and J. Rothhardt
Single-shot characterization of strongly focused coherent XUV and soft X-ray beams
Optics Letters 45, 4798 (2020)

Abstract: In this Letter, we present a novel, to the best of our knowledge, single-shot method for characterizing focused coherent beams. We utilize a dedicated amplitude-only mask, in combination with an iterative phase retrieval algorithm, to reconstruct the amplitude and phase of a focused beam from a single measured far-field diffraction pattern alone. In a proof-of-principle experiment at a wavelength of 13.5 nm, we demonstrate our new method and obtain an RMS phase error of better than $\lambda /70$. This method will find applications in the alignment of complex optical systems, real-time feedback to adaptive optics, and single-shot beam characterization, e.g., at free-electron lasers or high-order harmonic beamlines.

F. Tuitje, W. Eschen, G. Tadesse, J. Limpert, J. Rothhardt, and C. Spielmann
Reliability of ptychography on periodic structures
OSA Continuum 3, 1691 (2020)
R. Klas, W. Eschen, A. Kirsche, J. Rothhardt, and J. Limpert
Generation of coherent broadband high photon flux continua in the XUV with a sub-two-cycle fiber laser
Optics Express 28, 6188 (2020)

Abstract: High harmonic sources can provide ultrashort pulses of coherent radiation in the XUV and X-ray spectral region. In this paper we utilize a sub-two-cycle femtosecond fiber laser to efficiently generate a broadband continuum of high-order harmonics between 70 eV and 120 eV. The average power delivered by this source ranges from > 0.2 µW/eV at 80 eV to >0.03 µW/eV at 120 eV. At 92 eV (13.5 nm wavelength), we measured a coherent record-high average power of 0.1 µW/eV, which corresponds to 7 · 109 ph/s/eV, with a long-term stability of 0.8% rms deviation over a 20 min time period. The presented approach is average power scalable and promises up to 1011 ph/s/eV in the near future. With additional carrier-envelop phase control even isolated attosecond pulses can be expected from such sources. The combination of high flux, high photon energy and ultrashort (sub-) fs duration will enable photon-hungry time-resolved and multidimensional studies.

R. Klas, W. Eschen, A. Kirsche, J. Rothhardt, and J. Limpert
Power scalable fiber laser driven high-harmonic source for broadband high photon flux continua
(2020)

Abstract: A HHG source generating a broadband continuum from 70 eV to 120 eV with an average power of 2 µW is presented. At 92 eV (13.5 nm) 7 10^9 ph/s/eV are generated with an rms deviation of 0.8% over 20 minutes.

## 2019

G. Tadesse, W. Eschen, R. Klas, M. Tschernajew, T. Frederik, M. Steinert, M. Zilk, V. Schuster, M. Zürch, T. Pertsch, C. Spielmann, J. Limpert, and J. Rothhardt
Wavelength-scale ptychographic coherent diffractive imaging using a high-order harmonic source
Scientific Reports 9, 1735 (2019)

Abstract: Ptychography enables coherent diffractive imaging (CDI) of extended samples by raster scanning across the illuminating XUV/X-ray beam, thereby generalizing the unique advantages of CDI techniques. Table- top realizations of this method are urgently needed for many applications in sciences and industry. Previously, it was only possible to image features much larger than the illuminating wavelength with table-top ptychography although knife-edge tests suggested sub-wavelength resolution. However, most real-world imaging applications require resolving of the smallest and closely-spaced features of a sample in an extended field of view. In this work, resolving features as small as 2.5 \lambda (45 nm) using a table-top ptychography setup is demonstrated by employing a high-order harmonic XUV source with record-high photon flux. For the first time, a Rayleigh-type criterion is used as a direct and unambiguous resolution metric for high-resolution table-top setup. This reliably qualifies this imaging system for real-world applications e.g. in biological sciences, material sciences, imaging integrated circuits and semiconductor mask inspection.

## 2018

J. Rothhardt, G. Tadesse, W. Eschen, and J. Limpert
Table-top nanoscale coherent imaging with XUV light
Journal of Optics 20, 113001 (2018)

Abstract: Modern laser-based XUV light sources provide very high photon fluxes which have previously only been available at large scale facilities. This allows high-performance XUV nanoscale imaging to be implemented in a table-top manner, and thus qualifies XUV imaging as a novel imaging technique complementing electron and visible-light microscopy. This article presents the current state-of-the-art in table-top XUV light sources and matched coherent imaging schemes. Selected experiments demonstrate the unique capabilities of XUV imaging—namely, nanoscale (sub-20 nm) resolution, single shot imaging, imaging of extended samples and 3D imaging of µm-sized objects. In addition, future prospects will be discussed, including scaling to few-nm resolution, extension to the soft x-ray spectral region, chemically-specific imaging at absorption edges and time-resolved imaging on femtosecond time-scales.

G. Tadesse, W. Eschen, R. Klas, V. Hilbert, D. Schelle, A. Nathanael, M. Zilk, M. Steinert, F. Schrempel, T. Pertsch, A. Tünnermann, J. Limpert, and J. Rothhardt
High resolution XUV Fourier transform holography on a table top
Scientific Reports 8, 8677 (2018)

Abstract: Today, coherent imaging techniques provide the highest resolution in the extreme ultraviolet (XUV) and X-ray regions. Fourier transform holography (FTH) is particularly unique, providing robust and straightforward image reconstruction at the same time. Here, we combine two important advances: First, our experiment is based on a table-top light source which is compact, scalable and highly accessible. Second, we demonstrate the highest resolution ever achieved with FTH at any light source (34 nm) by utilizing a high photon flux source and cutting-edge nanofabrication technology. The performance, versatility and reliability of our approach allows imaging of complex wavelength-scale structures, including wave guiding effects within these structures, and resolving embedded nanoscale features, which are invisible for electron microscopes. Our work represents an important step towards real-world applications and a broad use of XUV imaging in many areas of science and technology. Even nanoscale studies of ultra-fast dynamics are within reach.